useless stores generated when returning a struct -- bug?

useless stores generated when returning a struct -- bug?

[Joshua Haberman]

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

Re: useless stores generated when returning a struct -- bug?

[Ian Lance Taylor]

Joshua Haberman <jhaberman@gmail.com> writes:

> 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 concur that this is a missed optimization bug.

Ian

Re: useless stores generated when returning a struct -- bug?

[Joshua Haberman]

Ian Lance Taylor <iant <at> google.com> writes:
 
> Joshua Haberman <jhaberman <at> gmail.com> writes:
> 
> > 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 concur that this is a missed optimization bug.

Thanks Ian.  I'll file it as a bug.

Josh