我想确保gcc对我的循环进行矢量化。结果，通过使用-march=znver1（或者-march=native）选项，即使一些可以被矢量化的循环，gcc也会跳过它们。为什么会发生这种情况？
在这段代码中，第二个循环将每个元素乘以一个标量，但未被向量化：
#include <stdio.h>
#include <inttypes.h>
int main() {
const size_t N = 1000;
uint64_t arr[N];
for (size_t i = 0; i < N; ++i)
arr[i] = 1;
for (size_t i = 0; i < N; ++i)
arr[i] *= 5;
for (size_t i = 0; i < N; ++i)
printf("%lu\n", arr[i]); // use the array so that it is not optimized away
}
gcc -O3 -fopt-info-vec-all -mavx2 main.c:
main.cpp:13:26: missed: couldn't vectorize loop
main.cpp:14:15: missed: statement clobbers memory: printf ("%lu\n", _3);
main.cpp:10:26: optimized: loop vectorized using 32 byte vectors
main.cpp:7:26: optimized: loop vectorized using 32 byte vectors
main.cpp:4:5: note: vectorized 2 loops in function.
main.cpp:14:15: missed: statement clobbers memory: printf ("%lu\n", _3);
main.cpp:15:1: note: ***** Analysis failed with vector mode V4DI
main.cpp:15:1: note: ***** Skipping vector mode V32QI, which would repeat the analysis for V4DI
gcc -O3 -fopt-info-vec-all -march=znver1 main.c:
main.cpp:13:26: missed: couldn't vectorize loop
main.cpp:14:15: missed: statement clobbers memory: printf ("%lu\n", _3);
main.cpp:10:26: missed: couldn't vectorize loop
main.cpp:10:26: missed: not vectorized: unsupported data-type
main.cpp:7:26: optimized: loop vectorized using 16 byte vectors
main.cpp:4:5: note: vectorized 1 loops in function.
main.cpp:14:15: missed: statement clobbers memory: printf ("%lu\n", _3);
main.cpp:15:1: note: ***** Analysis failed with vector mode V2DI
main.cpp:15:1: note: ***** Skipping vector mode V16QI, which would repeat the analysis for V2DI
-march=znver1 包括 -mavx2，因此我认为gcc由于某些原因选择不对其进行向量化：
~ $ gcc -march=znver1 -Q --help=target
The following options are target specific:
-m128bit-long-double [enabled]
-m16 [disabled]
-m32 [disabled]
-m3dnow [disabled]
-m3dnowa [disabled]
-m64 [enabled]
-m80387 [enabled]
-m8bit-idiv [disabled]
-m96bit-long-double [disabled]
-mabi= sysv
-mabm [enabled]
-maccumulate-outgoing-args [disabled]
-maddress-mode= long
-madx [enabled]
-maes [enabled]
-malign-data= compat
-malign-double [disabled]
-malign-functions= 0
-malign-jumps= 0
-malign-loops= 0
-malign-stringops [enabled]
-mamx-bf16 [disabled]
-mamx-int8 [disabled]
-mamx-tile [disabled]
-mandroid [disabled]
-march= znver1
-masm= att
-mavx [enabled]
-mavx2 [enabled]
-mavx256-split-unaligned-load [disabled]
-mavx256-split-unaligned-store [enabled]
-mavx5124fmaps [disabled]
-mavx5124vnniw [disabled]
-mavx512bf16 [disabled]
-mavx512bitalg [disabled]
-mavx512bw [disabled]
-mavx512cd [disabled]
-mavx512dq [disabled]
-mavx512er [disabled]
-mavx512f [disabled]
-mavx512ifma [disabled]
-mavx512pf [disabled]
-mavx512vbmi [disabled]
-mavx512vbmi2 [disabled]
-mavx512vl [disabled]
-mavx512vnni [disabled]
-mavx512vp2intersect [disabled]
-mavx512vpopcntdq [disabled]
