[llvm] [LoadStoreVectorizer] Propagate alignment through contiguous chain (PR #145733)

[Nikita Popov via llvm-commits]

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@@ -21,6 +21,43 @@
 
 using namespace llvm;
 
+static bool tryToPropagateAlign(Function &F, const DataLayout &DL) {
+  bool Changed = false;
+
+  for (BasicBlock &BB : F) {
+    // We need to reset the map for each block because alignment information
+    // can't be propagated across blocks. This is because control flow could
+    // be dependent on the address at runtime, making an alignment assumption
+    // within one block not true in another. Some sort of dominator tree
+    // approach could be better, but restricting within a basic block is correct
+    // too.
+    DenseMap<Value *, Align> BestBasePointerAligns;
+    for (Instruction &I : BB) {
+      if (auto *PtrOp = getLoadStorePointerOperand(&I)) {
+        Align LoadStoreAlign = getLoadStoreAlignment(&I);
+        APInt OffsetFromBase = APInt(
+            DL.getIndexSizeInBits(PtrOp->getType()->getPointerAddressSpace()),
+            0);
+        PtrOp = PtrOp->stripAndAccumulateInBoundsConstantOffsets(
+            DL, OffsetFromBase);
+        Align BasePointerAlign =
+            commonAlignment(LoadStoreAlign, OffsetFromBase.getLimitedValue());
+
+        if (BestBasePointerAligns.count(PtrOp) &&
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nikic wrote:

Avoid duplicate hash lookup (e.g. using try_emplace).

https://github.com/llvm/llvm-project/pull/145733