[JumpThreading] add a miscompile test based on discussion in D76332; NFC

Author: rotateright

llvm/test/Transforms/JumpThreading/select.ll

@@ -441,3 +441,31 @@ sw.default:                                       ; preds = %if.end, %sw.bb9
 ; CHECK: i32 2, label [[DEST1]]
 ; CHECK: i32 4, label [[DEST2]]
 }
+
+; FIXME: This is an invalid transform. If %b is false and %x is poison,
+; then the select produces poison (the result of the program is poison).
+; But with this transform, we may be branching on poison, and that is UB.
+
+define i32 @TryToUnfoldSelectInCurrBB(i1 %b, i1 %ui, i32 %s, i1 %x) {
+; CHECK-LABEL: @TryToUnfoldSelectInCurrBB(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[B:%.*]], label [[IF_END_THREAD:%.*]], label [[IF_END:%.*]]
+; CHECK:       if.end:
+; CHECK-NEXT:    br i1 [[X:%.*]], label [[TMP0:%.*]], label [[IF_END_THREAD]]
+; CHECK:       0:
+; CHECK-NEXT:    br label [[IF_END_THREAD]]
+; CHECK:       if.end.thread:
+; CHECK-NEXT:    [[TMP1:%.*]] = phi i32 [ [[S:%.*]], [[TMP0]] ], [ 42, [[IF_END]] ], [ 42, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret i32 [[TMP1]]
+;
+entry:
+  br i1 %b, label %if.end, label %if.else
+
+if.else:
+  br label %if.end
+
+if.end:
+  %v = phi i1 [ %x, %if.else ], [ false, %entry ]
+  %v1 = select i1 %v, i32 %s, i32 42
+  ret i32 %v1
+}