-mavxvnni [disabled]
-mbionic [disabled]
-mbmi [enabled]
-mbmi2 [enabled]
-mbranch-cost=<0,5> 3
-mcall-ms2sysv-xlogues [disabled]
-mcet-switch [disabled]
-mcld [disabled]
-mcldemote [disabled]
-mclflushopt [enabled]
-mclwb [disabled]
-mclzero [enabled]
-mcmodel= [default]
-mcpu=
-mcrc32 [disabled]
-mcx16 [enabled]
-mdispatch-scheduler [disabled]
-mdump-tune-features [disabled]
-menqcmd [disabled]
-mf16c [enabled]
-mfancy-math-387 [enabled]
-mfentry [disabled]
-mfentry-name=
-mfentry-section=
-mfma [enabled]
-mfma4 [disabled]
-mforce-drap [disabled]
-mforce-indirect-call [disabled]
-mfp-ret-in-387 [enabled]
-mfpmath= sse
-mfsgsbase [enabled]
-mfunction-return= keep
-mfused-madd -ffp-contract=fast
-mfxsr [enabled]
-mgeneral-regs-only [disabled]
-mgfni [disabled]
-mglibc [enabled]
-mhard-float [enabled]
-mhle [disabled]
-mhreset [disabled]
-miamcu [disabled]
-mieee-fp [enabled]
-mincoming-stack-boundary= 0
-mindirect-branch-register [disabled]
-mindirect-branch= keep
-minline-all-stringops [disabled]
-minline-stringops-dynamically [disabled]
-minstrument-return= none
-mintel-syntax -masm=intel
-mkl [disabled]
-mlarge-data-threshold=<number> 65536
-mlong-double-128 [disabled]
-mlong-double-64 [disabled]
-mlong-double-80 [enabled]
-mlwp [disabled]
-mlzcnt [enabled]
-mmanual-endbr [disabled]
-mmemcpy-strategy=
-mmemset-strategy=
-mmitigate-rop [disabled]
-mmmx [enabled]
-mmovbe [enabled]
-mmovdir64b [disabled]
-mmovdiri [disabled]
-mmpx [disabled]
-mms-bitfields [disabled]
-mmusl [disabled]
-mmwaitx [enabled]
-mneeded [disabled]
-mno-align-stringops [disabled]
-mno-default [disabled]
-mno-fancy-math-387 [disabled]
-mno-push-args [disabled]
-mno-red-zone [disabled]
-mno-sse4 [disabled]
-mnop-mcount [disabled]
-momit-leaf-frame-pointer [disabled]
-mpc32 [disabled]
-mpc64 [disabled]
-mpc80 [disabled]
-mpclmul [enabled]
-mpcommit [disabled]
-mpconfig [disabled]
-mpku [disabled]
-mpopcnt [enabled]
-mprefer-avx128 -mprefer-vector-width=128
-mprefer-vector-width= 128
-mpreferred-stack-boundary= 0
-mprefetchwt1 [disabled]
-mprfchw [enabled]
-mptwrite [disabled]
-mpush-args [enabled]
-mrdpid [disabled]
-mrdrnd [enabled]
-mrdseed [enabled]
-mrecip [disabled]
-mrecip=
-mrecord-mcount [disabled]
-mrecord-return [disabled]
-mred-zone [enabled]
-mregparm= 6
-mrtd [disabled]
-mrtm [disabled]
-msahf [enabled]
-mserialize [disabled]
-msgx [disabled]
-msha [enabled]
-mshstk [disabled]
-mskip-rax-setup [disabled]
-msoft-float [disabled]
-msse [enabled]
-msse2 [enabled]
-msse2avx [disabled]
-msse3 [enabled]
-msse4 [enabled]
-msse4.1 [enabled]
-msse4.2 [enabled]
-msse4a [enabled]
-msse5 -mavx
-msseregparm [disabled]
-mssse3 [enabled]
-mstack-arg-probe [disabled]
-mstack-protector-guard-offset=
-mstack-protector-guard-reg=
-mstack-protector-guard-symbol=
-mstack-protector-guard= tls
-mstackrealign [disabled]
