[llvm] [ConstraintElim] Derive signed facts for post-increment inductions. (PR #210079)
Florian Hahn via llvm-commits
llvm-commits at lists.llvm.org
Thu Jul 16 08:59:27 PDT 2026
https://github.com/fhahn updated https://github.com/llvm/llvm-project/pull/210079
>From 4d7508be7df78a32ccb34e2744f7cacaca04e795 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Thu, 16 Jul 2026 10:52:19 +0100
Subject: [PATCH 1/2] Precommit test
---
...nduction-condition-in-loop-exit-postinc.ll | 54 ++++++++++++++++++-
1 file changed, 52 insertions(+), 2 deletions(-)
diff --git a/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit-postinc.ll b/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit-postinc.ll
index 103fb73e4d37a..7054be680f66b 100644
--- a/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit-postinc.ll
+++ b/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit-postinc.ll
@@ -283,8 +283,8 @@ exit:
}
; Post-increment condition with signed predicates in body.
-define i1 @postinc_negative_start_signed_body_not_folded(i1 %c) {
-; CHECK-LABEL: define i1 @postinc_negative_start_signed_body_not_folded(
+define i1 @postinc_negative_start_signed_body_folded(i1 %c) {
+; CHECK-LABEL: define i1 @postinc_negative_start_signed_body_folded(
; CHECK-SAME: i1 [[C:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: br label %[[LOOP_HEADER:.*]]
@@ -576,3 +576,53 @@ loop.latch:
exit:
ret void
}
+
+; Similar to @postinc_signed_facts_when_unsigned_overflows: here the signed sum
+; StartValue + Step (127 + 1) overflows signed but not unsigned.
+define void @postinc_unsigned_facts_when_signed_overflows(ptr %p) {
+; CHECK-LABEL: define void @postinc_unsigned_facts_when_signed_overflows(
+; CHECK-SAME: ptr [[P:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*]]:
+; CHECK-NEXT: br label %[[LOOP_HEADER:.*]]
+; CHECK: [[LOOP_HEADER]]:
+; CHECK-NEXT: [[IV:%.*]] = phi i8 [ 127, %[[ENTRY]] ], [ [[IV_NEXT:%.*]], %[[LOOP_LATCH:.*]] ]
+; CHECK-NEXT: [[IV_NEXT]] = add i8 [[IV]], 1
+; CHECK-NEXT: [[DONE:%.*]] = icmp eq i8 [[IV_NEXT]], -100
+; CHECK-NEXT: br i1 [[DONE]], label %[[EXIT:.*]], label %[[LOOP_LATCH]]
+; CHECK: [[LOOP_LATCH]]:
+; CHECK-NEXT: [[SLB:%.*]] = icmp slt i8 [[IV]], 127
+; CHECK-NEXT: [[SUB:%.*]] = icmp slt i8 [[IV]], -100
+; CHECK-NEXT: [[Z0:%.*]] = zext i1 [[SLB]] to i8
+; CHECK-NEXT: [[Z1:%.*]] = zext i1 [[SUB]] to i8
+; CHECK-NEXT: [[Z2:%.*]] = zext i1 true to i8
+; CHECK-NEXT: store i8 [[Z0]], ptr [[P]], align 1
+; CHECK-NEXT: store i8 [[Z1]], ptr [[P]], align 1
+; CHECK-NEXT: store i8 [[Z2]], ptr [[P]], align 1
+; CHECK-NEXT: br label %[[LOOP_HEADER]]
+; CHECK: [[EXIT]]:
+; CHECK-NEXT: ret void
+;
+entry:
+ br label %loop.header
+
+loop.header:
+ %iv = phi i8 [ 127, %entry ], [ %iv.next, %loop.latch ]
+ %iv.next = add i8 %iv, 1
+ %done = icmp eq i8 %iv.next, -100
+ br i1 %done, label %exit, label %loop.latch
+
+loop.latch:
+ %slb = icmp slt i8 %iv, 127
+ %sub = icmp slt i8 %iv, -100
+ %uns = icmp ult i8 %iv, -100
+ %z0 = zext i1 %slb to i8
+ %z1 = zext i1 %sub to i8
+ %z2 = zext i1 %uns to i8
+ store i8 %z0, ptr %p, align 1
+ store i8 %z1, ptr %p, align 1
+ store i8 %z2, ptr %p, align 1
+ br label %loop.header
+
+exit:
+ ret void
+}
>From 6a3033b5da2e6855c5e7de4c1adf7ea454e9b7a5 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Thu, 9 Jul 2026 17:52:15 +0100
Subject: [PATCH 2/2] [ConstraintElim] Derive signed facts for post-increment
inductions.
Extend logic added in https://github.com/llvm/llvm-project/pull/209199
to also derive signed facts.
To do so, we keep track of whether LowerBound = Start + Step overflows
signed/unsigned. Code that previously bailed out if we were handling a
post-increment induction has been updated to use the computed
LowerBound, if it does not signed-wrap.
