Addition is simply XOR. If processed bit by bit, then, after shifting, a conditional XOR with 1B 16 should be performed if the shifted value is larger than FF 16 overflow must be corrected by subtraction of generating polynomial. This process is described further in the article Rijndael MixColumns. In the AddRoundKey step, the subkey is combined with the state.

For each round, a subkey is derived from the main key using Rijndael's key schedule ; each subkey is the same size as the state.

The subkey is added by combining each byte of the state with the corresponding byte of the subkey using bitwise XOR. On systems with bit or larger words, it is possible to speed up execution of this cipher by combining the SubBytes and ShiftRows steps with the MixColumns step by transforming them into a sequence of table lookups.

This requires four entry bit tables together occupying bytes. A round can then be performed with 16 table lookup operations and 12 bit exclusive-or operations, followed by four bit exclusive-or operations in the AddRoundKey step.

Using a byte-oriented approach, it is possible to combine the SubBytes , ShiftRows , and MixColumns steps into a single round operation. The National Security Agency NSA reviewed all the AES finalists, including Rijndael, and stated that all of them were secure enough for U. Government non-classified data.

In June , the U. Government announced that AES could be used to protect classified information :. The design and strength of all key lengths of the AES algorithm i. TOP SECRET information will require use of either the or key lengths. By , the best known attacks were on 7 rounds for bit keys, 8 rounds for bit keys, and 9 rounds for bit keys. For cryptographers, a cryptographic "break" is anything faster than a brute-force attack — i.

Despite being impractical, theoretical breaks can sometimes provide insight into vulnerability patterns. The largest successful publicly known brute-force attack against a widely implemented block-cipher encryption algorithm was against a bit RC5 key by distributed. net in The key space increases by a factor of 2 for each additional bit of key length, and if every possible value of the key is equiprobable, this translates into a doubling of the average brute-force key search time.

This implies that the effort of a brute-force search increases exponentially with key length. Key length in itself does not imply security against attacks, since there are ciphers with very long keys that have been found to be vulnerable.

AES has a fairly simple algebraic framework. During the AES selection process, developers of competing algorithms wrote of Rijndael's algorithm "we are concerned about [its] use in security-critical applications.

Until May , the only successful published attacks against the full AES were side-channel attacks on some specific implementations. In , a new related-key attack was discovered that exploits the simplicity of AES's key schedule and has a complexity of 2 In December it was improved to 2 Another attack was blogged by Bruce Schneier [22] on July 30, , and released as a preprint [23] on August 3, This new attack, by Alex Biryukov, Orr Dunkelman, Nathan Keller, Dmitry Khovratovich, and Adi Shamir , is against AES that uses only two related keys and 2 39 time to recover the complete bit key of a 9-round version, or 2 45 time for a round version with a stronger type of related subkey attack, or 2 70 time for an round version.

The practicality of these attacks with stronger related keys has been criticized, [24] for instance, by the paper on chosen-key-relations-in-the-middle attacks on AES authored by Vincent Rijmen in In November , the first known-key distinguishing attack against a reduced 8-round version of AES was released as a preprint. It works on the 8-round version of AES, with a time complexity of 2 48 , and a memory complexity of 2 The first key-recovery attacks on full AES were by Andrey Bogdanov, Dmitry Khovratovich, and Christian Rechberger, and were published in It requires 2 For AES and AES, 2 This result has been further improved to 2 This is a very small gain, as a bit key instead of bits would still take billions of years to brute force on current and foreseeable hardware.

Also, the authors calculate the best attack using their technique on AES with a bit key requires storing 2 88 bits of data. That works out to about 38 trillion terabytes of data, which is more than all the data stored on all the computers on the planet in As such, there are no practical implications on AES security.

According to the Snowden documents , the NSA is doing research on whether a cryptographic attack based on tau statistic may help to break AES. At present, there is no known practical attack that would allow someone without knowledge of the key to read data encrypted by AES when correctly implemented.

Side-channel attacks do not attack the cipher as a black box , and thus are not related to cipher security as defined in the classical context, but are important in practice. They attack implementations of the cipher on hardware or software systems that inadvertently leak data. There are several such known attacks on various implementations of AES. In April , D.