-mstringop-strategy= [default]
-mstv [enabled]
-mtbm [disabled]
-mtls-dialect= gnu
-mtls-direct-seg-refs [enabled]
-mtsxldtrk [disabled]
-mtune-ctrl=
-mtune= znver1
-muclibc [disabled]
-muintr [disabled]
-mvaes [disabled]
-mveclibabi= [default]
-mvect8-ret-in-mem [disabled]
-mvpclmulqdq [disabled]
-mvzeroupper [enabled]
-mwaitpkg [disabled]
-mwbnoinvd [disabled]
-mwidekl [disabled]
-mx32 [disabled]
-mxop [disabled]
-mxsave [enabled]
-mxsavec [enabled]
-mxsaveopt [enabled]
-mxsaves [enabled]
Known assembler dialects (for use with the -masm= option):
att intel
Known ABIs (for use with the -mabi= option):
ms sysv
Known code models (for use with the -mcmodel= option):
32 kernel large medium small
Valid arguments to -mfpmath=:
387 387+sse 387,sse both sse sse+387 sse,387
Known indirect branch choices (for use with the -mindirect-branch=/-mfunction-return= options):
keep thunk thunk-extern thunk-inline
Known choices for return instrumentation with -minstrument-return=:
call none nop5
Known data alignment choices (for use with the -malign-data= option):
abi cacheline compat
Known vectorization library ABIs (for use with the -mveclibabi= option):
acml svml
Known address mode (for use with the -maddress-mode= option):
long short
Known preferred register vector length (to use with the -mprefer-vector-width= option):
128 256 512 none
Known stack protector guard (for use with the -mstack-protector-guard= option):
global tls
Valid arguments to -mstringop-strategy=:
byte_loop libcall loop rep_4byte rep_8byte rep_byte unrolled_loop vector_loop
Known TLS dialects (for use with the -mtls-dialect= option):
gnu gnu2
Known valid arguments for -march= option:
i386 i486 i586 pentium lakemont pentium-mmx winchip-c6 winchip2 c3 samuel-2 c3-2 nehemiah c7 esther i686 pentiumpro pentium2 pentium3 pentium3m pentium-m pentium4 pentium4m prescott nocona core2 nehalem corei7 westmere sandybridge corei7-avx ivybridge core-avx-i haswell core-avx2 broadwell skylake skylake-avx512 cannonlake icelake-client rocketlake icelake-server cascadelake tigerlake cooperlake sapphirerapids alderlake bonnell atom silvermont slm goldmont goldmont-plus tremont knl knm intel geode k6 k6-2 k6-3 athlon athlon-tbird athlon-4 athlon-xp athlon-mp x86-64 x86-64-v2 x86-64-v3 x86-64-v4 eden-x2 nano nano-1000 nano-2000 nano-3000 nano-x2 eden-x4 nano-x4 k8 k8-sse3 opteron opteron-sse3 athlon64 athlon64-sse3 athlon-fx amdfam10 barcelona bdver1 bdver2 bdver3 bdver4 znver1 znver2 znver3 btver1 btver2 generic native
Known valid arguments for -mtune= option:
generic i386 i486 pentium lakemont pentiumpro pentium4 nocona core2 nehalem sandybridge haswell bonnell silvermont goldmont goldmont-plus tremont knl knm skylake skylake-avx512 cannonlake icelake-client icelake-server cascadelake tigerlake cooperlake sapphirerapids alderlake rocketlake intel geode k6 athlon k8 amdfam10 bdver1 bdver2 bdver3 bdver4 btver1 btver2 znver1 znver2 znver3
