Re: Elimination of duplicate sign extensions

[Mark Shinwell <shinwell at codesourcery dot com>]

Andrew Pinski wrote:
> On 9/11/07, Mark Shinwell <shinwell@codesourcery.com> wrote:
> > This patch, based on one by Nigel Stephens of MIPS Technologies,
> > eliminates some duplicate sign extensions upon function calls.
> > Currently, if we have something such as:
> >
> > static int f(short foobar)
> > {
> >    return foobar > 0;
> > }
>
> Does this optimization fix PR 23450 ?

Before my patch your second example in that PR gives:

f:
        pushl   %ebp
        addl    $1, %eax
        movl    %esp, %ebp
        movsbl  %al,%eax
        popl    %ebp
        ret
        .size   f, .-f
        .p2align 4,,15
.globl g
        .type   g, @function
g:
        pushl   %ebp
        movl    %esp, %ebp
        movzbl  8(%ebp), %eax
        addl    $1, %eax
        movsbl  %al,%eax
        call    f
        popl    %ebp
        movsbl  %al,%eax
        ret

and after the patch we get:

f:
        pushl   %ebp
        addl    $1, %eax
        movl    %esp, %ebp
        popl    %ebp
        ret
        .size   f, .-f
        .p2align 4,,15
.globl g
        .type   g, @function
g:
        pushl   %ebp
        movl    %esp, %ebp
        movzbl  8(%ebp), %eax
        popl    %ebp
        addl    $1, %eax
        movsbl  %al,%eax
        jmp     f

So it certainly helps -- but not being an x86 expert I'm not
sure offhand if this is addressing the entirety of that PR.

On x86_64-unknown-linux-gnu, it seems that the sign extensions at
the head of functions are already being elided for some reason.

Mark