[llvm] [DA] do not handle array accesses of different offsets (PR #123436)

[Sebastian Pop via llvm-commits]

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@@ -3569,6 +3569,123 @@ bool DependenceInfo::invalidate(Function &F, const PreservedAnalyses &PA,
          Inv.invalidate<LoopAnalysis>(F, PA);
 }
 
+// Check that memory access offsets in V are multiples of array element size
+// EltSize. Param records the first parametric expression. If the scalar
+// evolution V contains two or more parameters, we check that the subsequent
+// parametric expressions are multiples of the first parametric expression
+// Param.
+static bool checkOffsets(ScalarEvolution *SE, const SCEV *V, const SCEV *&Param,
+                         uint64_t EltSize) {
+  if (auto *AddRec = dyn_cast<SCEVAddRecExpr>(V)) {
+    if (!checkOffsets(SE, AddRec->getStart(), Param, EltSize))
+      return false;
+    return checkOffsets(SE, AddRec->getStepRecurrence(*SE), Param, EltSize);
+  }
+  if (auto *Cst = dyn_cast<SCEVConstant>(V)) {
+    APInt C = Cst->getAPInt();
+
+    // For example, alias_with_different_offsets in
+    // test/Analysis/DependenceAnalysis/DifferentOffsets.ll accesses "%A + 2":
+    //   %arrayidx = getelementptr inbounds i8, ptr %A, i64 2
+    //   store i32 42, ptr %arrayidx, align 1
+    // which is writing an i32, i.e., EltSize = 4 bytes, with an offset C = 2.
+    // checkOffsets returns false, as the offset C=2 is not a multiple of 4.
+    return C.srem(EltSize) == 0;
+  }
+
+  // Use a lambda helper function to check V for parametric expressions.
+  // Param records the first parametric expression. If the scalar evolution V
+  // contains two or more parameters, we check that the subsequent parametric
+  // expressions are multiples of the first parametric expression Param.
+  auto checkParamsMultipleOfSize = [&](const SCEV *V,
+                                       const SCEV *&Param) -> bool {
+    if (EltSize == 1)
+      return true;
+    if (!Param) {
+      Param = V;
+      return true;
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sebpop wrote:

I fixed these issues in 60ba4e688105c7663b8bb01eed7076f77b385935 by recording runtime assumptions for all expressions containing parameters.  These runtime assumptions can be generated as runtime if conditions to guard the code generated with the dependence info.

https://github.com/llvm/llvm-project/pull/123436