[ValueTracking] Handle trunc nuw in computeKnownBitsFromICmpCond

Author: dtcxzyw

llvm/lib/Analysis/ValueTracking.cpp

@@ -776,7 +776,10 @@ static void computeKnownBitsFromICmpCond(const Value *V, ICmpInst *Cmp,
   if (match(LHS, m_Trunc(m_Specific(V)))) {
     KnownBits DstKnown(LHS->getType()->getScalarSizeInBits());
     computeKnownBitsFromCmp(LHS, Pred, LHS, RHS, DstKnown, SQ);
-    Known = Known.unionWith(DstKnown.anyext(Known.getBitWidth()));
+    if (match(LHS, m_NUWTrunc(m_Value())))
+      Known = Known.unionWith(DstKnown.zext(Known.getBitWidth()));
+    else
+      Known = Known.unionWith(DstKnown.anyext(Known.getBitWidth()));
     return;
   }
 

llvm/test/Transforms/InstCombine/known-bits.ll

@@ -487,8 +487,7 @@ define i64 @test_icmp_trunc_nuw(i64 %a) {
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i32 [[CAST]], 0
 ; CHECK-NEXT:    br i1 [[CMP]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
 ; CHECK:       if.then:
-; CHECK-NEXT:    [[B:%.*]] = and i64 [[A]], 2147483647
-; CHECK-NEXT:    ret i64 [[B]]
+; CHECK-NEXT:    ret i64 [[A]]
 ; CHECK:       if.else:
 ; CHECK-NEXT:    ret i64 0
 ;