Bernstein announced a cache-timing attack that he used to break a custom server that used OpenSSL 's AES encryption. However, as Bernstein pointed out, "reducing the precision of the server's timestamps, or eliminating them from the server's responses, does not stop the attack: the client simply uses round-trip timings based on its local clock, and compensates for the increased noise by averaging over a larger number of samples".

In October , Dag Arne Osvik, Adi Shamir and Eran Tromer presented a paper demonstrating several cache-timing attacks against the implementations in AES found in OpenSSL and Linux's dm-crypt partition encryption function.

This attack requires the attacker to be able to run programs on the same system or platform that is performing AES. In December an attack on some hardware implementations was published that used differential fault analysis and allows recovery of a key with a complexity of 2 In November Endre Bangerter, David Gullasch and Stephan Krenn published a paper which described a practical approach to a "near real time" recovery of secret keys from AES without the need for either cipher text or plaintext.

The approach also works on AES implementations that use compression tables, such as OpenSSL. In March , Ashokkumar C.

Many modern CPUs have built-in hardware instructions for AES , which protect against timing-related side-channel attacks. The Cryptographic Module Validation Program CMVP is operated jointly by the United States Government's National Institute of Standards and Technology NIST Computer Security Division and the Communications Security Establishment CSE of the Government of Canada. The use of cryptographic modules validated to NIST FIPS is required by the United States Government for encryption of all data that has a classification of Sensitive but Unclassified SBU or above.

The Government of Canada also recommends the use of FIPS validated cryptographic modules in unclassified applications of its departments. Therefore, it is rare to find cryptographic modules that are uniquely FIPS validated and NIST itself does not generally take the time to list FIPS validated modules separately on its public web site.

Instead, FIPS validation is typically just listed as an "FIPS approved: AES" notation with a specific FIPS certificate number in the current list of FIPS validated cryptographic modules.

The Cryptographic Algorithm Validation Program CAVP [41] allows for independent validation of the correct implementation of the AES algorithm. Successful validation results in being listed on the NIST validations page. However, successful CAVP validation in no way implies that the cryptographic module implementing the algorithm is secure.

A cryptographic module lacking FIPS validation or specific approval by the NSA is not deemed secure by the US Government and cannot be used to protect government data. FIPS validation is challenging to achieve both technically and fiscally.

The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo. o and bar. The final invocation reads the GIMPLE bytecode from foo. o , merges the two files into a single internal image, and compiles the result as usual. Since both foo. o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one.

This means, for example, that the inliner is able to inline functions in bar. o into functions in foo. o and vice-versa. The above generates bytecode for foo. c and bar. c , merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog. The important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link step.

GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You can always override the automatic decision to do link-time optimization by passing -fno-lto to the link command. To make whole program optimization effective, it is necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit.

When supported by the linker, the linker plugin see -fuse-linker-plugin passes information to the compiler about used and externally visible symbols. When the linker plugin is not available, -fwhole-program should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions.

When a file is compiled with -flto without -fuse-linker-plugin , the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code see -ffat-lto-objects.

This means that object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster but you cannot perform a regular, non-LTO link on them. When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode.

Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing. Generally, options specified at link time override those specified at compile time, although in some cases GCC attempts to infer link-time options from the settings used to compile the input files.

If you do not specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files. Note that it is generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons. First, compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time. Second, some early optimization passes can be performed only at compile time and not at link time.

There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon , -fexceptions , -fnon-call-exceptions , -fgnu-tm and all the -m target flags. The following options -fPIC , -fpic , -fpie and -fPIE are combined based on the following scheme:.

Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return. Other options such as -ffp-contract , -fno-strict-overflow , -fwrapv , -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units.

You can override them at link time. Diagnostic options such as -Wstringop-overflow are passed through to the link stage and their setting matches that of the compile-step at function granularity.

Note that this matters only for diagnostics emitted during optimization. Note that code transforms such as inlining can lead to warnings being enabled or disabled for regions if code not consistent with the setting at compile time. When you need to pass options to the assembler via -Wa or -Xassembler make sure to either compile such translation units with -fno-lto or consistently use the same assembler options on all translation units.