我也尝试了clang，两种情况下我相信循环被向量化为32字节向量：
注意：矢量化循环（矢量化宽度：4，交错计数：4）
我正在使用gcc 11.2.0版本
编辑： 根据Peter Cordes的要求， 我意识到我实际上一直在进行乘以4的基准测试。
Makefile:
all:
gcc -O3 -mavx2 main.c -o 3
gcc -O3 -march=znver2 main.c -o 32
gcc -O3 -march=znver2 main.c -mprefer-vector-width=128 -o 32128
gcc -O3 -march=znver1 main.c -o 31
gcc -O2 -mavx2 main.c -o 2
gcc -O2 -march=znver2 main.c -o 22
gcc -O2 -march=znver2 main.c -mprefer-vector-width=128 -o 22128
gcc -O2 -march=znver1 main.c -o 21
hyperfine -r5 ./3 ./32 ./32128 ./31 ./2 ./22 ./22128 ./21
clean:
rm ./3 ./32 ./32128 ./31 ./2 ./22 ./22128 ./21
代码：
#include <stdio.h>
#include <inttypes.h>
#include <stdlib.h>
#include <time.h>
int main() {
const size_t N = 500;
uint64_t arr[N];
for (size_t i = 0; i < N; ++i)
arr[i] = 1;
for (int j = 0; j < 20000000; ++j)
for (size_t i = 0; i < N; ++i)
arr[i] *= 4;
srand(time(0));
printf("%lu\n", arr[rand() % N]); // use the array so that it is not optimized away
}
N = 500, arr[i] *= 4:
Benchmark 1: ./3
Time (mean ± σ): 1.780 s ± 0.011 s [User: 1.778 s, System: 0.000 s]
Range (min … max): 1.763 s … 1.791 s 5 runs
Benchmark 2: ./32
Time (mean ± σ): 1.785 s ± 0.016 s [User: 1.783 s, System: 0.000 s]
Range (min … max): 1.773 s … 1.810 s 5 runs
Benchmark 3: ./32128
Time (mean ± σ): 1.740 s ± 0.026 s [User: 1.737 s, System: 0.000 s]
Range (min … max): 1.724 s … 1.785 s 5 runs
Benchmark 4: ./31
Time (mean ± σ): 1.757 s ± 0.022 s [User: 1.754 s, System: 0.000 s]
Range (min … max): 1.727 s … 1.785 s 5 runs
Benchmark 5: ./2
Time (mean ± σ): 3.467 s ± 0.031 s [User: 3.462 s, System: 0.000 s]
Range (min … max): 3.443 s … 3.519 s 5 runs
Benchmark 6: ./22
Time (mean ± σ): 3.475 s ± 0.028 s [User: 3.469 s, System: 0.001 s]
Range (min … max): 3.447 s … 3.512 s 5 runs
Benchmark 7: ./22128
Time (mean ± σ): 3.464 s ± 0.034 s [User: 3.459 s, System: 0.001 s]
Range (min … max): 3.431 s … 3.509 s 5 runs
Benchmark 8: ./21
Time (mean ± σ): 3.465 s ± 0.013 s [User: 3.460 s, System: 0.001 s]
Range (min … max): 3.443 s … 3.475 s 5 runs
N = 500, arr[i] *= 5:
Benchmark 1: ./3
Time (mean ± σ): 1.789 s ± 0.004 s [User: 1.786 s, System: 0.001 s]
Range (min … max): 1.783 s … 1.793 s 5 runs
Benchmark 2: ./32
Time (mean ± σ): 1.772 s ± 0.017 s [User: 1.769 s, System: 0.000 s]
Range (min … max): 1.755 s … 1.800 s 5 runs
Benchmark 3: ./32128
Time (mean ± σ): 2.911 s ± 0.023 s [User: 2.907 s, System: 0.001 s]
Range (min … max): 2.880 s … 2.943 s 5 runs
Benchmark 4: ./31
Time (mean ± σ): 2.924 s ± 0.013 s [User: 2.921 s, System: 0.000 s]
Range (min … max): 2.906 s … 2.934 s 5 runs
Benchmark 5: ./2
Time (mean ± σ): 3.850 s ± 0.029 s [User: 3.846 s, System: 0.000 s]
Range (min … max): 3.823 s … 3.896 s 5 runs
Benchmark 6: ./22
Time (mean ± σ): 3.816 s ± 0.036 s [User: 3.812 s, System: 0.000 s]
