[LLVMdev] predicates vs. requirements [TableGen, X86InstrInfo.td]

Hal Finkel
Thu, 18 Sep 2014 21:40:44 -0000

----- Original Message -----
> From: "Sanjay Patel" <spatel at rotateright.com>
> To: llvmdev at cs.uiuc.edu
> Sent: Thursday, September 18, 2014 4:25:07 PM
> Subject: [LLVMdev] predicates vs. requirements [TableGen, X86InstrInfo.td]
>
>
>
>
> I tried to add an 'OptForSize' requirement to a pattern in
> X86InstrSSE.td, but it appears to be ignored. However, the condition
> was detected when specified as a predicate.

Without looking at this in detail, I've seen similar behavior before, so
I'll guess: When you use Requires with OptForSize, it will add OptForSize to
the Predicates list, but it will not do so if you explicitly set the Predicates
list (with a let clause). So if there are any other predicates in effect, the
Requires will be ignored. However, if you explicitly set Predicates (with the
let clause) you're erasing any other predicates that may have been in effect
from an outer block.

 -Hal

>
> So this doesn't work:
> def : Pat <( v2f64 ( X86VBroadcast ( loadf64 addr : $src ))), (
> VMOVDDUPrm addr : $src )>,
> Requires <[ OptForSize ]> ;
>
> But this does:
> let Predicates = [OptForSize] in {
> def : Pat <( v2f64 ( X86VBroadcast ( loadf64 addr : $src ))), (
> VMOVDDUPrm addr : $src )>;
> }
>
>
> I see both forms used on some patterns like this:
> let Predicates = [HasAVX] in {
> def : Pat<(X86Movddup (loadv2f64 addr:$src)),
> (VMOVDDUPrm addr:$src)>, Requires<[HasAVX]> ;
> }
>
>
> Is a predicate different than a requirement or is this a bug?
>
>
> There are existing patterns that specify 'OptForSize' with
> "Requires", but they are not behaving as I expected.
>
> Example:
> // For scalar unary operations, fold a load into the operation
> // only in OptForSize mode. It eliminates an instruction, but it also
> // eliminates a whole-register clobber (the load), so it introduces a
> // partial register update condition.
> def : Pat<(f32 (fsqrt (load addr:$src))),
> (VSQRTSSm (f32 (IMPLICIT_DEF)), addr:$src)>,
> Requires<[HasAVX, OptForSize]>;
>
>
> This is generated:
>
> vsqrtss (%rdi), %xmm0, %xmm0
>
>
> regardless of whether I specify -Os or -O1 with clang.
>
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--
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory