[InstCombine] weaken assertions for icmp folds (PR35846)

Because of potential UB (known bits conflicts with an llvm.assume),
we have to check rather than assert here because InstSimplify doesn't
kill the compare:
https://bugs.llvm.org/show_bug.cgi?id=35846

llvm-svn: 322104

2 files changed, 21 insertions, 10 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 4fe653b4bd0..6df09dfb3a4 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1893,11 +1893,8 @@ Instruction *InstCombiner::foldICmpShlConstant(ICmpInst &Cmp,
       APInt ShiftedC = C.ashr(*ShiftAmt);
       return new ICmpInst(Pred, X, ConstantInt::get(ShType, ShiftedC));
     }
-    if (Pred == ICmpInst::ICMP_EQ || Pred == ICmpInst::ICMP_NE) {
-      // This is the same code as the SGT case, but assert the pre-condition
-      // that is needed for this to work with equality predicates.
-      assert(C.ashr(*ShiftAmt).shl(*ShiftAmt) == C &&
-             "Compare known true or false was not folded");
+    if ((Pred == ICmpInst::ICMP_EQ || Pred == ICmpInst::ICMP_NE) &&
+        C.ashr(*ShiftAmt).shl(*ShiftAmt) == C) {
       APInt ShiftedC = C.ashr(*ShiftAmt);
       return new ICmpInst(Pred, X, ConstantInt::get(ShType, ShiftedC));
     }
@@ -1926,11 +1923,8 @@ Instruction *InstCombiner::foldICmpShlConstant(ICmpInst &Cmp,
       APInt ShiftedC = C.lshr(*ShiftAmt);
       return new ICmpInst(Pred, X, ConstantInt::get(ShType, ShiftedC));
     }
-    if (Pred == ICmpInst::ICMP_EQ || Pred == ICmpInst::ICMP_NE) {
-      // This is the same code as the UGT case, but assert the pre-condition
-      // that is needed for this to work with equality predicates.
-      assert(C.lshr(*ShiftAmt).shl(*ShiftAmt) == C &&
-             "Compare known true or false was not folded");
+    if ((Pred == ICmpInst::ICMP_EQ || Pred == ICmpInst::ICMP_NE) &&
+        C.lshr(*ShiftAmt).shl(*ShiftAmt) == C) {
       APInt ShiftedC = C.lshr(*ShiftAmt);
       return new ICmpInst(Pred, X, ConstantInt::get(ShType, ShiftedC));
     }
diff --git a/llvm/test/Transforms/InstCombine/assume.ll b/llvm/test/Transforms/InstCombine/assume.ll
index 13fa6339e85..7016dfc577f 100644
--- a/llvm/test/Transforms/InstCombine/assume.ll
+++ b/llvm/test/Transforms/InstCombine/assume.ll
@@ -275,6 +275,23 @@ define i1 @nonnull4(i32** %a) {
   ret i1 %rval
 }
 
+; PR35846 - https://bugs.llvm.org/show_bug.cgi?id=35846
+
+define i32 @assumption_conflicts_with_known_bits(i32 %a, i32 %b) {
+; CHECK-LABEL: @assumption_conflicts_with_known_bits(
+; CHECK-NEXT:    tail call void @llvm.assume(i1 false)
+; CHECK-NEXT:    ret i32 0
+;
+  %and1 = and i32 %b, 3
+  %B1 = lshr i32 %and1, %and1
+  %B3 = shl nuw nsw i32 %and1, %B1
+  %cmp = icmp eq i32 %B3, 1
+  tail call void @llvm.assume(i1 %cmp)
+  %cmp2 = icmp eq i32 %B1, %B3
+  tail call void @llvm.assume(i1 %cmp2)
+  ret i32 %and1
+}
+
 
 attributes #0 = { nounwind uwtable }
 attributes #1 = { nounwind }