[llvm] [GVN] Load-store forwaring of scalable store to fixed load. (PR #124748)

[Nikita Popov via llvm-commits]

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@@ -13,32 +13,52 @@ static bool isFirstClassAggregateOrScalableType(Type *Ty) {
   return Ty->isStructTy() || Ty->isArrayTy() || isa<ScalableVectorType>(Ty);
 }
 
+static std::optional<unsigned> getKnownVScale(Function *F) {
+  const auto &Attrs = F->getAttributes().getFnAttrs();
+  unsigned MinVScale = Attrs.getVScaleRangeMin();
+  if (Attrs.getVScaleRangeMax() == MinVScale)
+    return MinVScale;
+  return std::nullopt;
+}
+
 /// Return true if coerceAvailableValueToLoadType will succeed.
 bool canCoerceMustAliasedValueToLoad(Value *StoredVal, Type *LoadTy,
-                                     const DataLayout &DL) {
+                                     Function *F) {
   Type *StoredTy = StoredVal->getType();
-
   if (StoredTy == LoadTy)
     return true;
 
+  const DataLayout &DL = F->getDataLayout();
+  TypeSize StoreSize = DL.getTypeSizeInBits(StoredTy);
+  TypeSize LoadSize = DL.getTypeSizeInBits(LoadTy);
   if (isa<ScalableVectorType>(StoredTy) && isa<ScalableVectorType>(LoadTy) &&
-      DL.getTypeSizeInBits(StoredTy) == DL.getTypeSizeInBits(LoadTy))
+      StoreSize == LoadSize)
     return true;
 
-  // If the loaded/stored value is a first class array/struct, or scalable type,
-  // don't try to transform them. We need to be able to bitcast to integer.
-  if (isFirstClassAggregateOrScalableType(LoadTy) ||
-      isFirstClassAggregateOrScalableType(StoredTy))
+  // If the loaded/stored value is a first class array/struct, don't try to
+  // transform them. We need to be able to bitcast to integer. For scalable
+  // vectors forwarded to fixed-sized vectors @llvm.vector.extract is used.
+  if (isa<ScalableVectorType>(StoredTy) && isa<FixedVectorType>(LoadTy)) {
+    if (StoredTy->getScalarType() != LoadTy->getScalarType())
+      return false;
+
+    // If the VScale is known at compile-time, use that information to
+    // allow for wider loads.
+    std::optional<unsigned> VScale = getKnownVScale(F);
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nikic wrote:

This could be generalized to only look at getVScaleRangeMin to determine the minimum store size (without knowing the exact store size), as we only need StoreSize >= LoadSize. (Don't know if we expect that pattern to occur.)

https://github.com/llvm/llvm-project/pull/124748