I have a case where I think useless stores are being generated, but I
want to be sure before I file a bug. This is with gcc 4.4.3 on Ubuntu
10.04, x86-64.
I have been experimenting with returning structs from functions instead
of passing pointers to "out" parameters. This seems like it should be
more optimizer-friendly because you can avoid taking addresses of local
variables, which prevents them from possibly being aliased.
However in this test case, gcc is generating four stores that appear to
be completely useless:
#include <stdint.h>
struct twoints { uint64_t a, b; } foo();
void bar(uint64_t a, uint64_t b);
void func() {
struct twoints s = foo();
bar(s.a, s.b);
}
$ gcc -Wall -c -o testbad.o -msse2 -O3 -fomit-frame-pointer testbad.c-
$ objdump -d -r -M intel testbad.o
testbad.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <func>:
0: 48 83 ec 28 sub rsp,0x28
4: 31 c0 xor eax,eax
6: e8 00 00 00 00 call b <func+0xb>
7: R_X86_64_PC32 foo-0x4
b: 48 89 04 24 mov QWORD PTR [rsp],rax
f: 48 89 54 24 08 mov QWORD PTR [rsp+0x8],rdx
14: 48 89 d6 mov rsi,rdx
17: 48 89 44 24 10 mov QWORD PTR [rsp+0x10],rax
1c: 48 89 54 24 18 mov QWORD PTR [rsp+0x18],rdx
21: 48 89 c7 mov rdi,rax
24: 48 83 c4 28 add rsp,0x28
28: e9 00 00 00 00 jmp 2d <func+0x2d>
29: R_X86_64_PC32 bar-0x4
Why is it storing rax and rdx to the stack twice? These stores are never
used AFAICS.
Josh