You can alternatively also specify assembler options at LTO link time. To enable debug info generation you need to supply -g at compile time.

If any of the input files at link time were built with debug info generation enabled the link will enable debug info generation as well. Any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line and mixing different settings in different translation units is discouraged. If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together undefined behavior according to ISO C99 6.

The behavior is still undefined at run time. Similar diagnostics may be raised for other languages. Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages:. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular non-LTO compilation. If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.

a , it is possible to extract and use them in an LTO link if you are using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib ; to show the symbols of object files with GIMPLE bytecode, use gcc-nm.

Those commands require that ar , ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:.

With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo. a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized. a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects. Link-time optimizations do not require the presence of the whole program to operate.

If the program does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization opportunities. Use of -fwhole-program is not needed when linker plugin is active see -fuse-linker-plugin. The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC.

Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF. If you specify the optional n , the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used. This is useful when the Makefile calling GCC is already executing in parallel.

This option likely only works if MAKE is GNU make. Specify the partitioning algorithm used by the link-time optimizer. This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode -flto.

GCC currently supports two LTO compression algorithms. For zstd, valid values are 0 no compression to 19 maximum compression , while zlib supports values from 0 to 9. Values outside this range are clamped to either minimum or maximum of the supported values. If the option is not given, a default balanced compression setting is used. Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.

This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally by non-LTO object or during dynamic linking. Resulting code quality improvements on binaries and shared libraries that use hidden visibility are similar to -fwhole-program.

See -flto for a description of the effect of this flag and how to use it. This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins GNU ld 2. Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link time.

It requires a linker with linker plugin support for basic functionality. Additionally, nm , ar and ranlib need to support linker plugins to allow a full-featured build environment capable of building static libraries etc.

GCC provides the gcc-ar , gcc-nm , gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO makefiles need to be modified to use them. Note that modern binutils provide plugin auto-load mechanism. After register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic.

If possible, eliminate the explicit comparison operation. This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete. After register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy. Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates.

When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected. With -fprofile-use all portions of programs not executed during train run are optimized agressively for size rather than speed. In some cases it is not practical to train all possible hot paths in the program. For example, program may contain functions specific for a given hardware and trianing may not cover all hardware configurations program is run on.

With -fprofile-partial-training profile feedback will be ignored for all functions not executed during the train run leading them to be optimized as if they were compiled without profile feedback. This leads to better performance when train run is not representative but also leads to significantly bigger code. Enable profile feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:.

Before you can use this option, you must first generate profiling information. See Instrumentation Options , for information about the -fprofile-generate option. By default, GCC emits an error message if the feedback profiles do not match the source code.

Note this may result in poorly optimized code. Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist see -Wmissing-profile. If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir. Enable sampling-based feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:.

path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata. afdo in the current directory. You must also supply the unstripped binary for your program to this tool. The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled.

Do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point value is taken from a register or memory. This option prevents undesirable excess precision on machines such as the where the floating registers of the keep more precision than a double is supposed to have.

Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables. This option allows further control over excess precision on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types.

It may, however, yield faster code for programs that do not require the guarantees of these specifications. Do not set errno after calling math functions that are executed with a single instruction, e. A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility. On Darwin systems, the math library never sets errno. There is therefore no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default.

Allow optimizations for floating-point arithmetic that a assume that arguments and results are valid and b may violate IEEE or ANSI standards. When used at link time, it may include libraries or startup files that change the default FPU control word or other similar optimizations. Enables -fno-signed-zeros , -fno-trapping-math , -fassociative-math and -freciprocal-math. Allow re-association of operands in series of floating-point operations.

May also reorder floating-point comparisons and thus may not be used when ordered comparisons are required. This option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect. Allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. Note that this loses precision and increases the number of flops operating on the value.

Allow optimizations for floating-point arithmetic that ignore the signedness of zero. Compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation.

This option requires that -fno-signaling-nans be in effect. Disable transformations and optimizations that assume default floating-point rounding behavior. This is round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode.