Range (min … max): 3.777 s … 3.855 s 5 runs
Benchmark 7: ./22128
Time (mean ± σ): 3.813 s ± 0.026 s [User: 3.809 s, System: 0.000 s]
Range (min … max): 3.780 s … 3.834 s 5 runs
Benchmark 8: ./21
Time (mean ± σ): 3.783 s ± 0.010 s [User: 3.779 s, System: 0.000 s]
Range (min … max): 3.773 s … 3.798 s 5 runs
N = 512时，arr[i]会乘以4。
Benchmark 1: ./3
Time (mean ± σ): 1.849 s ± 0.015 s [User: 1.847 s, System: 0.000 s]
Range (min … max): 1.831 s … 1.873 s 5 runs
Benchmark 2: ./32
Time (mean ± σ): 1.846 s ± 0.013 s [User: 1.844 s, System: 0.001 s]
Range (min … max): 1.832 s … 1.860 s 5 runs
Benchmark 3: ./32128
Time (mean ± σ): 1.756 s ± 0.012 s [User: 1.754 s, System: 0.000 s]
Range (min … max): 1.744 s … 1.771 s 5 runs
Benchmark 4: ./31
Time (mean ± σ): 1.788 s ± 0.012 s [User: 1.785 s, System: 0.001 s]
Range (min … max): 1.774 s … 1.801 s 5 runs
Benchmark 5: ./2
Time (mean ± σ): 3.476 s ± 0.015 s [User: 3.472 s, System: 0.001 s]
Range (min … max): 3.458 s … 3.494 s 5 runs
Benchmark 6: ./22
Time (mean ± σ): 3.449 s ± 0.002 s [User: 3.446 s, System: 0.000 s]
Range (min … max): 3.446 s … 3.452 s 5 runs
Benchmark 7: ./22128
Time (mean ± σ): 3.456 s ± 0.007 s [User: 3.453 s, System: 0.000 s]
Range (min … max): 3.446 s … 3.462 s 5 runs
Benchmark 8: ./21
Time (mean ± σ): 3.547 s ± 0.044 s [User: 3.542 s, System: 0.001 s]
Range (min … max): 3.482 s … 3.600 s 5 runs
N = 512, arr[i] *= 5
Benchmark 1: ./3
Time (mean ± σ): 1.847 s ± 0.013 s [User: 1.845 s, System: 0.000 s]
Range (min … max): 1.836 s … 1.863 s 5 runs
Benchmark 2: ./32
Time (mean ± σ): 1.830 s ± 0.007 s [User: 1.827 s, System: 0.001 s]
Range (min … max): 1.820 s … 1.837 s 5 runs
Benchmark 3: ./32128
Time (mean ± σ): 2.983 s ± 0.017 s [User: 2.980 s, System: 0.000 s]
Range (min … max): 2.966 s … 3.012 s 5 runs
Benchmark 4: ./31
Time (mean ± σ): 3.026 s ± 0.039 s [User: 3.021 s, System: 0.001 s]
Range (min … max): 2.989 s … 3.089 s 5 runs
Benchmark 5: ./2
Time (mean ± σ): 4.000 s ± 0.021 s [User: 3.994 s, System: 0.001 s]
Range (min … max): 3.982 s … 4.035 s 5 runs
Benchmark 6: ./22
Time (mean ± σ): 3.940 s ± 0.041 s [User: 3.934 s, System: 0.001 s]
Range (min … max): 3.890 s … 3.981 s 5 runs
Benchmark 7: ./22128
Time (mean ± σ): 3.928 s ± 0.032 s [User: 3.922 s, System: 0.001 s]
Range (min … max): 3.898 s … 3.979 s 5 runs
Benchmark 8: ./21
Time (mean ± σ): 3.908 s ± 0.029 s [User: 3.904 s, System: 0.000 s]
Range (min … max): 3.879 s … 3.954 s 5 runs
我认为使用 -O2 -march=znver1 和 -O3 -march=znver1 的运行速度相同是我的问题，原因是文件命名错误，当时我还没有创建 makefile 文件，而是使用了 shell 历史记录。
*=4情况（它将矢量化为Zen1），-O3 -march=znver2（256位向量）比-O3 -march=znver1（128位向量）略慢，这似乎验证了GCC的调优选择。数组上的_Alignas(32)是否可以恢复-O3 -march=znver2*=4情况下的性能？如果是这样，那就表明即使每半个都是16字节对齐，Zen1上的未对齐32字节存储（甚至加载）也具有额外的成本。我希望这不是这种情况，因为它们作为单独的uop运行，而Sandybridge则使用相同的uop一次使用AGU。 - Peter Cordes