[InstCombine] Fold zext(X) + C2 u< C -> X + trunc(C2) u< trunc(C)

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -3165,6 +3165,26 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,
                         Builder.CreateAdd(X, ConstantInt::get(Ty, *C2 - C - 1)),
                         ConstantInt::get(Ty, ~C));
 
+  // zext(V) + C2 <u C -> V + trunc(C2) <u trunc(C) iff C2 s<0 && C s>0
+  Value *V;
+  if (Pred == ICmpInst::ICMP_ULT && match(X, m_ZExt(m_Value(V)))) {
+    Type *NewCmpTy = V->getType();
+    unsigned NewCmpBW = NewCmpTy->getScalarSizeInBits();
+    if (shouldChangeType(Ty, NewCmpTy) &&
+        C2->getSignificantBits() <= NewCmpBW &&
+        C.getSignificantBits() <= NewCmpBW) {
+      APInt TruncatedOffset = C2->trunc(NewCmpBW);
+      APInt TruncatedRHS = C.trunc(NewCmpBW);
+      if (TruncatedOffset.isNegative() && TruncatedRHS.isNonNegative()) {
+        Value *TruncatedOffsetV = ConstantInt::get(NewCmpTy, TruncatedOffset);
+        Value *TruncatedRV = ConstantInt::get(NewCmpTy, TruncatedRHS);
+        return new ICmpInst(ICmpInst::ICMP_ULT,
+                            Builder.CreateAdd(V, TruncatedOffsetV),
+                            TruncatedRV);
+      }
+    }
+  }
+
   return nullptr;
 }
 

llvm/test/Transforms/InstCombine/icmp-add.ll

@@ -3187,9 +3187,8 @@ define i1 @icmp_of_ucmp_plus_const_with_const(i32 %x, i32 %y) {
 define i1 @zext_range_check_ult(i8 %x) {
 ; CHECK-LABEL: @zext_range_check_ult(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CONV:%.*]] = zext i8 [[X:%.*]] to i32
-; CHECK-NEXT:    [[ADD:%.*]] = add nsw i32 [[CONV]], -4
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i32 [[ADD]], 3
+; CHECK-NEXT:    [[TMP0:%.*]] = add i8 [[X:%.*]], -4
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i8 [[TMP0]], 3
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
 entry:
@@ -3233,11 +3232,7 @@ entry:
 
 define i1 @zext_range_check_mergable(i8 %x) {
 ; CHECK-LABEL: @zext_range_check_mergable(
-; CHECK-NEXT:    [[CONV:%.*]] = zext i8 [[X:%.*]] to i32
-; CHECK-NEXT:    [[ADD:%.*]] = add nsw i32 [[CONV]], -4
-; CHECK-NEXT:    [[CMP1:%.*]] = icmp ult i32 [[ADD]], 3
-; CHECK-NEXT:    [[CMP2:%.*]] = icmp slt i8 [[X]], 4
-; CHECK-NEXT:    [[COND:%.*]] = select i1 [[CMP2]], i1 true, i1 [[CMP1]]
+; CHECK-NEXT:    [[COND:%.*]] = icmp slt i8 [[X:%.*]], 7
 ; CHECK-NEXT:    ret i1 [[COND]]
 ;
   %conv = zext i8 %x to i32