[llvm] [InstCombine] Allow folding cross-lane operations into PHIs/selects (PR #164388)

[Benjamin Maxwell via llvm-commits]

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@@ -4003,18 +4003,29 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
 
   // Try to fold intrinsic into select/phi operands. This is legal if:
   //  * The intrinsic is speculatable.
-  //  * The select condition is not a vector, or the intrinsic does not
-  //    perform cross-lane operations.
-  if (isSafeToSpeculativelyExecuteWithVariableReplaced(&CI) &&
-      isNotCrossLaneOperation(II))
+  //  * The operand is one of the following:
+  //    - a phi.
+  //    - a select with a scalar condition.
+  //    - a select with a vector condition and II is not a cross lane operation.
+  if (isSafeToSpeculativelyExecuteWithVariableReplaced(&CI)) {
     for (Value *Op : II->args()) {
-      if (auto *Sel = dyn_cast<SelectInst>(Op))
-        if (Instruction *R = FoldOpIntoSelect(*II, Sel))
+      if (auto *Sel = dyn_cast<SelectInst>(Op)) {
+        bool IsVectorCond = Sel->getCondition()->getType()->isVectorTy();
+        if (IsVectorCond && !isNotCrossLaneOperation(II))
+          continue;
+        // Don't replace a scalar select with a more expensive vector select if
+        // we can't simplify both arms of the select.
+        bool SimplifyBothArms =
+            !Op->getType()->isVectorTy() && II->getType()->isVectorTy();
+        if (Instruction *R = FoldOpIntoSelect(
+                *II, Sel, /*FoldWithMultiUse=*/false, SimplifyBothArms))
           return R;
+      }
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MacDue wrote:

Added some test cases, I don't think it's entirely unexpected as `isNotCrossLaneOperation` is ~the same as `isTriviallyVectorizable()`, and the overflow intrinsics currently are not trivially vectorizable. 

So now `isNotCrossLaneOperation` is only checked for vector select conditions, it means these can also get folded. 

https://github.com/llvm/llvm-project/pull/164388