[llvm] [SCCP] Simplify [us]cmp(X, Y) into X - Y (PR #144717)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-function-specialization

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

If the difference between [us]cmp's operands is not greater than 1, we can simplify it into `X - Y`.
Alive2: https://alive2.llvm.org/ce/z/JS55so
llvm-opt-benchmark diff: https://github.com/dtcxzyw/llvm-opt-benchmark/pull/2464/files


---
Full diff: https://github.com/llvm/llvm-project/pull/144717.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Utils/SCCPSolver.cpp (+36-1) 
- (added) llvm/test/Transforms/SCCP/uscmp.ll (+78) 


``````````diff
diff --git a/llvm/lib/Transforms/Utils/SCCPSolver.cpp b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
index f4b378b82daec..0e3f0647d6284 100644
--- a/llvm/lib/Transforms/Utils/SCCPSolver.cpp
+++ b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
@@ -245,11 +245,46 @@ static Value *simplifyInstruction(SCCPSolver &Solver,
   const APInt *RHSC;
   // Remove masking operations.
   if (match(&Inst, m_And(m_Value(X), m_LowBitMask(RHSC)))) {
-    ConstantRange LRange = GetRange(Inst.getOperand(0));
+    ConstantRange LRange = GetRange(X);
     if (LRange.getUnsignedMax().ule(*RHSC))
       return X;
   }
 
+  if (auto *II = dyn_cast<IntrinsicInst>(&Inst)) {
+    Intrinsic::ID IID = II->getIntrinsicID();
+    // Check if we can simplify [us]cmp(X, Y) to X - Y.
+    if (IID == Intrinsic::scmp || IID == Intrinsic::ucmp) {
+      Value *LHS = II->getOperand(0);
+      Value *RHS = II->getOperand(1);
+      unsigned BitWidth = LHS->getType()->getScalarSizeInBits();
+      ConstantRange LRange = GetRange(LHS);
+      if (LRange.isSizeLargerThan(3))
+        return nullptr;
+      ConstantRange RRange = GetRange(RHS);
+      if (RRange.isSizeLargerThan(3))
+        return nullptr;
+      ConstantRange RHSLower = RRange.sub(APInt(BitWidth, 1));
+      ConstantRange RHSUpper = RRange.add(APInt(BitWidth, 1));
+      ICmpInst::Predicate Pred =
+          IID == Intrinsic::scmp ? CmpInst::ICMP_SLE : CmpInst::ICMP_ULE;
+      if (!RHSLower.icmp(Pred, LRange) || !LRange.icmp(Pred, RHSUpper))
+        return nullptr;
+      Instruction *Sub = BinaryOperator::CreateSub(LHS, RHS, Inst.getName(),
+                                                   Inst.getIterator());
+      if (IID == Intrinsic::scmp)
+        Sub->setHasNoSignedWrap(true);
+      Sub->setDebugLoc(Inst.getDebugLoc());
+      InsertedValues.insert(Sub);
+      if (Sub->getType() != Inst.getType()) {
+        Sub = CastInst::CreateIntegerCast(Sub, Inst.getType(), true, "",
+                                          Inst.getIterator());
+        Sub->setDebugLoc(Inst.getDebugLoc());
+        InsertedValues.insert(Sub);
+      }
+      return Sub;
+    }
+  }
+
   return nullptr;
 }
 
