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

[Eli Friedman]

On Fri, Sep 30, 2011 at 5:05 PM, Nick Lewycky <nlewycky at google.com> wrote:
> 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!

The given testcase works fine for me at -O2 without your patch... is
there some case which doesn't work?

-Eli