[llvm] [VectorCombine] Fold chain of (scalar load)->ext->ext to load->ext. (PR #141109)

[Matt Arsenault via llvm-commits]

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@@ -2777,6 +2778,55 @@ bool VectorCombine::foldShuffleToIdentity(Instruction &I) {
   return true;
 }
 
+bool VectorCombine::foldShuffleExtExtracts(Instruction &I) {
+  // Try to fold vector zero- and sign-extends split across multiple operations
+  // into a single extend, removing redundant inserts and shuffles.
+
+  // Check if we have an extended shuffle that selects the first vector, which
+  // itself is another extend fed by a load.
+  Instruction *L;
+  if (!match(
+          &I,
+          m_OneUse(m_Shuffle(
+              m_OneUse(m_ZExtOrSExt(m_OneUse(m_BitCast(m_OneUse(m_InsertElt(
+                  m_Value(), m_OneUse(m_Instruction(L)), m_SpecificInt(0))))))),
+              m_Value()))) ||
+      !cast<ShuffleVectorInst>(&I)->isIdentityWithExtract() ||
+      !isa<LoadInst>(L))
+    return false;
+  auto *InnerExt = cast<Instruction>(I.getOperand(0));
+  auto *OuterExt = dyn_cast<Instruction>(*I.user_begin());
+  if (!isa<SExtInst, ZExtInst>(OuterExt))
+    return false;
+
+  // If the inner extend is a sign extend and the outer one isnt (i.e. a
+  // zero-extend), don't fold. If the first one is zero-extend, it doesn't
+  // matter if the second one is a sign- or zero-extend.
+  if (isa<SExtInst>(InnerExt) && !isa<SExtInst>(OuterExt))
+    return false;
+
+  // Don't try to convert the load if it has an odd size.
+  if (!DL->typeSizeEqualsStoreSize(L->getType()))
+    return false;
+  auto *DstTy = cast<FixedVectorType>(OuterExt->getType());
+  auto *SrcTy =
+      FixedVectorType::get(InnerExt->getOperand(0)->getType()->getScalarType(),
+                           DstTy->getNumElements());
+  if (DL->getTypeStoreSize(SrcTy) != DL->getTypeStoreSize(L->getType()))
+    return false;
+
+  // Convert to a vector load feeding a single wide extend.
+  Builder.SetInsertPoint(*L->getInsertionPointAfterDef());
+  auto *NewLoad = cast<LoadInst>(
+      Builder.CreateLoad(SrcTy, L->getOperand(0), L->getName() + ".vec"));
+  auto *NewExt = isa<ZExtInst>(InnerExt) ? Builder.CreateZExt(NewLoad, DstTy)
----------------
arsenm wrote:

Loses nneg flag on the cast?

https://github.com/llvm/llvm-project/pull/141109