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

[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.
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ayalz wrote:

Should this be taken care of by VPRecipeWithIRFlags::dropPoisonGeneratingFlags(), i.e., have it replace the opcode of a disjoint Or with Add?

nit (independent): `collectPoisonGeneratingInstrsInBackwardSlice()` does more than collect Instrs, it also updates them.

> Note that replacing all disjoint ORs with ADDs instead of dropping the flags is not strictly necessary. It is only needed for disjoint ORs that SCEV treated as ADDs, but those are not tracked.

Should code based on such SCEV expressions be generated (expanded(?)) directly?

(This case is reminiscent of dropping assumptions based on conditions subject to predication.)

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