[Bug target/36661] New: x86 asm "+r" operands cause unnecessary spills/copies

[astrange at ithinksw dot com]

Compiling the attached source on i386 with:
> gcc -O3  -fomit-frame-pointer -fno-pic -S asm-spills.i
produces:
        .text
        .align 4,0x90
.globl _get_cabac_noinline
_get_cabac_noinline:
        subl    $76, %esp
        movl    %esi, 64(%esp)
        movl    %edi, 68(%esp)
        movl    %ebp, 72(%esp)
        movl    %ebx, 60(%esp)
        movl    80(%esp), %esi
        movl    84(%esp), %edi
        movl    (%esi), %edx
        movl    4(%esi), %ebx
        movl    %edx, 28(%esp) # unused spill
        movl    %edx, %ebp # pointless move
# 24 "../strange-spills.i" 1
        #%eax %bp %ebx 16(%esi) %edx (%edi)
# 0 "" 2
        movl    %ebp, (%esi)
        movl    %ebx, 4(%esi)
        andl    $1, %eax
        movl    60(%esp), %ebx
        movl    %eax, 44(%esp) #unused spill
        movl    64(%esp), %esi
        movl    68(%esp), %edi
        movl    72(%esp), %ebp
        addl    $76, %esp
        ret
        .subsections_via_symbols

which has several unnecessary stack spills.

Reading through RTL dumps:
- everything is fine before asmcons
- asmcons inserts copies of c->low/range after they're loaded. There's no point
to this, since the original is never used later, but I guess there isn't a
problem as long as it's cleaned up.
- Somehow, the RA gets confused by the asmcons copy and the later one to copy
the return value into eax. Instead of assigning both sides of the copy to the
same register (which is obviously possible), or even using mov, it spills and
reloads them into different registers.
- Later passes optimize away the reloads but keep the stores.

This isn't a regression (gcc 3.4 isn't much better) and still happens in the
IRA branch. For some reason, changing "=&q"(tmp) to "=&d" improves IRA but not
trunk.

This is about the same source as
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36539.


-- 
           Summary: x86 asm "+r" operands cause unnecessary spills/copies
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: astrange at ithinksw dot com
GCC target triplet: i?86-*-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36661