[clang] [llvm] [RISCV] Improve casting between i1 scalable vectors and i8 fixed vectors for -mrvv-vector-bits (PR #139190)

[Paul Walker via cfe-commits]

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@@ -1476,8 +1486,14 @@ CoerceScalableToFixed(CodeGenFunction &CGF, llvm::FixedVectorType *ToTy,
   // If we are casting a scalable i1 predicate vector to a fixed i8
   // vector, first bitcast the source.
   if (FromTy->getElementType()->isIntegerTy(1) &&
-      FromTy->getElementCount().isKnownMultipleOf(8) &&
       ToTy->getElementType() == CGF.Builder.getInt8Ty()) {
+    if (!FromTy->getElementCount().isKnownMultipleOf(8)) {
+      FromTy = llvm::ScalableVectorType::get(
+          FromTy->getElementType(),
+          llvm::alignTo<8>(FromTy->getElementCount().getKnownMinValue()));
+      llvm::Value *ZeroVec = llvm::Constant::getNullValue(FromTy);
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paulwalker-arm wrote:

If it's a choice between the LangRef and an old mailing list post then I think the LangRef should win. At least the bitcast part of the LangRef suggests this is not true, which is backed up by https://godbolt.org/z/n67WTcf1v that shows effort is put in to preserving the known bits?

https://github.com/llvm/llvm-project/pull/139190