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101 lines
3.5 KiB
Markdown
101 lines
3.5 KiB
Markdown
# cpu features
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A cross platform C89 library to get cpu features at runtime.
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### Checking features at runtime.
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```c
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#include "cpuinfo_x86.h"
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static const X86Features features = GetX86Info().features;
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void Compute(void) {
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if(features.aes && features.sse4_2) {
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// Run optimized code.
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} else {
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// Run standard code.
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}
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}
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```
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### Caching for faster evaluation of complex checks.
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Features are bit packed and some compilers generate poor code when combining
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them. A simple solution is to read them once and store the result in a global
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variable.
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```c
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#include "cpuinfo_arm.h"
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static const ArmFeatures features = GetArmInfo().features;
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static const bool has_aes_and_neon = features.aes && features.neon;
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// use has_aes_and_neon.
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```
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### Checking compile time flags as well.
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```c
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#include "cpuinfo_x86.h"
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static const X86Features features = GetX86Info().features;
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static const bool has_avx = CPU_FEATURES_COMPILED_X86_AVX || features.avx;
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// use has_avx.
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```
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`CPU_FEATURES_COMPILED_X86_AVX` is set to 1 if the compiler is instructed to use
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AVX, 0 otherwise. This allows combining compile time and runtime knowledge.
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### Use x86 microarchitecture to reject poor hardware implementations.
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```c
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#include "cpuinfo_x86.h"
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static const X86Info info = GetX86Info();
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static const X86Microarchitecture uarch = GetX86Microarchitecture(&info);
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static const bool has_fast_avx = info.features.avx && uarch != INTEL_SNB;
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// use has_fast_avx.
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```
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On x86, the first incarnation of a feature in a microarchitecture may not be
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very efficient (e.g. AVX on Sandybridge). We provide a function to retrieve the
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underlying microarchitecture so clients can decide whether they want to use it
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or not.
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## What does it currently support
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| | x86 | ARM | aarch64 | mips | POWER |
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|---------------------------- | :-: | :-: | :-----: | :----: | :-----: |
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|Features From cpu | yes | no* | no* | no yet | not yet |
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|Features From Linux | no | yes | yes | yes | not yet |
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|Micro Architecture Detection | yes | no | no | no | not yet |
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|Windows support | yes | no | no | no | not yet |
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- **Features From Cpuid**: features are retrieved by using the cpuid
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instruction. (*) Unfortunately this instruction is privileged for some
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architectures; in this case we fall back to Linux.
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- **Features From Linux**: we gather data from several sources depending on
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what's available:
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+ from glibc's
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[getauxval](https://www.gnu.org/software/libc/manual/html_node/Auxiliary-Vector.html)
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+ by parsing `/proc/self/auxv`
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+ by parsing `/proc/cpuinfo`
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- **Micro Architecture Detection**: On x86 some features are not always
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implemented efficiently in hardware (e.g. AVX on Sandybridge). Exposing the
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microarchitecture allows the client to reject some microarchitectures.
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## Design Rationale
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- Simple to use, API should be straightforward from the snippets above.
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- Unit tested.
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- Extensible. It should be easy to add missing features or architectures.
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- Compatible with old compilers and available on many architectures so it can
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be used widely. We target gnu89.
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- Works in sandboxed environment: some architectures rely on parsing files
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that may not be available in a sandboxed environment. It is useful when
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running integration tests in hermetic environments.
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- Thread safe, no allocation, no exception: suitable for implementation of
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fundamental libc functions like malloc, memcpy, memcmp...
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