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

[David Sherwood via llvm-commits]

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@@ -747,6 +781,655 @@ 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;
+};
+
+/// Recursion cap for addScaledStencilTerm. Depth counts how deep a term
+/// sits inside the offset expression. Example, the offset
+///   8 + (64 * (s1 + s2 + (4 * s3)))
+/// is visited like this:
+///   depth 0: the whole add
+///   depth 1: its operands 8 and (64 * (s1 + s2 + (4 * s3)))
+///   depth 2: (s1 + s2 + (4 * s3)), the operand of the multiply
+///   depth 3: s1, s2 and (4 * s3), the operands of that add
+/// At depth 3 addScaledStencilTerm stops going deeper. s1 and s2 are plain
+/// strides anyway. (4 * s3) is not split into 4 times s3: it becomes one
+/// stride key as it is, with coefficient 64. The result is Constant = 8
+/// and coefficients {s1: 64, s2: 64, (4 * s3): 64}.
+/// Three levels cover the stencil offsets we care about: a top-level add,
+/// a constant times a sum inside it, and the strides in that sum. A deeper
+/// term is kept whole as one stride key. The merge does not care what is
+/// inside a key. It only needs a loop-invariant value with a
+/// positive-stride predicate, and a whole term has both. The only cost is
+/// precision, when another member uses a part of that term, here s3 alone,
+/// as a key of its own. isNeverAbove sees two unrelated keys, so a member
+/// that is in fact always lower or higher may stay a candidate.
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david-arm wrote:

This is fantastic, thanks @igogo-x86! This really helps me personally to understand what's going on.

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