diff --git a/llvm/test/Transforms/SCCP/uscmp.ll b/llvm/test/Transforms/SCCP/uscmp.ll
new file mode 100644
index 0000000000000..ea5a65556ac55
--- /dev/null
+++ b/llvm/test/Transforms/SCCP/uscmp.ll
@@ -0,0 +1,78 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -passes=sccp -S < %s | FileCheck %s
+
+define i32 @scmp_to_sub(i32 range(i32 -1, 2) %a) {
+; CHECK-LABEL: define i32 @scmp_to_sub(
+; CHECK-SAME: i32 range(i32 -1, 2) [[A:%.*]]) {
+; CHECK-NEXT:    [[SCMP:%.*]] = sub nsw i32 [[A]], 0
+; CHECK-NEXT:    ret i32 [[SCMP]]
+;
+  %scmp = call i32 @llvm.scmp(i32 %a, i32 0)
+  ret i32 %scmp
+}
+
+define i32 @scmp_zext_to_sub(i1 %a, i1 %b) {
+; CHECK-LABEL: define i32 @scmp_zext_to_sub(
+; CHECK-SAME: i1 [[A:%.*]], i1 [[B:%.*]]) {
+; CHECK-NEXT:    [[ZEXT_A:%.*]] = zext i1 [[A]] to i32
+; CHECK-NEXT:    [[ZEXT_B:%.*]] = zext i1 [[B]] to i32
+; CHECK-NEXT:    [[SCMP:%.*]] = sub nsw i32 [[ZEXT_A]], [[ZEXT_B]]
+; CHECK-NEXT:    ret i32 [[SCMP]]
+;
+  %zext_a = zext i1 %a to i32
+  %zext_b = zext i1 %b to i32
+  %scmp = call i32 @llvm.scmp(i32 %zext_a, i32 %zext_b)
+  ret i32 %scmp
+}
+
+define i8 @scmp_to_sub_trunc(i32 range(i32 -1, 2) %a) {
+; CHECK-LABEL: define i8 @scmp_to_sub_trunc(
+; CHECK-SAME: i32 range(i32 -1, 2) [[A:%.*]]) {
+; CHECK-NEXT:    [[SCMP1:%.*]] = sub nsw i32 [[A]], 0
+; CHECK-NEXT:    [[SCMP:%.*]] = trunc i32 [[SCMP1]] to i8
+; CHECK-NEXT:    ret i8 [[SCMP]]
+;
+  %scmp = call i8 @llvm.scmp(i32 %a, i32 0)
+  ret i8 %scmp
+}
+
+define i64 @scmp_to_sub_sext(i32 range(i32 -1, 2) %a) {
+; CHECK-LABEL: define i64 @scmp_to_sub_sext(
+; CHECK-SAME: i32 range(i32 -1, 2) [[A:%.*]]) {
+; CHECK-NEXT:    [[SCMP1:%.*]] = sub nsw i32 [[A]], 0
+; CHECK-NEXT:    [[SCMP:%.*]] = sext i32 [[SCMP1]] to i64
+; CHECK-NEXT:    ret i64 [[SCMP]]
+;
+  %scmp = call i64 @llvm.scmp(i32 %a, i32 0)
+  ret i64 %scmp
+}
+
+define i32 @scmp_to_sub_small_range(i32 range(i32 -1, 1) %a) {
+; CHECK-LABEL: define i32 @scmp_to_sub_small_range(
+; CHECK-SAME: i32 range(i32 -1, 1) [[A:%.*]]) {
+; CHECK-NEXT:    [[SCMP:%.*]] = sub nsw i32 [[A]], 0
+; CHECK-NEXT:    ret i32 [[SCMP]]
+;
+  %scmp = call i32 @llvm.scmp(i32 %a, i32 0)
+  ret i32 %scmp
+}
+
+define i32 @ucmp_to_sub(i32 range(i32 0, 3) %a) {
+; CHECK-LABEL: define i32 @ucmp_to_sub(
+; CHECK-SAME: i32 range(i32 0, 3) [[A:%.*]]) {
+; CHECK-NEXT:    [[SCMP:%.*]] = sub nsw i32 [[A]], 1
+; CHECK-NEXT:    ret i32 [[SCMP]]
+;
+  %scmp = call i32 @llvm.scmp(i32 %a, i32 1)
+  ret i32 %scmp
+}
+
+define i32 @scmp_to_sub_large_range(i32 range(i32 -1, 3) %a) {
+; CHECK-LABEL: define i32 @scmp_to_sub_large_range(
+; CHECK-SAME: i32 range(i32 -1, 3) [[A:%.*]]) {
+; CHECK-NEXT:    [[SCMP:%.*]] = call i32 @llvm.scmp.i32.i32(i32 [[A]], i32 0)
+; CHECK-NEXT:    ret i32 [[SCMP]]
+;
+  %scmp = call i32 @llvm.scmp(i32 %a, i32 0)
+  ret i32 %scmp
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/144717