[llvm] [LAA] Fix WAW dependency analysis with negative distances (PR #155266)

[Ryotaro Kasuga via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -passes='print<access-info>' -disable-output %s 2>&1 | FileCheck %s
+
+; Test that LAA correctly identifies Write-After-Write dependencies with negative
+; distances as unsafe. Previously, LAA would incorrectly classify negative distance
+; WAW dependencies as safe Forward dependencies, allowing inappropriate vectorization.
+;
+; This corresponds to the loop:
+;   for(int i = 0; i < n; ++i) {
+;       A[(i+1)*4] = 10;   // First store: A[4, 8, 12, 16, ...]
+;       A[i] = 100;        // Second store: A[0, 1, 2, 3, 4, ...]
+;   }
+;
+; The dependence distance from first store to second store is negative:
+; A[i] - A[(i+1)*4] = {0,+,4} - {16,+,16} = {-16,+,-12}
+; However, the dependence from second store to first store in the next iteration
+; would be positive: A[(i+1)*4] - A[i] = {16,+,16} - {0,+,4} = {16,+,12}
+;
+; This bidirectional dependence pattern (negative in one direction, positive in the
+; other) creates a Write-After-Write dependency that is unsafe for vectorization.
+; DependenceAnalysis would report this as "output [<>]!" indicating the complex
+; dependence direction. LAA must detect this as unsafe even when only checking
+; the negative distance direction.
+
+define void @test_waw_negative_dependence(i64 %n, ptr nocapture %A) {
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kasuga-fj wrote:

Wait, hold on. LAA already reports Unsafe for this case.

https://godbolt.org/z/PvxsMdEa1

https://github.com/llvm/llvm-project/pull/155266