[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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@@ -3509,9 +3509,9 @@ static SDValue matchSplatAsGather(SDValue SplatVal, MVT VT, const SDLoc &DL,
     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();
+  if (auto *CIdx = dyn_cast<ConstantSDNode>(Idx))
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preames wrote:

As written this doesn't work.  If you have a non-constant index which is out of bounds, this patch allows it to proceed.  That's illegal.

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