[llvm-commits] patch: use addressing modes for "m" constraints fed by bitcast/GEP

[Nick Lewycky]

On 30 September 2011 16:30, Eli Friedman <eli.friedman at gmail.com> wrote:

> On Fri, Sep 30, 2011 at 3:26 PM, Nick Lewycky <nlewycky at google.com> wrote:
> > The attached patch eagerly evaluates bitcasts and GEPs feeding inline
> > assembly constraints. That reduces the chance that we'll copy the
> > result into a register. On this testcase:
> >
> > void test(short *a,short *b,float *c) {
> >    char tmp[256];
> >    __asm__ volatile(
> >        "%0 %1 %2 %3 %4"
> >        :
> >        :"m"(a),"m"(b),"m"(c),"m"(tmp[0]), "m"(tmp[128])
> >        :"memory","%eax","%ebx","%ecx","%edx","%esi","%edi");
> > }
> >
> > previously we'd run out of registers. Now, we emit:
> >
> >        pushl   %ebx
> >        pushl   %edi
> >        pushl   %esi
> >        subl    $268, %esp              # imm = 0x10C
> >        movl    292(%esp), %eax
> >        movl    288(%esp), %ecx
> >        movl    284(%esp), %edx
> >        movl    %edx, 264(%esp)
> >        movl    %ecx, 260(%esp)
> >        movl    %eax, 256(%esp)
> >        #APP
> >        264(%esp) 260(%esp) 256(%esp) (%esp) 128(%esp)
> >        #NO_APP
> >        addl    $268, %esp              # imm = 0x10C
> >        popl    %esi
> >        popl    %edi
> >        popl    %ebx
> >        ret
> >
> > which clearly has an extra copy, but that's a bug which existed
> > without my patch anyways. The tmp[0] and tmp[128] don't have any
> > copies.
> >
> > Please review!
>
> I follow the concept, but I don't think the place you're hacking it in
> is really appropriate.  This is a fast-isel-specific problem, so it
> seems like the fix should also be in fast-isel code.
>

Thanks. I don't like where it is now, but I don't see where else to put
it. It is not fast-isel specific, the same testcase fails at -O2 before and
works after. Can't explain that!

Nick
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