[JT] check xor operand is exactly the same in processBranchOnXOR

[inclyc]

Summary

Reproducer:

; RUN: opt -S -jump-threading < %s
define void @test() {
entry:
br i1 false, label %loop, label %exit

loop:
%bool = phi i1 [ %xor, %loop.latch ], [ false, %entry ]
%cmp = icmp eq i16 0, 1
%xor = xor i1 %cmp, %bool
br i1 %bool, label %loop.latch, label %exit

loop.latch:
%dummy = phi i16 [ 0, %loop ]
br label %loop

exit:
ret void
}

On this occassion, phi node %bool is actually %xor, and doing substitution causes assertion failure.

Fixes: https://github.com/llvm/llvm-project/issues/58812

[inclyc]

ping :)

[nikic]

LGTM

llvm/lib/Transforms/Scalar/JumpThreading.cpp

bool JumpThreadingPass::processBranchOnXOR(BinaryOperator *BO) {
   // If we inferred a value for all of the predecessors, then duplication won't
   // help us.  However, we can just replace the LHS or RHS with the constant.
   if (BlocksToFoldInto.size() ==
       cast<PHINode>(BB->front()).getNumIncomingValues()) {
     if (!SplitVal) {
       // If all preds provide undef, just nuke the xor, because it is undef too.
       BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
       BO->eraseFromParent();
-    } else if (SplitVal->isZero()) {
+    } else if (SplitVal->isZero() && BO != BO->getOperand(isLHS)) {
       // If all preds provide 0, replace the xor with the other input.
       BO->replaceAllUsesWith(BO->getOperand(isLHS));
       BO->eraseFromParent();
     } else {
       // If all preds provide 1, set the computed value to 1.
       BO->setOperand(!isLHS, SplitVal);
     }
 

llvm/test/Transforms/JumpThreading/phi-xor-branch.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -passes=jump-threading %s | FileCheck %s
+
+; https://github.com/llvm/llvm-project/issues/58812
+
+define void @test() {
+; CHECK-LABEL: @test(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[EXIT:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[DUMMY:%.*]] = phi i16 [ 0, [[LOOP:%.*]] ]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i16 0, 1
+; CHECK-NEXT:    [[XOR:%.*]] = xor i1 false, [[XOR]]
+; CHECK-NEXT:    br i1 [[XOR]], label [[LOOP]], label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret void
+;
+entry:
+  br i1 false, label %loop, label %exit
+
+loop:
+  %bool = phi i1 [ %xor, %loop.latch ], [ false, %entry ]
+  %cmp = icmp eq i16 0, 1
+  %xor = xor i1 %cmp, %bool
+  br i1 %bool, label %loop.latch, label %exit
+
+loop.latch:
+  %dummy = phi i16 [ 0, %loop ]
+  br label %loop
+
+exit:
+  ret void
+}