Alive2 Proof: https://alive2.llvm.org/ce/z/frskVt
---
.../Scalar/ConstraintElimination.cpp | 32 +++++++++++--------
...nduction-condition-in-loop-exit-postinc.ll | 6 ++--
.../monotonic-int-phis-signed.ll | 3 +-
3 files changed, 21 insertions(+), 20 deletions(-)
diff --git a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
index f1f800cd69d60..676006a9e6201 100644
--- a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
@@ -1072,38 +1072,42 @@ void State::addInfoForInductions(BasicBlock &BB) {
}
Value *LowerBound = StartValue;
+ bool UnsignedInBounds = true, SignedInBounds = true;
if (IncStep) {
- // Adjust lower bound when dealing with a post-increment value.
auto *StartC = dyn_cast<ConstantInt>(StartValue);
if (!StartC)
return;
- bool Overflow = false;
- APInt Sum = StartC->getValue().uadd_ov(*StepOffset, Overflow);
- if (Overflow)
- return;
+ bool UOverflow = false, SOverflow = false;
+ APInt Sum = StartC->getValue().uadd_ov(*StepOffset, UOverflow);
+ (void)StartC->getValue().sadd_ov(*StepOffset, SOverflow);
LowerBound = ConstantInt::get(StartValue->getType(), Sum);
+ UnsignedInBounds = !UOverflow;
+ SignedInBounds = !SOverflow;
}
- // AR may wrap. Add PN >= StartValue conditional on LowerBound <= B which
+ // AR may wrap. Add PN >= StartValue conditional on LowerBound <= B, which
// guarantees that the loop exits before wrapping in combination with the
// restrictions on B and the step above.
- if (!MonotonicallyIncreasingUnsigned)
+ if (!MonotonicallyIncreasingUnsigned && UnsignedInBounds)
WorkList.push_back(FactOrCheck::getConditionFact(
DTN, CmpInst::ICMP_UGE, PN, StartValue,
ConditionTy(CmpInst::ICMP_ULE, LowerBound, B)));
- // Only unsigned facts are derived for the post-increment path.
- if (!MonotonicallyIncreasingSigned && !IncStep)
+ if (!MonotonicallyIncreasingSigned && SignedInBounds)
WorkList.push_back(FactOrCheck::getConditionFact(
DTN, CmpInst::ICMP_SGE, PN, StartValue,
- ConditionTy(CmpInst::ICMP_SLE, StartValue, B)));
+ ConditionTy(CmpInst::ICMP_SLE, LowerBound, B)));
+
+ if (SignedInBounds)
+ WorkList.push_back(FactOrCheck::getConditionFact(
+ DTN, CmpInst::ICMP_SLT, PN, B,
+ ConditionTy(CmpInst::ICMP_SLE, LowerBound, B)));
+
+ if (!UnsignedInBounds)
+ return;
WorkList.push_back(FactOrCheck::getConditionFact(
DTN, CmpInst::ICMP_ULT, PN, B,
ConditionTy(CmpInst::ICMP_ULE, LowerBound, B)));
- if (!IncStep)
- WorkList.push_back(FactOrCheck::getConditionFact(
- DTN, CmpInst::ICMP_SLT, PN, B,
- ConditionTy(CmpInst::ICMP_SLE, StartValue, B)));
// Try to add condition from header to the dedicated exit blocks. When exiting
// either with EQ or NE in the header, we know that the induction value must
diff --git a/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit-postinc.ll b/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit-postinc.ll
index 7054be680f66b..b1ac763592750 100644
--- a/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit-postinc.ll
+++ b/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit-postinc.ll
@@ -294,10 +294,9 @@ define i1 @postinc_negative_start_signed_body_folded(i1 %c) {
; CHECK-NEXT: [[DONE:%.*]] = icmp eq i64 [[IV_NEXT]], 10
; CHECK-NEXT: br i1 [[DONE]], label %[[EXIT:.*]], label %[[LOOP_LATCH]]
; CHECK: [[LOOP_LATCH]]:
-; CHECK-NEXT: [[RES:%.*]] = icmp slt i64 [[IV]], 10
; CHECK-NEXT: br i1 [[C]], label %[[EXIT_0:.*]], label %[[LOOP_HEADER]]
; CHECK: [[EXIT_0]]:
-; CHECK-NEXT: ret i1 [[RES]]
+; CHECK-NEXT: ret i1 true
; CHECK: [[EXIT]]:
; CHECK-NEXT: ret i1 false
;
@@ -540,10 +539,9 @@ define void @postinc_signed_facts_when_unsigned_overflows(ptr %p, i1 %c) {
; CHECK-NEXT: [[DONE:%.*]] = icmp eq i8 [[IV_NEXT]], 100
; CHECK-NEXT: br i1 [[DONE]], label %[[EXIT:.*]], label %[[LOOP_LATCH]]
; CHECK: [[LOOP_LATCH]]:
-; CHECK-NEXT: [[UB:%.*]] = icmp slt i8 [[IV]], 100
; CHECK-NEXT: [[UNS:%.*]] = icmp ult i8 [[IV]], 100
; CHECK-NEXT: [[Z0:%.*]] = zext i1 false to i8
-; CHECK-NEXT: [[Z1:%.*]] = zext i1 [[UB]] to i8
+; CHECK-NEXT: [[Z1:%.*]] = zext i1 true to i8
; CHECK-NEXT: [[Z2:%.*]] = zext i1 [[UNS]] to i8
; CHECK-NEXT: store i8 [[Z0]], ptr [[P]], align 1
; CHECK-NEXT: store i8 [[Z1]], ptr [[P]], align 1
diff --git a/llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-signed.ll b/llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-signed.ll
index 5aa179a25ccac..a5c0702896efd 100644
--- a/llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-signed.ll
+++ b/llvm/test/Transforms/ConstraintElimination/monotonic-int-phis-signed.ll
@@ -302,8 +302,7 @@ define void @signed_postinc_iv_step_1(i64 %end) {
; CHECK-NEXT: [[CMP_I_NOT:%.*]] = icmp eq i64 [[IV_NEXT]], [[END]]
; CHECK-NEXT: br i1 [[CMP_I_NOT]], label [[EXIT]], label [[LOOP_LATCH]]
; CHECK: loop.latch:
-; CHECK-NEXT: [[CMP2:%.*]] = icmp slt i64 [[IV]], [[END]]
-; CHECK-NEXT: call void @use(i1 [[CMP2]])
+; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: call void @use(i1 true)
; CHECK-NEXT: br label [[LOOP]]
; CHECK: exit:
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