This option disables constant folding of floating-point expressions at compile time which may be affected by rounding mode and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes. This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode.

Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies -ftrapping-math.

This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior. The default is -ffp-int-builtin-inexact , allowing the exception to be raised, unless C2X or a later C standard is selected. This option does nothing unless -ftrapping-math is in effect. Treat floating-point constants as single precision instead of implicitly converting them to double-precision constants. When enabled, this option states that a range reduction step is not needed when performing complex division.

The default is -fno-cx-limited-range , but is enabled by -ffast-math. Nevertheless, the option applies to all languages. Complex multiplication and division follow Fortran rules. The following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code. After running a program compiled with -fprofile-arcs see Instrumentation Options , you can compile it a second time using -fbranch-probabilities , to improve optimizations based on the number of times each branch was taken.

When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename. gcda for each source file. The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. See details about the file naming in -fprofile-arcs.

These can be used to improve optimization. Currently, they are only used in one place: in reorg. If combined with -fprofile-arcs , it adds code so that some data about values of expressions in the program is gathered.

With -fbranch-probabilities , it reads back the data gathered from profiling values of expressions for usage in optimizations. Enabled by -fprofile-generate , -fprofile-use , and -fauto-profile. Function reordering based on profile instrumentation collects first time of execution of a function and orders these functions in ascending order.

If combined with -fprofile-arcs , this option instructs the compiler to add code to gather information about values of expressions. With -fbranch-probabilities , it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator.

Attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization most benefits processors with lots of registers. Performs a target dependent pass over the instruction stream to schedule instructions of same type together because target machine can execute them more efficiently if they are adjacent to each other in the instruction flow.

Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do a better job. Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop. It also turns on complete loop peeling i. complete removal of loops with a small constant number of iterations. This option makes code larger, and may or may not make it run faster.

Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. Peels loops for which there is enough information that they do not roll much from profile feedback or static analysis. complete removal of loops with small constant number of iterations. Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1 and higher, except for -Og.

Enables the loop store motion pass in the GIMPLE loop optimizer. This moves invariant stores to after the end of the loop in exchange for carrying the stored value in a register across the iteration. Note for this option to have an effect -ftree-loop-im has to be enabled as well. Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches modified according to result of the condition.

If a loop iterates over an array with a variable stride, create another version of the loop that assumes the stride is always one. This is particularly useful for assumed-shape arrays in Fortran where for example it allows better vectorization assuming contiguous accesses. Place each function or data item into its own section in the output file if the target supports arbitrary sections.

Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations.

On AIX, the linker rearranges sections CSECTs based on the call graph. The performance impact varies. Together with a linker garbage collection linker --gc-sections option these options may lead to smaller statically-linked executables after stripping. Only use these options when there are significant benefits from doing so.

When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets.

usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors , it accesses the variables from a common anchor point instead. Zero call-used registers at function return to increase program security by either mitigating Return-Oriented Programming ROP attacks or preventing information leakage through registers. In some places, GCC uses various constants to control the amount of optimization that is done.

For example, GCC does not inline functions that contain more than a certain number of instructions. You can control some of these constants on the command line using the --param option.

The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases. In each case, the value is an integer. The following choices of name are recognized for all targets:. When branch is predicted to be taken with probability lower than this threshold in percent , then it is considered well predictable. RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions.

This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable.

RTL if-conversion will try to remove conditional branches around a block and replace them with conditionally executed instructions. These parameters give the maximum permissible cost for the sequence that would be generated by if-conversion depending on whether the branch is statically determined to be predictable or not.

The maximum number of incoming edges to consider for cross-jumping. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size. The minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched.

The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction. The maximum number of instructions to duplicate to a block that jumps to a computed goto. Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored. The maximum number of instructions to consider when looking for an instruction to fill a delay slot.

If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time.

When trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.

The approximate maximum amount of memory in kB that can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done. If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream. The maximum number of pending dependencies scheduling allows before flushing the current state and starting over.

Large functions with few branches or calls can create excessively large lists which needlessly consume memory and resources. The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can exponentially increase compilation time.

