[Bug middle-end/47581] [4.6 regression] Unnecessary adjustments to stack pointer

[hjl.tools at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47581

--- Comment #8 from H.J. Lu <hjl.tools at gmail dot com> 2011-02-02 14:57:22 UTC ---
For this tescase, gcc 4.4 generates the most efficient code
while 4.5/4.6 become worse and worse:

[hjl@gnu-6 gcc]$ cat x.i 
unsigned mulh(unsigned a, unsigned b)
{
  unsigned long long l __attribute__ ((aligned(32)))
    =((unsigned long long)a * (unsigned long long)b) >> 32;
  return l;
}
[hjl@gnu-6 gcc]$ /usr/gcc-4.4/bin/gcc -O2 -S -fomit-frame-pointer x.i -m32
[hjl@gnu-6 gcc]$ cat x.s 
    .file    "x.i"
    .text
    .p2align 4,,15
.globl mulh
    .type    mulh, @function
mulh:
    movl    8(%esp), %eax
    mull    4(%esp)
    movl    %edx, %eax
    ret
    .size    mulh, .-mulh
    .ident    "GCC: (GNU) 4.4.4"
    .section    .note.GNU-stack,"",@progbits
[hjl@gnu-6 gcc]$ /usr/gcc-4.5/bin/gcc -O2 -S -fomit-frame-pointer x.i -m32
[hjl@gnu-6 gcc]$ cat x.s
    .file    "x.i"
    .text
    .p2align 4,,15
.globl mulh
    .type    mulh, @function
mulh:
    pushl    %ebp
    movl    %esp, %ebp
    movl    12(%ebp), %eax
    mull    8(%ebp)
    popl    %ebp
    movl    %edx, %eax
    ret
    .size    mulh, .-mulh
    .ident    "GCC: (GNU) 4.5.1 20100507 (prerelease) [gcc-4_5-branch revision
159167]"
    .section    .note.GNU-stack,"",@progbits
[hjl@gnu-6 gcc]$ ./xgcc -B./  -O2 -S -fomit-frame-pointer x.i -m32
[hjl@gnu-6 gcc]$ cat x.s
    .file    "x.i"
    .text
    .p2align 4,,15
    .globl    mulh
    .type    mulh, @function
mulh:
.LFB0:
    .cfi_startproc
    pushl    %ebp
    .cfi_def_cfa_offset 8
    .cfi_offset 5, -8
    movl    %esp, %ebp
    .cfi_def_cfa_register 5
    andl    $-32, %esp
    movl    12(%ebp), %eax
    mull    8(%ebp)
    leave
    .cfi_restore 5
    .cfi_def_cfa 4, 4
    movl    %edx, %eax
    ret
    .cfi_endproc
.LFE0:
    .size    mulh, .-mulh
    .ident    "GCC: (GNU) 4.6.0 20110131 (experimental)"
    .section    .note.GNU-stack,"",@progbits
[hjl@gnu-6 gcc]$