[InstCombine] Fold usub_sat((sub nuw C1, A), C2) to usub_sat(C1 - C2, A) or 0

Author: elhewaty

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -2139,6 +2139,23 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
       }
     }
 
+    // usub_sat((sub nuw C1, A), C2) -> usub_sat(C1 - C2, A) if C2 u< C1
+    // usub_sat((sub nuw C1, A), C2) -> 0 otherwise
+    const APInt *C1, *C2;
+    Value *A;
+    if (IID == Intrinsic::usub_sat &&
+        match(Arg0, m_OneUse(m_NUWSub(m_APInt(C1), m_Value(A)))) &&
+        match(Arg1, m_APInt(C2)) && SI->hasOneUse()) {
+
+      if (C2->ult(*C1)) {
+        auto *New = Builder.CreateBinaryIntrinsic(
+            Intrinsic::usub_sat, ConstantInt::get(A->getType(), *C1 - *C2), A);
+        return replaceInstUsesWith(*SI, New);
+      } else {
+        return replaceInstUsesWith(*SI, ConstantInt::get(Ty, 0));
+      }
+    }
+
     // ssub.sat(X, C) -> sadd.sat(X, -C) if C != MIN
     Constant *C;
     if (IID == Intrinsic::ssub_sat && match(Arg1, m_Constant(C)) &&

llvm/test/Transforms/InstCombine/unsigned_saturated_sub.ll

@@ -14,8 +14,7 @@ declare i16 @llvm.usub.sat.i16(i16, i16)
 
 define i32 @usub_sat_C1_C2(i32 %a){
 ; CHECK-LABEL: @usub_sat_C1_C2(
-; CHECK-NEXT:    [[ADD:%.*]] = sub nuw i32 64, [[A:%.*]]
-; CHECK-NEXT:    [[COND:%.*]] = call i32 @llvm.usub.sat.i32(i32 [[ADD]], i32 14)
+; CHECK-NEXT:    [[COND:%.*]] = call i32 @llvm.usub.sat.i32(i32 50, i32 [[A:%.*]])
 ; CHECK-NEXT:    ret i32 [[COND]]
 ;
   %add = sub nuw i32 64, %a
@@ -25,9 +24,7 @@ define i32 @usub_sat_C1_C2(i32 %a){
 
 define i32 @usub_sat_C1_C2_produce_0(i32 %a){
 ; CHECK-LABEL: @usub_sat_C1_C2_produce_0(
-; CHECK-NEXT:    [[ADD:%.*]] = sub nuw i32 14, [[A:%.*]]
-; CHECK-NEXT:    [[COND:%.*]] = call i32 @llvm.usub.sat.i32(i32 [[ADD]], i32 14)
-; CHECK-NEXT:    ret i32 [[COND]]
+; CHECK-NEXT:    ret i32 0
 ;
   %add = sub nuw i32 14, %a
   %cond = call i32 @llvm.usub.sat.i32(i32 %add, i32 14)
@@ -36,9 +33,7 @@ define i32 @usub_sat_C1_C2_produce_0(i32 %a){
 
 define i32 @usub_sat_C1_C2_produce_0_too(i32 %a){
 ; CHECK-LABEL: @usub_sat_C1_C2_produce_0_too(
-; CHECK-NEXT:    [[ADD:%.*]] = sub nuw i32 12, [[A:%.*]]
-; CHECK-NEXT:    [[COND:%.*]] = call i32 @llvm.usub.sat.i32(i32 [[ADD]], i32 14)
-; CHECK-NEXT:    ret i32 [[COND]]
+; CHECK-NEXT:    ret i32 0
 ;
   %add = sub nuw i32 12, %a
   %cond = call i32 @llvm.usub.sat.i32(i32 %add, i32 14)