[llvm] [X86] getFauxShuffleMask - only handle VTRUNC nodes with matching src/dst sizes (PR #134161)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

<details>
<summary>Changes</summary>

Cleanup work for #<!-- -->133947 - we need to handle VTRUNC nodes with large source vectors directly to allow us to widen the size of the shuffle combine

We currently discard these results in combineX86ShufflesRecursively anyhow as we don't allow inputs from getTargetShuffleInputs to be larger than the shuffle value type

---
Full diff: https://github.com/llvm/llvm-project/pull/134161.diff


1 Files Affected:

- (modified) llvm/lib/Target/X86/X86ISelLowering.cpp (+1-3) 


``````````diff
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 8e6a891444bf1..aea80120a0481 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -6413,9 +6413,7 @@ static bool getFauxShuffleMask(SDValue N, const APInt &DemandedElts,
   case X86ISD::VTRUNC: {
     SDValue Src = N.getOperand(0);
     EVT SrcVT = Src.getValueType();
-    // Truncated source must be a simple vector.
-    if (!SrcVT.isSimple() || (SrcVT.getSizeInBits() % 128) != 0 ||
-        (SrcVT.getScalarSizeInBits() % 8) != 0)
+    if (SrcVT.getSizeInBits() != NumSizeInBits)
       return false;
     unsigned NumSrcElts = SrcVT.getVectorNumElements();
     unsigned NumBitsPerSrcElt = SrcVT.getScalarSizeInBits();

``````````

</details>


https://github.com/llvm/llvm-project/pull/134161