AVX scalar operations are much faster - c

Question

I test the following simple function
void mul(double *a, double *b) {
for (int i = 0; i<N; i++) a[i] *= b[i];
}
with very large arrays so that it is memory bandwidth bound. The test code I use is below. When I compile with -O2 it takes 1.7 seconds. When I compile with -O2 -mavx it takes only 1.0 seconds. The non vex-encoded scalar operations are 70% slower! Why is this?
Here is the the assembly for -O2 and -O2 -mavx.
https://godbolt.org/g/w4p60f
System: i7-6700HQ#2.60GHz (Skylake) 32 GB mem, Ubuntu 16.10, GCC 6.3
Test code
//gcc -O2 -fopenmp test.c
//or
//gcc -O2 -mavx -fopenmp test.c
#include <string.h>
#include <stdio.h>
#include <x86intrin.h>
#include <omp.h>
#define N 1000000
#define R 1000
void mul(double *a, double *b) {
for (int i = 0; i<N; i++) a[i] *= b[i];
}
int main() {
double *a = (double*)_mm_malloc(sizeof *a * N, 32);
double *b = (double*)_mm_malloc(sizeof *b * N, 32);
//b must be initialized to get the correct bandwidth!!!
memset(a, 1, sizeof *a * N);
memset(b, 1, sizeof *b * N);
double dtime;
const double mem = 3*sizeof(double)*N*R/1024/1024/1024;
const double maxbw = 34.1;
dtime = -omp_get_wtime();
for(int i=0; i<R; i++) mul(a,b);
dtime += omp_get_wtime();
printf("time %.2f s, %.1f GB/s, efficency %.1f%%\n", dtime, mem/dtime, 100*mem/dtime/maxbw);
_mm_free(a), _mm_free(b);
}

Answer 1

The problem is related to a dirty upper half of an AVX register after calling omp_get_wtime(). This is a problem particularly for Skylake processors.
The first time I read about this problem was here. Since then other people have observed this problem: here and here.
Using gdb I found that omp_get_wtime() calls clock_gettime. I rewrote my code to use clock_gettime() and I see the same problem.
void fix_avx() { __asm__ __volatile__ ( "vzeroupper" : : : ); }
void fix_sse() { }
void (*fix)();
double get_wtime() {
struct timespec time;
clock_gettime(CLOCK_MONOTONIC, &time);
#ifndef __AVX__
fix();
#endif
return time.tv_sec + 1E-9*time.tv_nsec;
}
void dispatch() {
fix = fix_sse;
#if defined(__INTEL_COMPILER)
if (_may_i_use_cpu_feature (_FEATURE_AVX)) fix = fix_avx;
#else
#if defined(__GNUC__) && !defined(__clang__)
__builtin_cpu_init();
#endif
if(__builtin_cpu_supports("avx")) fix = fix_avx;
#endif
}
Stepping through code with gdb I see that the first time clock_gettime is called it calls _dl_runtime_resolve_avx(). I believe the problem is in this function based on this comment. This function appears to only be called the first time clock_gettime is called.
With GCC the problem goes away using //__asm__ __volatile__ ( "vzeroupper" : : : ); after the first call with clock_gettime however with Clang (using clang -O2 -fno-vectorize since Clang vectorizes even at -O2) it only goes away using it after every call to clock_gettime.
Here is the code I used to test this (with GCC 6.3 and Clang 3.8)
#include <string.h>
#include <stdio.h>
#include <x86intrin.h>
#include <time.h>
void fix_avx() { __asm__ __volatile__ ( "vzeroupper" : : : ); }
void fix_sse() { }
void (*fix)();
double get_wtime() {
struct timespec time;
clock_gettime(CLOCK_MONOTONIC, &time);
#ifndef __AVX__
fix();
#endif
return time.tv_sec + 1E-9*time.tv_nsec;
}
void dispatch() {
fix = fix_sse;
#if defined(__INTEL_COMPILER)
if (_may_i_use_cpu_feature (_FEATURE_AVX)) fix = fix_avx;
#else
#if defined(__GNUC__) && !defined(__clang__)
__builtin_cpu_init();
#endif
if(__builtin_cpu_supports("avx")) fix = fix_avx;
#endif
}
#define N 1000000
#define R 1000
void mul(double *a, double *b) {
for (int i = 0; i<N; i++) a[i] *= b[i];
}
int main() {
dispatch();
const double mem = 3*sizeof(double)*N*R/1024/1024/1024;
const double maxbw = 34.1;
double *a = (double*)_mm_malloc(sizeof *a * N, 32);
double *b = (double*)_mm_malloc(sizeof *b * N, 32);
//b must be initialized to get the correct bandwidth!!!
memset(a, 1, sizeof *a * N);
memset(b, 1, sizeof *b * N);
double dtime;
//dtime = get_wtime(); // call once to fix GCC
//printf("%f\n", dtime);
//fix = fix_sse;
dtime = -get_wtime();
for(int i=0; i<R; i++) mul(a,b);
dtime += get_wtime();
printf("time %.2f s, %.1f GB/s, efficency %.1f%%\n", dtime, mem/dtime, 100*mem/dtime/maxbw);
_mm_free(a), _mm_free(b);
}
If I disable lazy function call resolution with -z now (e.g. clang -O2 -fno-vectorize -z now foo.c) then Clang only needs __asm__ __volatile__ ( "vzeroupper" : : : ); after the first call to clock_gettime just like GCC.
I expected that with -z now I would only need __asm__ __volatile__ ( "vzeroupper" : : : ); right after main() but I still need it after the first call to clock_gettime.