[llvm] [RISCV] Update matchSplatAsGather to use the index of extract_elt if in-bounds (PR #118873)

[Philip Reames via llvm-commits]

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@@ -3517,27 +3517,44 @@ static SDValue matchSplatAsGather(SDValue SplatVal, MVT VT, const SDLoc &DL,
   // different
   // FIXME: Support i1 vectors, maybe by promoting to i8?
   MVT EltTy = VT.getVectorElementType();
-  if (EltTy == MVT::i1 ||
-      EltTy != Vec.getSimpleValueType().getVectorElementType())
+  MVT VecVT = Vec.getSimpleValueType();
+  if (EltTy == MVT::i1 || EltTy != VecVT.getVectorElementType())
     return SDValue();
   SDValue Idx = SplatVal.getOperand(1);
   // The index must be a legal type.
   if (Idx.getValueType() != Subtarget.getXLenVT())
     return SDValue();
 
-  // Check that Index lies within VT
-  // TODO: Can we check if the Index is constant and known in-bounds?
-  if (!TypeSize::isKnownLE(Vec.getValueSizeInBits(), VT.getSizeInBits()))
-    return SDValue();
+  // Check that we know Idx lies within VT
+  if (!TypeSize::isKnownLE(Vec.getValueSizeInBits(), VT.getSizeInBits())) {
+    auto *CIdx = dyn_cast(Idx);
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preames wrote:

Does this compile?  dyn_cast<X> would be more typical?

https://github.com/llvm/llvm-project/pull/118873