[llvm] [ConstantFold] Support byte values in `bitcast` constant folding (PR #188030)

[Yingwei Zheng via llvm-commits]

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@@ -105,6 +106,68 @@ static Constant *foldConstVectorToAPInt(APInt &Result, Type *DestTy,
   return nullptr;
 }
 
+/// Check whether folding this bitcast into a byte vector would mix poison and
+/// non-poison bits in the same output lane. While integer types track poison on
+/// a per-value basis, byte types track it on a per-bit basis. However,
+/// `ConstantByte` cannot represent values with both poison and non-poison bits.
+///
+/// Source elements are grouped by the output lane they map to. Returns true if
+/// any group contains both poison and non-poison elements.
+static bool foldMixesPoisonBits(Constant *C, unsigned NumSrcElt,
+                                unsigned NumDstElt) {
+  unsigned Ratio = NumSrcElt / NumDstElt;
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dtcxzyw wrote:

As I pointed out in https://github.com/llvm/llvm-project/pull/185205, this is incorrect for some bitcasts like `<8 x b3> -> <3 x b8>`. But it should be fine for now.


https://github.com/llvm/llvm-project/pull/188030