Maximal loop depth of a call considered by inline heuristics that tries to inline all functions called once. Several parameters control the tree inliner used in GCC.

When you use -finline-functions included in -O3 , a lot of functions that would otherwise not be considered for inlining by the compiler are investigated. To those functions, a different more restrictive limit compared to functions declared inline can be applied --param max-inline-insns-auto.

This is bound applied to calls which are considered relevant with -finline-small-functions. This is bound applied to calls which are optimized for size. Small growth may be desirable to anticipate optimization oppurtunities exposed by inlining. Number of instructions accounted by inliner for function overhead such as function prologue and epilogue. Extra time accounted by inliner for function overhead such as time needed to execute function prologue and epilogue. The scale in percents applied to inline-insns-single , inline-insns-single-O2 , inline-insns-auto when inline heuristics hints that inlining is very profitable will enable later optimizations.

Same as --param uninlined-function-insns and --param uninlined-function-time but applied to function thunks. The limit specifying really large functions. For functions larger than this limit after inlining, inlining is constrained by --param large-function-growth.

This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms used by the back end. Specifies maximal growth of large function caused by inlining in percents. For example, parameter value limits large function growth to 2.

The limit specifying large translation unit. Growth caused by inlining of units larger than this limit is limited by --param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. For very large units consisting of small inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code size.

Thus for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth. Specifies maximal overall growth of the compilation unit caused by inlining.

For example, parameter value 20 limits unit growth to 1. Cold functions either marked cold via an attribute or by profile feedback are not accounted into the unit size. Specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation.

For example, parameter value 10 limits unit growth to 1. The limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit too much. Specifies maximal growth of large stack frames caused by inlining in percents. For example, parameter value limits large stack frame growth to 11 times the original size.

Specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by performing recursive inlining. For functions not declared inline, recursive inlining happens only when -finline-functions included in -O3 is enabled; --param max-inline-insns-recursive-auto applies instead.

For functions not declared inline, recursive inlining happens only when -finline-functions included in -O3 is enabled; --param max-inline-recursive-depth-auto applies instead.

Recursive inlining is profitable only for function having deep recursion in average and can hurt for function having little recursion depth by increasing the prologue size or complexity of function body to other optimizers. When profile feedback is available see -fprofile-generate the actual recursion depth can be guessed from the probability that function recurses via a given call expression. This parameter limits inlining only to call expressions whose probability exceeds the given threshold in percents.

Specify growth that the early inliner can make. In effect it increases the amount of inlining for code having a large abstraction penalty. Limit of iterations of the early inliner. This basically bounds the number of nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining. This parameter ought to be bigger than --param modref-max-bases and --param modref-max-refs.

Specifies the maximum depth of DFS walk used by modref escape analysis. Setting to 0 disables the analysis completely. A parameter to control whether to use function internal id in profile database lookup. If the value is 0, the compiler uses an id that is based on function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering etc. The minimum number of iterations under which loops are not vectorized when -ftree-vectorize is used.

The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization. Scaling factor in calculation of maximum distance an expression can be moved by GCSE optimizations.

This is currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting is with simple expressions, i. Specifying 0 disables hoisting of simple expressions. Cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel.

The lesser the cost, the more aggressive code hoisting is. Specifying 0 allows all expressions to travel unrestricted distances. The depth of search in the dominator tree for expressions to hoist.

This is used to avoid quadratic behavior in hoisting algorithm. The value of 0 does not limit on the search, but may slow down compilation of huge functions. The maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior in tree tail merging. The maximum amount of iterations of the pass over the function. This is used to limit compilation time in tree tail merging.

The maximum number of store chains to track at the same time in the attempt to merge them into wider stores in the store merging pass. The maximum number of stores to track at the same time in the attemt to to merge them into wider stores in the store merging pass. The maximum number of instructions that a loop may have to be unrolled.

If a loop is unrolled, this parameter also determines how many times the loop code is unrolled. The maximum number of instructions biased by probabilities of their execution that a loop may have to be unrolled.

