[llvm] [InstCombine] Fold icmp eq/ne (X and C), 0 when partial bits are known (PR #85863)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

Fixes https://github.com/llvm/llvm-project/issues/85823.
Alive2: https://alive2.llvm.org/ce/z/QUvHVj

---
Full diff: https://github.com/llvm/llvm-project/pull/85863.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp (+6-3) 
- (modified) llvm/test/Transforms/InstCombine/icmp-dom.ll (+85) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 0dce0077bf1588..7cb5e98dea2581 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1771,11 +1771,14 @@ Instruction *InstCombinerImpl::foldICmpAndConstConst(ICmpInst &Cmp,
       return new ICmpInst(NewPred, X, Zero);
     }
 
-    APInt NewC2 = *C2;
     KnownBits Know = computeKnownBits(And->getOperand(0), 0, And);
+    if (Know.One.intersects(*C2))
+      return replaceInstUsesWith(
+          Cmp, ConstantInt::getBool(Cmp.getType(), isICMP_NE));
+
     // Set high zeros of C2 to allow matching negated power-of-2.
-    NewC2 = *C2 | APInt::getHighBitsSet(C2->getBitWidth(),
-                                        Know.countMinLeadingZeros());
+    APInt NewC2 = *C2 | APInt::getHighBitsSet(C2->getBitWidth(),
+                                              Know.countMinLeadingZeros());
 
     // Restrict this fold only for single-use 'and' (PR10267).
     // ((%x & C) == 0) --> %x u< (-C)  iff (-C) is power of two.
diff --git a/llvm/test/Transforms/InstCombine/icmp-dom.ll b/llvm/test/Transforms/InstCombine/icmp-dom.ll
index f4b9022d14349b..d97a2e707ae235 100644
--- a/llvm/test/Transforms/InstCombine/icmp-dom.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-dom.ll
@@ -403,3 +403,88 @@ truelabel:
 falselabel:
   ret i8 0
 }
+
+define i1 @and_mask1_eq(i32 %conv) {
+; CHECK-LABEL: @and_mask1_eq(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[AND:%.*]] = and i32 [[CONV:%.*]], 1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[AND]], 0
+; CHECK-NEXT:    br i1 [[CMP]], label [[THEN:%.*]], label [[ELSE:%.*]]
+; CHECK:       then:
+; CHECK-NEXT:    ret i1 false
+; CHECK:       else:
+; CHECK-NEXT:    call void @dummy()
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  %and = and i32 %conv, 1
+  %cmp = icmp eq i32 %and, 0
+  br i1 %cmp, label %then, label %else
+
+then:
+  ret i1 0
+
+else:
+  call void @dummy()
+  %and1 = and i32 %conv, 3
+  %cmp1 = icmp eq i32 %and1, 0
+  ret i1 %cmp1
+}
+
+define i1 @and_mask1_ne(i32 %conv) {
+; CHECK-LABEL: @and_mask1_ne(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[AND:%.*]] = and i32 [[CONV:%.*]], 1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[AND]], 0
+; CHECK-NEXT:    br i1 [[CMP]], label [[THEN:%.*]], label [[ELSE:%.*]]
+; CHECK:       then:
+; CHECK-NEXT:    ret i1 false
+; CHECK:       else:
+; CHECK-NEXT:    call void @dummy()
+; CHECK-NEXT:    ret i1 true
+;
+entry:
+  %and = and i32 %conv, 1
+  %cmp = icmp eq i32 %and, 0
+  br i1 %cmp, label %then, label %else
+
+then:
+  ret i1 0
+
+else:
+  call void @dummy()
+  %and1 = and i32 %conv, 3
+  %cmp1 = icmp ne i32 %and1, 0
+  ret i1 %cmp1
+}
+
+define i1 @and_mask2(i32 %conv) {
+; CHECK-LABEL: @and_mask2(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[AND:%.*]] = and i32 [[CONV:%.*]], 4
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[AND]], 0
+; CHECK-NEXT:    br i1 [[CMP]], label [[THEN:%.*]], label [[ELSE:%.*]]
+; CHECK:       then:
+; CHECK-NEXT:    ret i1 false
+; CHECK:       else:
+; CHECK-NEXT:    call void @dummy()
+; CHECK-NEXT:    [[AND1:%.*]] = and i32 [[CONV]], 3
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp eq i32 [[AND1]], 0
+; CHECK-NEXT:    ret i1 [[CMP1]]
+;
+entry:
+  %and = and i32 %conv, 4
+  %cmp = icmp eq i32 %and, 0
+  br i1 %cmp, label %then, label %else
+
+then:
+  ret i1 0
+
+else:
+  call void @dummy()
+  %and1 = and i32 %conv, 3
+  %cmp1 = icmp eq i32 %and1, 0
+  ret i1 %cmp1
+}
+
+declare void @dummy()

``````````

</details>


https://github.com/llvm/llvm-project/pull/85863