[X86] getFauxShuffleMask - OR(BITCAST(SHUFFLE()),BITCAST(SHUFFLE())) patterns should return bitcasted source values (#134993)

RKSimon · web-flow · commit 747214e59be6 · 2025-04-09T12:54:16.000+01:00
Noticed while investigating #133947 regressions - if we peek through
bitcasts we can lose track of oneuse/combined nodes in shuffle combining

Currently the same codegen as combineX86ShufflesRecursively still peeks
through the bitcasts itself, but we will soon handle this consistently
as another part of #133947
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -6155,8 +6155,8 @@ static bool getFauxShuffleMask(SDValue N, const APInt &DemandedElts,
       else
         return false;
     }
-    Ops.push_back(N0);
-    Ops.push_back(N1);
+    Ops.push_back(N.getOperand(0));
+    Ops.push_back(N.getOperand(1));
     return true;
   }
   case ISD::CONCAT_VECTORS: {

Original file line number	Diff line number	Diff line change
`@@ -6155,8 +6155,8 @@ static bool getFauxShuffleMask(SDValue N, const APInt &DemandedElts,`
`6155`	`6155`	`else`
`6156`	`6156`	`return false;`
`6157`	`6157`	`}`
`6158`		`- Ops.push_back(N0);`
`6159`		`- Ops.push_back(N1);`
	`6158`	`+ Ops.push_back(N.getOperand(0));`
	`6159`	`+ Ops.push_back(N.getOperand(1));`
`6160`	`6160`	`return true;`
`6161`	`6161`	`}`
`6162`	`6162`	`case ISD::CONCAT_VECTORS: {`