The maximum number of instructions that a loop may have to be peeled. If a loop is peeled, this parameter also determines how many times the loop code is peeled. When FDO profile information is available, min-loop-cond-split-prob specifies minimum threshold for probability of semi-invariant condition statement to trigger loop split.

Bound on number of candidates for induction variables, below which all candidates are considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones are considered to avoid quadratic time complexity. If the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one.

Maximum size in bytes of objects tracked bytewise by dead store elimination. Larger values may result in larger compilation times. Maximum number of queries into the alias oracle per store. Larger values result in larger compilation times and may result in more removed dead stores. Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer. Bound on the complexity of the expressions in the scalar evolutions analyzer.

Complex expressions slow the analyzer. Maximum number of arguments in a PHI supported by TREE if conversion unless the loop is marked with simd pragma.

The maximum number of possible vector layouts such as permutations to consider when optimizing to-be-vectorized code. The maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer. The maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer. The maximum number of loop peels to enhance access alignment for vectorizer.

Value -1 means no limit. The maximum number of iterations of a loop the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate. Used in non-LTO mode. It gives you the ability to download multiple files at one time and download large files quickly and reliably. It also allows you to suspend active downloads and resume downloads that have failed. Microsoft Download Manager is free and available for download now. Windows Service Pack 2, Windows ME, Windows Server , Windows XP Home Edition , Windows XP Professional Edition.

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An HTML form is a section of a document containing normal content, markup, special elements called controls checkboxes, radio buttons, menus, etc. Users generally "complete" a form by modifying its controls entering text, selecting menu items, etc. Here's a simple form that includes labels, radio buttons, and push buttons reset the form or submit it :. This specification includes more detailed information about forms in the subsections on form display issues.

Users interact with forms through named controls. A control's "control name" is given by its name attribute. The scope of the name attribute for a control within a FORM element is the FORM element. Each control has both an initial value and a current value, both of which are character strings. Please consult the definition of each control for information about initial values and possible constraints on values imposed by the control.

In general, a control's "initial value" may be specified with the control element's value attribute. However, the initial value of a TEXTAREA element is given by its contents, and the initial value of an OBJECT element in a form is determined by the object implementation i.

The control's "current value" is first set to the initial value. Thereafter, the control's current value may be modified through user interaction and scripts. A control's initial value does not change. Thus, when a form is reset, each control's current value is reset to its initial value. If a control does not have an initial value, the effect of a form reset on that control is undefined. When a form is submitted for processing, some controls have their name paired with their current value and these pairs are submitted with the form.

Authors should specify the scripting language of a push button script through a default script declaration with the META element. Authors create buttons with the BUTTON element or the INPUT element. Please consult the definitions of these elements for details about specifying different button types. Authors should note that the BUTTON element offers richer rendering capabilities than the INPUT element. Several checkboxes in a form may share the same control name.

Thus, for example, checkboxes allow users to select several values for the same property. The INPUT element is used to create a checkbox control. Since user agent behavior differs, authors should ensure that in each set of radio buttons that one is initially "on".

The elements used to create controls generally appear inside a FORM element, but may also appear outside of a FORM element declaration when they are used to build user interfaces. This is discussed in the section on intrinsic events.

Note that controls outside a form cannot be successful controls. The default value for this attribute is the reserved string "UNKNOWN". User agents may interpret this value as the character encoding that was used to transmit the document containing this FORM element. The FORM element acts as a container for controls. It specifies:. A form can contain text and markup paragraphs, lists, etc.

in addition to form controls. The following example shows a form that is to be processed by the "adduser" program when submitted. The form will be sent to the program using the HTTP "post" method. Please consult the section on form submission for information about how user agents must prepare form data for servers and how user agents should handle expected responses.

Further discussion on the behavior of servers that receive form data is beyond the scope of this specification. The control type defined by the INPUT element depends on the value of the type attribute:. Application designers should note that this mechanism affords only light security protection.

Although the password is masked by user agents from casual observers, it is transmitted to the server in clear text, and may be read by anyone with low-level access to the network. When a pointing device is used to click on the image, the form is submitted and the click coordinates passed to the server. The x value is measured in pixels from the left of the image, and the y value in pixels from the top of the image. The submitted data includes name. If the server takes different actions depending on the location clicked, users of non-graphical browsers will be disadvantaged.

