mirror of
https://github.com/RGBCube/serenity
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bcf124c07d
This commit addresses the following shortcomings of our current, simple and elegant memset function: - REP STOSB/STOSQ has considerable startup overhead, it's impractical to use for smaller sizes. - Up until very recently, AMD CPUs didn't have support for "Enhanced REP MOVSB/STOSB", so it performed pretty poorly on them. With this commit applied, I could measure a ~5% decrease in `test-js`'s runtime when I used qemu's TCG backend. The implementation is based on the following article from Microsoft: https://msrc-blog.microsoft.com/2021/01/11/building-faster-amd64-memset-routines Two versions of the routine are implemented: one that uses the ERMS extension mentioned above, and one that performs plain SSE stores. The version appropriate for the CPU is selected at load time using an IFUNC.
59 lines
1.7 KiB
C++
59 lines
1.7 KiB
C++
/*
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* Copyright (c) 2022, Daniel Bertalan <dani@danielbertalan.dev>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#include <AK/Types.h>
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#include <cpuid.h>
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#include <string.h>
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extern "C" {
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extern void* memset_sse2(void*, int, size_t);
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extern void* memset_sse2_erms(void*, int, size_t);
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constexpr u32 tcg_signature_ebx = 0x54474354;
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constexpr u32 tcg_signature_ecx = 0x43544743;
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constexpr u32 tcg_signature_edx = 0x47435447;
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// Bit 9 of ebx in cpuid[eax = 7] indicates support for "Enhanced REP MOVSB/STOSB"
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constexpr u32 cpuid_7_ebx_bit_erms = 1 << 9;
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namespace {
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[[gnu::used]] decltype(&memset) resolve_memset()
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{
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u32 eax, ebx, ecx, edx;
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__cpuid(0x40000000, eax, ebx, ecx, edx);
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bool is_tcg = ebx == tcg_signature_ebx && ecx == tcg_signature_ecx && edx == tcg_signature_edx;
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// Although TCG reports ERMS support, testing shows that rep stosb performs strictly worse than
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// SSE copies on all data sizes except <= 4 bytes.
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if (is_tcg)
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return memset_sse2;
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__cpuid_count(7, 0, eax, ebx, ecx, edx);
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if (ebx & cpuid_7_ebx_bit_erms)
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return memset_sse2_erms;
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return memset_sse2;
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}
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}
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#if !defined(__clang__) && !defined(_DYNAMIC_LOADER)
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[[gnu::ifunc("resolve_memset")]] void* memset(void*, int, size_t);
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#else
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// DynamicLoader can't self-relocate IFUNCs.
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// FIXME: There's a circular dependency between LibC and libunwind when built with Clang,
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// so the IFUNC resolver could be called before LibC has been relocated, returning bogus addresses.
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void* memset(void* dest_ptr, int c, size_t n)
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{
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static decltype(&memset) s_impl = nullptr;
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if (s_impl == nullptr)
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s_impl = resolve_memset();
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return s_impl(dest_ptr, c, n);
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}
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#endif
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}
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