[llvm] [InstCombine] Fold usub_sat((sub nuw C1, A), C2) to usub_sat(C1 - C2, A) or 0 (PR #82280)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: None (elhewaty)

<details>
<summary>Changes</summary>

- [InstCombine] Add test coverage for folding usub_sat((sub nuw C1, A), C2) to usub_sat(C1 - C2, A) or 0(NFC)
- [InstCombine] Fold usub_sat((sub nuw C1, A), C2) to usub_sat(C1 - C2, A) or 0
- Fixes: https://github.com/llvm/llvm-project/issues/82177
- Alive2: https://alive2.llvm.org/ce/z/4w-77P


---
Full diff: https://github.com/llvm/llvm-project/pull/82280.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp (+17) 
- (modified) llvm/test/Transforms/InstCombine/unsigned_saturated_sub.ll (+71) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 5266808c5abab4..7f59ad1d61d91e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/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)) &&
diff --git a/llvm/test/Transforms/InstCombine/unsigned_saturated_sub.ll b/llvm/test/Transforms/InstCombine/unsigned_saturated_sub.ll
index 5cece931b8d987..4d60a7cc5a54bf 100644
--- a/llvm/test/Transforms/InstCombine/unsigned_saturated_sub.ll
+++ b/llvm/test/Transforms/InstCombine/unsigned_saturated_sub.ll
@@ -7,6 +7,77 @@
 declare void @use(i64)
 declare void @usei32(i32)
 declare void @usei1(i1)
+declare i32 @llvm.usub.sat.i32(i32, i32)
+declare i16 @llvm.usub.sat.i16(i16, i16)
+
+; usub_sat((sub nuw C1, A), C2) to usub_sat(C1 - C2, A) or 0
+
+define i32 @usub_sat_C1_C2(i32 %a){
+; CHECK-LABEL: @usub_sat_C1_C2(
+; CHECK-NEXT:    [[COND:%.*]] = call i32 @llvm.usub.sat.i32(i32 50, i32 [[A:%.*]])
+; CHECK-NEXT:    ret i32 [[COND]]
+;
+  %add = sub nuw i32 64, %a
+  %cond = call i32 @llvm.usub.sat.i32(i32 %add, i32 14)
+  ret i32 %cond
+}
+
+define i32 @usub_sat_C1_C2_produce_0(i32 %a){
+; CHECK-LABEL: @usub_sat_C1_C2_produce_0(
+; CHECK-NEXT:    ret i32 0
+;
+  %add = sub nuw i32 14, %a
+  %cond = call i32 @llvm.usub.sat.i32(i32 %add, i32 14)
+  ret i32 %cond
+}
+
+define i32 @usub_sat_C1_C2_produce_0_too(i32 %a){
+; CHECK-LABEL: @usub_sat_C1_C2_produce_0_too(
+; CHECK-NEXT:    ret i32 0
+;
+  %add = sub nuw i32 12, %a
+  %cond = call i32 @llvm.usub.sat.i32(i32 %add, i32 14)
+  ret i32 %cond
+}
+
+; negative tests this souldn't work
+
+define i32 @usub_sat_C1_C2_without_nuw(i32 %a){
+; CHECK-LABEL: @usub_sat_C1_C2_without_nuw(
+; CHECK-NEXT:    [[ADD:%.*]] = sub i32 12, [[A:%.*]]
+; CHECK-NEXT:    [[COND:%.*]] = call i32 @llvm.usub.sat.i32(i32 [[ADD]], i32 14)
+; CHECK-NEXT:    ret i32 [[COND]]
+;
+  %add = sub i32 12, %a
+  %cond = call i32 @llvm.usub.sat.i32(i32 %add, i32 14)
+  ret i32 %cond
+}
+
+define i32 @usub_sat_C1_C2_more_than_one_use_with_add(i32 %a){
+; CHECK-LABEL: @usub_sat_C1_C2_more_than_one_use_with_add(
+; CHECK-NEXT:    [[ADD:%.*]] = sub nuw i32 12, [[A:%.*]]
+; CHECK-NEXT:    [[COND:%.*]] = call i32 @llvm.usub.sat.i32(i32 [[ADD]], i32 14)
+; CHECK-NEXT:    call void @usei32(i32 [[ADD]])
+; CHECK-NEXT:    ret i32 [[COND]]
+;
+  %add = sub nuw i32 12, %a
+  %cond = call i32 @llvm.usub.sat.i32(i32 %add, i32 14)
+  call void @usei32(i32 %add)
+  ret i32 %cond
+}
+
+define i32 @usub_sat_C1_C2_more_than_one_use_with_cond(i32 %a){
+; CHECK-LABEL: @usub_sat_C1_C2_more_than_one_use_with_cond(
+; CHECK-NEXT:    [[ADD:%.*]] = sub nuw i32 12, [[A:%.*]]
+; CHECK-NEXT:    [[COND:%.*]] = call i32 @llvm.usub.sat.i32(i32 [[ADD]], i32 14)
+; CHECK-NEXT:    call void @usei32(i32 [[COND]])
+; CHECK-NEXT:    ret i32 [[COND]]
+;
+  %add = sub nuw i32 12, %a
+  %cond = call i32 @llvm.usub.sat.i32(i32 %add, i32 14)
+  call void @usei32(i32 %cond)
+  ret i32 %cond
+}
 
 ; (a > b) ? a - b : 0 -> usub.sat(a, b)
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/82280