[llvm] 8f79754 - [SCEV] Better preserve wrapping info in SimplifyICmpOperands for UGE. (#144404)

[via llvm-commits]

Author: Florian Hahn
Date: 2025-06-17T15:30:08+01:00
New Revision: 8f797542258f6e682eb251d0851922a1ac08fb44

URL: https://github.com/llvm/llvm-project/commit/8f797542258f6e682eb251d0851922a1ac08fb44
DIFF: https://github.com/llvm/llvm-project/commit/8f797542258f6e682eb251d0851922a1ac08fb44.diff

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

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.

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

Added: 
    

Modified: 
    llvm/lib/Analysis/ScalarEvolution.cpp
    llvm/test/Transforms/IndVarSimplify/simplify-icmp-operands-order.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 2dfe625eb0dcc..dd309bc2c54a8 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -10892,7 +10892,12 @@ 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.
+    // Otherwise prefer LHS to preserve the nuw flag.
+    if ((isa<SCEVConstant>(RHS) ||
+         (isa<SCEVAddExpr, SCEVAddRecExpr>(RHS) &&
+          isa<SCEVConstant>(cast<SCEVNAryExpr>(RHS)->getOperand(0)))) &&
+        !getUnsignedRangeMin(RHS).isMinValue()) {
       RHS = getAddExpr(getConstant(RHS->getType(), (uint64_t)-1, true), RHS);
       Pred = ICmpInst::ICMP_UGT;
       Changed = true;
@@ -10901,6 +10906,10 @@ 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
 ;