[Bug c/43640] New: Struct with two floats generates poor code

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

struct u1
{
        float x;
        float y;
};

float foo(struct u1 u)
{
        return u.x + u.y;
}

compiles into
gcc-4.5 -O3 tgcc.c -c -o tgcc.o

   0x0000000000000000 <+0>:     movq   %xmm0,-0x20(%rsp)
   0x0000000000000006 <+6>:     mov    -0x20(%rsp),%rax
   0x000000000000000b <+11>:    mov    %eax,-0x14(%rsp)
   0x000000000000000f <+15>:    shr    $0x20,%rax
   0x0000000000000013 <+19>:    mov    %eax,-0x10(%rsp)
   0x0000000000000017 <+23>:    movss  -0x14(%rsp),%xmm0
   0x000000000000001d <+29>:    addss  -0x10(%rsp),%xmm0
   0x0000000000000023 <+35>:    retq

The instructions dealing with rax/eax can be elided if the movss and addss load
from the correct stack locations.

A better sequence, avoiding memory, might be

pshufd %xmm0, %xmm1, 1
addss %xmm1, %xmm0
retq


-- 
           Summary: Struct with two floats generates poor code
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: svfuerst at gmail dot com
 GCC build triplet: x86_64-linux
  GCC host triplet: x86_64-linux
GCC target triplet: x86_64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43640