[llvm] [SCEV] Better preserve wrapping info in SimplifyICmpOperands for UGE. (PR #144404)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: Florian Hahn (fhahn)

<details>
<summary>Changes</summary>

Update SimplifyICmpOperands to only try subtracting 1 from RHS first, if RHS is an op we can fold the subtract directly into. Otherwise try adding to LHS first, as we can preserve NUW flags.

This improves results in a few cases, including the modified test case from berkeley-abc and new code to be added in
https://github.com/llvm/llvm-project/pull/128061.

Note that there are more cases where the results can be improved by better ordering here which I'll try to investigate as follow-up.

---
Full diff: https://github.com/llvm/llvm-project/pull/144404.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/ScalarEvolution.cpp (+9-1) 
- (modified) llvm/test/Transforms/IndVarSimplify/simplify-icmp-operands-order.ll (+3-6) 


``````````diff
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 2dfe625eb0dcc..ea7c7ece5632f 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -10892,7 +10892,11 @@ bool ScalarEvolution::SimplifyICmpOperands(CmpPredicate &Pred, const SCEV *&LHS,
     }
     break;
   case ICmpInst::ICMP_UGE:
-    if (!getUnsignedRangeMin(RHS).isMinValue()) {
+    // If RHS is an op we can fold the -1, try that first.
+    if ((isa<SCEVConstant>(RHS) ||
+         match(RHS,
+               m_scev_AffineAddRec(m_SCEVConstant(), m_SCEV(), m_Loop()))) &&
+        !getUnsignedRangeMin(RHS).isMinValue()) {
       RHS = getAddExpr(getConstant(RHS->getType(), (uint64_t)-1, true), RHS);
       Pred = ICmpInst::ICMP_UGT;
       Changed = true;
@@ -10901,6 +10905,10 @@ bool ScalarEvolution::SimplifyICmpOperands(CmpPredicate &Pred, const SCEV *&LHS,
                        SCEV::FlagNUW);
       Pred = ICmpInst::ICMP_UGT;
       Changed = true;
+    } else if (!getUnsignedRangeMin(RHS).isMinValue()) {
+      RHS = getAddExpr(getConstant(RHS->getType(), (uint64_t)-1, true), RHS);
+      Pred = ICmpInst::ICMP_UGT;
+      Changed = true;
     }
     break;
   default:
diff --git a/llvm/test/Transforms/IndVarSimplify/simplify-icmp-operands-order.ll b/llvm/test/Transforms/IndVarSimplify/simplify-icmp-operands-order.ll
index b0dbbd5eaedf4..fb2fdb116f904 100644
--- a/llvm/test/Transforms/IndVarSimplify/simplify-icmp-operands-order.ll
+++ b/llvm/test/Transforms/IndVarSimplify/simplify-icmp-operands-order.ll
@@ -53,15 +53,12 @@ loop.latch:
 
 define void @test_simplifycompare_rhs_not_constant1() {
 ; CHECK-LABEL: define void @test_simplifycompare_rhs_not_constant1() {
-; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:  [[ENTRY:.*:]]
 ; CHECK-NEXT:    [[P:%.*]] = alloca i64, align 8
 ; CHECK-NEXT:    br label %[[LOOP:.*]]
 ; CHECK:       [[LOOP]]:
-; CHECK-NEXT:    [[PTR_IV:%.*]] = phi ptr [ [[P]], %[[ENTRY]] ], [ [[PTR_IV_NEXT:%.*]], %[[LOOP]] ]
-; CHECK-NEXT:    [[PTR_IV_NEXT]] = getelementptr i8, ptr [[PTR_IV]], i64 -8
-; CHECK-NEXT:    call void @use(ptr [[PTR_IV]])
-; CHECK-NEXT:    [[EC:%.*]] = icmp ult ptr [[PTR_IV_NEXT]], [[P]]
-; CHECK-NEXT:    br i1 [[EC]], label %[[EXIT:.*]], label %[[LOOP]]
+; CHECK-NEXT:    call void @use(ptr [[P]])
+; CHECK-NEXT:    br i1 true, label %[[EXIT:.*]], label %[[LOOP]]
 ; CHECK:       [[EXIT]]:
 ; CHECK-NEXT:    ret void
 ;

``````````

</details>


https://github.com/llvm/llvm-project/pull/144404