[llvm] [GISel] Add support for scalable vectors in getLCMType (PR #80306)

[Craig Topper via llvm-commits]

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@@ -1071,49 +1071,73 @@ void llvm::getSelectionDAGFallbackAnalysisUsage(AnalysisUsage &AU) {
 }
 
 LLT llvm::getLCMType(LLT OrigTy, LLT TargetTy) {
-  const unsigned OrigSize = OrigTy.getSizeInBits();
-  const unsigned TargetSize = TargetTy.getSizeInBits();
-
-  if (OrigSize == TargetSize)
+  if (OrigTy.getSizeInBits() == TargetTy.getSizeInBits())
     return OrigTy;
 
-  if (OrigTy.isVector()) {
-    const LLT OrigElt = OrigTy.getElementType();
-
-    if (TargetTy.isVector()) {
-      const LLT TargetElt = TargetTy.getElementType();
+  if (OrigTy.isVector() && TargetTy.isVector()) {
+    LLT OrigElt = OrigTy.getElementType();
+    LLT TargetElt = TargetTy.getElementType();
 
-      if (OrigElt.getSizeInBits() == TargetElt.getSizeInBits()) {
-        int GCDElts =
-            std::gcd(OrigTy.getNumElements(), TargetTy.getNumElements());
-        // Prefer the original element type.
-        ElementCount Mul = OrigTy.getElementCount() * TargetTy.getNumElements();
-        return LLT::vector(Mul.divideCoefficientBy(GCDElts),
-                           OrigTy.getElementType());
-      }
-    } else {
-      if (OrigElt.getSizeInBits() == TargetSize)
-        return OrigTy;
+    // TODO: The docstring for this function says the intention is to use this
+    // function to build MERGE/UNMERGE instructions. It won't be the case that
+    // we generate a MERGE/UNMERGE between fixed and scalable vector types. We
+    // could implement getLCMType between the two in the future if there was a
+    // need, but it is not worth it now as this function should not be used in
+    // that way.
+    if ((OrigTy.isScalableVector() && TargetTy.isFixedVector()) ||
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topperc wrote:

Use an assert.

https://github.com/llvm/llvm-project/pull/80306