[llvm] [LAA] Add stencil group merging to reduce runtime pointer checks (PR #187252)

[Igor Kirillov via llvm-commits]

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@@ -737,6 +763,415 @@ void RuntimePointerChecking::groupChecks(
   }
 }
 
+/// Result of decomposing a SCEV expression into stencil offset form:
+///   Offset = Constant + sum(Coefficients[stride] * stride)
+/// where each stride is a loop-invariant SCEV expression.
+struct StencilDecomposition {
+  int64_t Constant = 0;
+  /// Map from loop-invariant stride SCEV to its integer coefficient.
+  SmallMapVector<const SCEV *, int64_t, 4> Coefficients;
+};
+
+/// Try to decompose \p Expr into a stencil offset function of loop-invariant
+/// strides: C + a1*s1 + a2*s2 + ...
+/// Relies on SCEV's canonical form: AddExpr operands are flattened (N-ary),
+/// MulExpr has the constant operand first when present.
+/// Returns std::nullopt if the expression contains non-stencil terms or any
+/// SCEV constant doesn't fit in int64_t (we commit to the signed
+/// interpretation; values that need more than 64 significant bits are
+/// out of scope).
+static std::optional<StencilDecomposition>
+decomposeStencilOffset(const SCEV *Expr, ScalarEvolution &SE, const Loop &L) {
+  StencilDecomposition D;
+
+  // The only caller passes a difference of two pointer Starts, and a Start is
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igogo-x86 wrote:

I updated the doc comment of `decomposeStencilOffset` explaining all that

https://github.com/llvm/llvm-project/pull/187252