[PATCH] D56636: [CodeGen][X86] Expand vector USUBSAT to UMAX+SUB

Sat Jan 12 13:59:23 PST 2019

RKSimon added inline comments.

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Comment at: lib/Target/X86/X86ISelLowering.cpp:1224
+    if (!HasInt256)
+      setOperationAction(ISD::USUBSAT, MVT::v8i32, Custom);

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nikic wrote:
> RKSimon wrote:
> > nikic wrote:
> > > RKSimon wrote:
> > > > LowerADDSAT_SUBSAT supports AVX1 splitting, so why not:
> > > > ```
> > > > setOperationAction(ISD::USUBSAT, MVT::v8i32, HasInt256 ? Legal : Custom);
> > > > ```
> > > > 
> > > > Also, v4i64  should be ok as well.
> > > I've added this to get the splitting, but I don't think v8i32 would be legal, as we need it to expand. Maybe `hasInt256 ? Expand : Custom` would make it more obvious what the intention here is?
> > Better for the existing x86 lowering code to handle it IMO - that's what we do for nearly all 256-bit integer code on AVX1 targets.
> That's what this is intended to do. v8i32 is custom so lowering splits it into two v4i32s, which then get expanded.
> 
> If I change this to `HasInt256 ? Legal : Custom`, then I get:
> 
>     LLVM ERROR: Cannot select: t5: v8i32 = usubsat t2, t4
> 
> Or am I misunderstanding what you have in mind here?
If you look at LowerABS just below LowerADDSAT_SUBSAT you'll see the approach that I took.

Repository:
  rL LLVM

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D56636/new/

https://reviews.llvm.org/D56636