Why is it necessary to use edi constraint in this inline assembly? - gcc

Question

centos 6.5 64bit vps, 500MB ram gcc 4.8.2
I have the following function that works only if I use edi as the constraint to hold the string pointer. If I try to use any other register or constraintg or q etc, it segfaults.
BUT this problem only occurs when both link time optimization and o3 are used together. If o2 it's fine. If I don't use -flto, it's fine. But both together then the only register I can use that doesn't crash is edi
gcc -flto
CFLAGS=-I. -flto -std=gnu11 -msse4.2 -fno-builtin-printf -Wall -Winline -Wstrict-aliasing -g -pg -O3 -lrt -lpthread
It seems like there might be some sort of register clobbering going on or something else. I'm really at a loss to understand why and how to fix this. Another interesting aspect is the generated assembly puts rdi into rdx before using the pointer but if I try to use either register as the input constraint... it segfaults! If it fails under aggressive compiling options it suggests to me either the compiler is stuffing up somehow, or more likely I'm doing something wrong.
char *sse4_strCRLF(char *str)
{
__m128i M = _mm_set1_epi8(13);
char *res;
__asm__ __volatile__(
"xor %0,%0\n\t"
"sub $1, %1\n\t"
"1:" "sub $15,%1\n\t"
".align 16\n\t"
"2:" "add $16, %1\n\t"
"pcmpistri $0x08,(%1),%2\n\t"
"ja 2b\n\t"
"jnc 2f\n\t"
"cmpb $10,1(%1,%%rcx)\n\t"
"jne 1b\n\t"
"add %%rcx,%1\n\t"
"mov %1,%0\n\t"
"2:"
:"=q"(res)
:"edi"(str),"x"(M) //<-- if use anything except edi, it segfaults
:"rcx"
);
return (char*) res;
}
Disassembled output:
00000000000002e0 <sse4_strCRLF>:
2e0: 55 push rbp
2e1: 48 89 e5 mov rbp,rsp
2e4: e8 00 00 00 00 call 2e9 <sse4_strCRLF+0x9>
2e9: 66 0f 6f 05 00 00 00 00 movdqa xmm0,[rip+0x0] # 2f1 <sse4_strCRLF+0x11>
2f1: 48 89 fa mov rdx,rdi //<--- puts rdi into rdx!
2f4: 48 31 c0 xor rax,rax
2f7: 48 83 ea 01 sub rdx,0x1
2fb: 48 83 ea 0f sub rdx,0xf
2ff: 90 nop
300: 48 83 c2 10 add rdx,0x10
304: 66 0f 3a 63 02 08 pcmpistri xmm0,[rdx],0x8
30a: 77 f4 ja 300 <sse4_strCRLF+0x20>
30c: 73 0d jae 31b <sse4_strCRLF+0x3b>
30e: 80 7c 0a 01 0a cmp byte[rdx+rcx*1+0x1],0xa
313: 75 e6 jne 2fb <sse4_strCRLF+0x1b>
315: 48 01 ca add rdx,rcx
318: 48 89 d0 mov rax,rdx
31b: 5d pop rbp
31c: c3 ret

Answer 1

#David Wohlferd gave me the answer. It was 2 dumb mistakes I was making due to ignorance and assumptions. The below code is modified such that the input variable char pointer is not modified by the routine. It's copied into a register and that register is used. Also I was mistakenly thinking I could directly specify a particular register as opposed to a b etc.
gcc still seems to be fussy about what constraints I use. e.g. If I use a and b for res and str respectively, it compiles fine but segfaults on running. But using S and D seems to work fine.
#David Wohlferd, I'd like to credit you as the answerer but I don't think I can do that to a comment.
char *sse4_strCRLF(char *str)
{
__m128i M = _mm_set1_epi8(13);
char *res;
__asm__ __volatile__(
"xor %0,%0\n\t"
"mov %1,%%rdx\n\t"
"sub $1,%%rdx\n\t"
"1:" "sub $15,%%rdx\n\t"
".align 16\n\t"
"2:" "add $16, %%rdx\n\t"
"pcmpistri $0x08,(%%rdx),%2\n\t"
"ja 2b\n\t"
"jnc 2f\n\t"
"cmpb $10,1(%%rdx,%%rcx)\n\t"
"jne 1b\n\t"
"add %%rcx,%%rdx\n\t"
"mov %%rdx,%0\n\t"
"2:"
:"=S"(res)
:"D"(str),"x"(M)
:"rcx","rdx"
);
return (char*) res;
}