I'm trying to write a strcmp version that takes advantage of SSE4.2 new instructions leveraging GCC intrinsics.
This is the code I have so far:
#include <stdio.h>
#include <smmintrin.h>
int main(int argc, char const *argv[])
{
int n;
const char str1[16] = "foo bar";
const char str2[16] = "foo quxx";
/* Safety check for SSE4.2 support */
__builtin_cpu_init();
if(__builtin_cpu_supports("sse4.2"))
puts("Ok SSE4.2");
else
{
puts("Nok SSE4.2");
return -__LINE__;
}
/* Load strings into registers */
__v16qi xmm1 = __builtin_ia32_loaddqu(str1);
__v16qi xmm2 = __builtin_ia32_loaddqu(str2);
/* Print to check registers were loaded correctly */
printf("xmm1: %s\nxmm2: %s\n", (const char *) &xmm1, (const char *) &xmm2);
/* Perform compare */
n = __builtin_ia32_pcmpistri128(xmm1, xmm2, (_SIDD_CMP_EQUAL_EACH | _SIDD_LEAST_SIGNIFICANT));
/* Print result */
printf("n: %d\n", n);
return 0;
}
It should print the index of the first different byte, but instead it always prints 0.
I've tried to debug it for hours until I saw this in the generated assembly:
call printf
movdqa -64(%rbp), %xmm1
movdqa -80(%rbp), %xmm0
pcmpistri $8, %xmm1, %xmm0
movl %ecx, %eax
pcmpistrm $8, %xmm1, %xmm0
movl %eax, -84(%rbp)
movl -84(%rbp), %eax
According to Wikibooks in case of instructions that output the index (just like pcmpistri I'm trying to use) the result is saved in ECX register, but, if I remeber correctly, the instruction immediately following pcmpistri overrides that register with EAX!
I think that might be the bug that is driving me crazy, but I have no experience in assembly and I am probably wrong.
Anyone is experiencing this issue? Does anyone know how to solve this?
I've tried with GCC 5.4 and 6.2 under Ubuntu 16.04 (actually, bash on Windows) with either -O0, -O1 and -O2 (and obviously -msse4.2).
What makes me think it's a GCC bug is that a similar code compiled undex MSVC from Visual Studio 2017 works correctly:
#include <stdio.h>
#include <nmmintrin.h>
int main()
{
__m128i a, b;
const int mode = _SIDD_CMP_EQUAL_EACH | _SIDD_LEAST_SIGNIFICANT;
a.m128i_u16[7] = 0xFFFF;
a.m128i_u16[6] = 0xFFFF;
a.m128i_u16[5] = 0xFFFF;
a.m128i_u16[4] = 0xFFFF;
a.m128i_u16[3] = 0xFFFF;
a.m128i_u16[2] = 0xFFFF;
a.m128i_u16[1] = 0x0001;
a.m128i_u16[0] = 0xFFFF;
b.m128i_u16[7] = 0x0001;
b.m128i_u16[6] = 0x0001;
b.m128i_u16[5] = 0x0001;
b.m128i_u16[4] = 0x0001;
b.m128i_u16[3] = 0x0001;
b.m128i_u16[2] = 0x0001;
b.m128i_u16[1] = 0x0001;
b.m128i_u16[0] = 0x0001;
int returnValue = _mm_cmpistri(a, b, mode);
printf_s("%i\n", returnValue);
return 0;
}
