[llvm] [LAA] Perform checks for no-wrap separately from getPtrStride. (PR #126971)

[Ramkumar Ramachandra via llvm-commits]

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@@ -813,16 +813,102 @@ static bool hasComputableBounds(PredicatedScalarEvolution &PSE, Value *Ptr,
   return AR->isAffine();
 }
 
+/// Try to compute the stride for \p AR. Used by getPtrStride.
+static std::optional<int64_t>
+getStrideFromAddRec(const SCEVAddRecExpr *AR, const Loop *Lp, Type *AccessTy,
+                    Value *Ptr, PredicatedScalarEvolution &PSE) {
+  // The access function must stride over the innermost loop.
+  if (Lp != AR->getLoop()) {
+    LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not striding over innermost loop "
+                      << *Ptr << " SCEV: " << *AR << "\n");
+    return std::nullopt;
+  }
+
+  // Check the step is constant.
+  const SCEV *Step = AR->getStepRecurrence(*PSE.getSE());
+
+  // Calculate the pointer stride and check if it is constant.
+  const SCEVConstant *C = dyn_cast<SCEVConstant>(Step);
+  if (!C) {
+    LLVM_DEBUG(dbgs() << "LAA: Bad stride - Not a constant strided " << *Ptr
+                      << " SCEV: " << *AR << "\n");
+    return std::nullopt;
+  }
+
+  const auto &DL = Lp->getHeader()->getDataLayout();
+  TypeSize AllocSize = DL.getTypeAllocSize(AccessTy);
+  int64_t Size = AllocSize.getFixedValue();
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artagnon wrote:

I don't mind making the improvements as a separate NFC.

https://github.com/llvm/llvm-project/pull/126971