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Date: Tue, 07 Jul 2009 02:25:00 -0000
Subject: [Bug regression/40667]  New: Performance regression: stack frames are generated even with -fomit-frame-pointer
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From: "mikulas at artax dot karlin dot mff dot cuni dot cz" <gcc-bugzilla@gcc.gnu.org>
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This is performance regression from 4.3 (which was better).

On i386, when -O2 -fomit-frame-pointer -mpreferred-stack-boundary=2 is used,
and function operates with long long values, stack frame is generated, although
it doesn't have to be.

Example:
int f(long long x);

int g(long long x)
{
        f(x);
        return 0;
}

Generated code:
        .text
        .p2align 4,,15
.globl g
        .type   g, @function
g:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $8, %esp
        movl    8(%ebp), %eax
        movl    12(%ebp), %edx
        movl    %eax, (%esp)
        movl    %edx, 4(%esp)
        call    f
        xorl    %eax, %eax
        leave
        ret
        .size   g, .-g

Gcc 4.3 didn't generate stack frame in this case. On i386, spurious stack frame
is especially bad because there are few registers and one register is lost for
the stack frame. One my program doing heavy 64-bit math shows almost 1% code
size increase because of these unneeded frames.


-- 
           Summary: Performance regression: stack frames are generated even
                    with -fomit-frame-pointer
           Product: gcc
           Version: 4.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: regression
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mikulas at artax dot karlin dot mff dot cuni dot cz
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40667