[Bug rtl-optimization/44194] New: struct returned by value generates useless stores

["jhaberman at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Test case:

--

#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 -save-temps -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

--

As you can see above, rax and rdx are stored to the stack twice, but these
stores are unnecessary.

$ gcc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.4.3-4ubuntu5'
--with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--enable-multiarch --enable-linker-build-id --with-system-zlib
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 --enable-nls
--enable-clocale=gnu --enable-libstdcxx-debug --enable-plugin --enable-objc-gc
--disable-werror --with-arch-32=i486 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5)


-- 
           Summary: struct returned by value generates useless stores
           Product: gcc
           Version: 4.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jhaberman at gmail dot com
 GCC build triplet: x86_64-linux-gnu
  GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44194