[llvm] [VPlan] Replace disjoint or with add instead of dropping disjoint. (PR #83821)

[Florian Hahn via llvm-commits]

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@@ -1216,6 +1216,23 @@ void VPlanTransforms::dropPoisonGeneratingRecipes(
       // load/store. If the underlying instruction has poison-generating flags,
       // drop them directly.
       if (auto *RecWithFlags = dyn_cast<VPRecipeWithIRFlags>(CurRec)) {
+        VPValue *A, *B;
+        using namespace llvm::VPlanPatternMatch;
+        // Dropping disjoint from an OR may yield incorrect results, as some
+        // analysis may have converted it to an Add implicitly (e.g. SCEV used
+        // for dependence analysis). Instead, replace it with an equivalent Add.
+        // This is possible as all users of the disjoint OR only access lanes
+        // where the operands are disjoint or poison otherwise.
+        if (match(RecWithFlags, m_Or(m_VPValue(A), m_VPValue(B))) &&
+            RecWithFlags->isDisjoint()) {
+          VPBuilder Builder(RecWithFlags);
+          VPInstruction *New = Builder.createOverflowingOp(
+              Instruction::Add, {A, B}, {false, false},
+              RecWithFlags->getDebugLoc());
+          RecWithFlags->replaceAllUsesWith(New);
+          RecWithFlags->eraseFromParent();
+          CurRec = New;
+        }
         RecWithFlags->dropPoisonGeneratingFlags();
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fhahn wrote:

Missed that this was reverted, recommitted with a fix.

https://github.com/llvm/llvm-project/pull/83821