For this reason, authors should consider alternate approaches:. The following sample HTML fragment defines a simple form that allows the user to enter a first name, last name, email address, and gender. When the submit button is activated, the form will be sent to the program specified by the action attribute.

In the section on the LABEL element, we discuss marking up labels such as "First name". In this next example, the JavaScript function name verify is triggered when the "onclick" event occurs:. Please consult the section on intrinsic events for more information about scripting and events. The following example shows how the contents of a user-specified file may be submitted with a form. The user is prompted for his or her name and a list of file names whose contents should be submitted with the form.

Buttons created with the BUTTON element function just like buttons created with the INPUT element, but they offer richer rendering possibilities: the BUTTON element may have content.

For example, a BUTTON element that contains an image functions like and may resemble an INPUT element whose type is set to "image", but the BUTTON element type allows content. The following example expands a previous example, but creates submit and reset buttons with BUTTON instead of INPUT.

The buttons contain images by way of the IMG element. Recall that authors must provide alternate text for an IMG element. It is illegal to associate an image map with an IMG that appears as the contents of a BUTTON element. The SELECT element creates a menu. Each choice offered by the menu is represented by an OPTION element. A SELECT element must contain at least one OPTION element.

The OPTGROUP element allows authors to group choices logically. This is particularly helpful when the user must choose from a long list of options; groups of related choices are easier to grasp and remember than a single long list of options.

In HTML 4, all OPTGROUP elements must be specified directly within a SELECT element i. Zero or more choices may be pre-selected for the user. User agents should determine which choices are pre-selected as follows:.

Since user agent behavior differs, authors should ensure that each menu includes a default pre-selected OPTION. Implementors are advised that future versions of HTML may extend the grouping mechanism to allow for nested groups i. This will allow authors to represent a richer hierarchy of choices.

When rendering a menu choice , user agents should use the value of the label attribute of the OPTION element as the choice. If this attribute is not specified, user agents should use the contents of the OPTION element.

The label attribute of the OPTGROUP element specifies the label for a group of choices. In this example, we create a menu that allows the user to select which of seven software components to install. The first and second components are pre-selected but may be deselected by the user.

The remaining components are not pre-selected. The size attribute states that the menu should only have 4 rows even though the user may select from among 7 options. The other options should be made available through a scrolling mechanism. The SELECT is followed by submit and reset buttons. Only selected options will be successful using the control name "component-select".

When no options are selected, the control is not successful and neither the name nor any values are submitted to the server when the form is submitted. Note that where the value attribute is set, it determines the control's initial value , otherwise it's the element's contents. In this example we use the OPTGROUP element to group choices.

The following markup:. Visual user agents may allow users to select from option groups through a hierarchical menu or some other mechanism that reflects the structure of choices. This image shows a SELECT element rendered as cascading menus. The top label of the menu displays the currently selected value PortMaster 3, 3. The user has unfurled two cascading menus, but has not yet selected the new value PortMaster 2, 3. Note that each cascading menu displays the label of an OPTGROUP or OPTION element.

The TEXTAREA element creates a multi-line text input control. User agents should use the contents of this element as the initial value of the control and should render this text initially. This example creates a TEXTAREA control that is 20 rows by 80 columns and contains two lines of text initially. The TEXTAREA is followed by submit and reset buttons. Setting the readonly attribute allows authors to display unmodifiable text in a TEXTAREA.

This differs from using standard marked-up text in a document because the value of TEXTAREA is submitted with the form. ISINDEX is deprecated. This element creates a single-line text input control. Authors should use the INPUT element to create text input controls. The ISINDEX element creates a single-line text input control that allows any number of characters.

Example Pipelined Table Function Transforms Each Row to Two Rows. Đó là hệ thống bus nhằm kết nối các thiết bị với nhau để truyền hoặc nhận dữ liệu. SET NAMES indicates what character set the client uses to send SQL statements to the server. However, successful CAVP validation in no way implies that the cryptographic module implementing the algorithm is secure. If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream.

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