[llvm] [ConstraintElim] Use isKnownNonNegative for signed->unsigned transfer. (PR #72879)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Florian Hahn (fhahn)

<details>
<summary>Changes</summary>

Use isKnownNonNegative for SLT information transfer. This can improve results, in cases where ValueTracking can infer additional non-negative info, e.g. for phi nodes.

This allows simplifying the check from
https://github.com/llvm/llvm-project/issues/63126 by ConstraintElimination. It is also simplifyed by IndVarSimplify now; note the changes in llvm/test/Transforms/PhaseOrdering/loop-access-checks.ll, due to this now being simplified earlier.

---
Full diff: https://github.com/llvm/llvm-project/pull/72879.diff


3 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/ConstraintElimination.cpp (+2-1) 
- (modified) llvm/test/Transforms/ConstraintElimination/transfer-signed-facts-to-unsigned.ll (+3-6) 
- (modified) llvm/test/Transforms/PhaseOrdering/loop-access-checks.ll (+4-4) 


``````````diff
diff --git a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
index 8ec2ee47db1b70c..350b561ca53deaf 100644
--- a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
@@ -804,7 +804,8 @@ void ConstraintInfo::transferToOtherSystem(
     }
     break;
   case CmpInst::ICMP_SLT:
-    if (doesHold(CmpInst::ICMP_SGE, A, ConstantInt::get(B->getType(), 0)))
+    if (doesHold(CmpInst::ICMP_SGE, A, ConstantInt::get(B->getType(), 0)) ||
+        isKnownNonNegative(A, DL, /*Depth=*/MaxAnalysisRecursionDepth - 1))
       addFact(CmpInst::ICMP_ULT, A, B, NumIn, NumOut, DFSInStack);
     break;
   case CmpInst::ICMP_SGT: {
diff --git a/llvm/test/Transforms/ConstraintElimination/transfer-signed-facts-to-unsigned.ll b/llvm/test/Transforms/ConstraintElimination/transfer-signed-facts-to-unsigned.ll
index 41596dbf3e3365e..46cc306466f182c 100644
--- a/llvm/test/Transforms/ConstraintElimination/transfer-signed-facts-to-unsigned.ll
+++ b/llvm/test/Transforms/ConstraintElimination/transfer-signed-facts-to-unsigned.ll
@@ -750,8 +750,7 @@ define i8 @iv_known_non_negative_constant_trip_count(ptr %dst, i8 %N) {
 ; CHECK-NEXT:    call void @use(i1 [[F_1]])
 ; CHECK-NEXT:    [[F_2:%.*]] = icmp sle i8 [[N]], [[IV]]
 ; CHECK-NEXT:    call void @use(i1 [[F_2]])
-; CHECK-NEXT:    [[C_0:%.*]] = icmp ugt i8 [[IV]], 2
-; CHECK-NEXT:    call void @use(i1 [[C_0]])
+; CHECK-NEXT:    call void @use(i1 false)
 ; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i8 [[IV]], 1
 ; CHECK-NEXT:    br label [[LOOP_HEADER]]
 ; CHECK:       exit.1:
@@ -842,11 +841,9 @@ define i8 @iv_known_non_negative_variable_trip_count(ptr %dst, i8 %N) {
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i8 [[IV]], [[N:%.*]]
 ; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP_LATCH]], label [[EXIT_1:%.*]]
 ; CHECK:       loop.latch:
-; CHECK-NEXT:    [[T_1:%.*]] = icmp ugt i8 [[N]], [[IV]]
-; CHECK-NEXT:    call void @use(i1 [[T_1]])
 ; CHECK-NEXT:    call void @use(i1 true)
-; CHECK-NEXT:    [[F_1:%.*]] = icmp ule i8 [[N]], [[IV]]
-; CHECK-NEXT:    call void @use(i1 [[F_1]])
+; CHECK-NEXT:    call void @use(i1 true)
+; CHECK-NEXT:    call void @use(i1 false)
 ; CHECK-NEXT:    call void @use(i1 false)
 ; CHECK-NEXT:    [[C_0:%.*]] = icmp ugt i8 [[IV]], 2
 ; CHECK-NEXT:    call void @use(i1 [[C_0]])
diff --git a/llvm/test/Transforms/PhaseOrdering/loop-access-checks.ll b/llvm/test/Transforms/PhaseOrdering/loop-access-checks.ll
index e1eac5f80485494..dbe29a071092e40 100644
--- a/llvm/test/Transforms/PhaseOrdering/loop-access-checks.ll
+++ b/llvm/test/Transforms/PhaseOrdering/loop-access-checks.ll
@@ -280,18 +280,18 @@ define void @loop_with_signed_induction(ptr noundef nonnull align 8 dereferencea
 ; CHECK-NEXT:    [[SUB_PTR_SUB_I_I:%.*]] = sub i64 [[SUB_PTR_LHS_CAST_I_I]], [[SUB_PTR_RHS_CAST_I_I]]
 ; CHECK-NEXT:    [[SUB_PTR_DIV_I_I:%.*]] = ashr exact i64 [[SUB_PTR_SUB_I_I]], 3
 ; CHECK-NEXT:    [[CMP9:%.*]] = icmp sgt i64 [[SUB_PTR_DIV_I_I]], 0
-; CHECK-NEXT:    br i1 [[CMP9]], label [[OPERATOR_ACC_EXIT:%.*]], label [[FOR_COND_CLEANUP:%.*]]
+; CHECK-NEXT:    br i1 [[CMP9]], label [[FOR_BODY:%.*]], label [[FOR_COND_CLEANUP:%.*]]
 ; CHECK:       for.cond.cleanup:
 ; CHECK-NEXT:    ret void
-; CHECK:       operator_acc.exit:
-; CHECK-NEXT:    [[I_010:%.*]] = phi i64 [ [[INC:%.*]], [[OPERATOR_ACC_EXIT]] ], [ 0, [[ENTRY:%.*]] ]
+; CHECK:       for.body:
+; CHECK-NEXT:    [[I_010:%.*]] = phi i64 [ [[INC:%.*]], [[FOR_BODY]] ], [ 0, [[ENTRY:%.*]] ]
 ; CHECK-NEXT:    [[ADD_PTR_I:%.*]] = getelementptr inbounds double, ptr [[TMP1]], i64 [[I_010]]
 ; CHECK-NEXT:    [[TMP2:%.*]] = load double, ptr [[ADD_PTR_I]], align 8, !tbaa [[TBAA6:![0-9]+]]
 ; CHECK-NEXT:    [[ADD:%.*]] = fadd double [[TMP2]], 1.000000e+00
 ; CHECK-NEXT:    store double [[ADD]], ptr [[ADD_PTR_I]], align 8, !tbaa [[TBAA6]]
 ; CHECK-NEXT:    [[INC]] = add nuw nsw i64 [[I_010]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i64 [[INC]], [[SUB_PTR_DIV_I_I]]
-; CHECK-NEXT:    br i1 [[CMP]], label [[OPERATOR_ACC_EXIT]], label [[FOR_COND_CLEANUP]]
+; CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY]], label [[FOR_COND_CLEANUP]]
 ;
 entry:
   %vec.addr = alloca ptr, align 8

``````````

</details>


https://github.com/llvm/llvm-project/pull/72879