[llvm] [ConstraintElim] Check if a single row implies a condition before FM. (PR #228369)
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Fri Oct 2 02:24:02 PDT 2026
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<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
@llvm/pr-subscribers-llvm-analysis
Author: Florian Hahn (fhahn)
<details>
<summary>Changes</summary>
If a single row of the system implies the queried condition, the condition holds without copying the system and running Fourier-Motzkin elimination.
This overall can be slightly cheaper (although changes mostly in noise), this also catches cases where FM gives up due to overflow, either when combining rows with large coefficients or when negating a condition with a constant of INT64_MAX.
In https://github.com/dtcxzyw/llvm-opt-benchmark-nightly/pull/1555, it leads to additional successful queries with end-to-end impact in ~900 files, mostly strengthening flags.
https://llvm-compile-time-tracker.com/compare.php?from=a90d9d26f746d913da12adf3bbb95a6735741e05&to=5f771faeedd4e65f180d288700f00b55ff00e6e3&stat=instructions:u
---
Full diff: https://github.com/llvm/llvm-project/pull/228369.diff
6 Files Affected:
- (modified) llvm/lib/Analysis/ConstraintSystem.cpp (+4)
- (modified) llvm/test/Transforms/ConstraintElimination/add-nsw.ll (+4-6)
- (modified) llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit-latch-counted.ll (+1-1)
- (modified) llvm/test/Transforms/ConstraintElimination/large-constant-ints.ll (+24)
- (modified) llvm/test/Transforms/PhaseOrdering/X86/pr38280.ll (+1-1)
- (modified) llvm/unittests/Analysis/ConstraintSystemTest.cpp (+4-1)
``````````diff
diff --git a/llvm/lib/Analysis/ConstraintSystem.cpp b/llvm/lib/Analysis/ConstraintSystem.cpp
index c659b4de3bb13..1aabb648a3638 100644
--- a/llvm/lib/Analysis/ConstraintSystem.cpp
+++ b/llvm/lib/Analysis/ConstraintSystem.cpp
@@ -300,6 +300,10 @@ bool ConstraintSystem::isConditionImplied(RowTy R) const {
if (isConstantOnly(R))
return getConstant(R) >= 0;
+ // R is trivially implied if a single row of the system implies it.
+ if (isImpliedBySingleRow(R))
+ return true;
+
// If there is no solution with the negation of R added to the system, the
// condition must hold based on the existing constraints.
R = ConstraintSystem::negate(std::move(R));
diff --git a/llvm/test/Transforms/ConstraintElimination/add-nsw.ll b/llvm/test/Transforms/ConstraintElimination/add-nsw.ll
index 9b6ec5f10aa7a..07ad9a51f1912 100644
--- a/llvm/test/Transforms/ConstraintElimination/add-nsw.ll
+++ b/llvm/test/Transforms/ConstraintElimination/add-nsw.ll
@@ -906,9 +906,8 @@ define i1 @add_neg_1_slt(i64 %x, i64 %b) {
; CHECK-NEXT: call void @llvm.assume(i1 [[X_NON_NEG]])
; CHECK-NEXT: [[C:%.*]] = icmp slt i64 [[X]], [[B:%.*]]
; CHECK-NEXT: call void @llvm.assume(i1 [[C]])
-; CHECK-NEXT: [[SUB:%.*]] = add i64 [[X]], -1
-; CHECK-NEXT: [[T:%.*]] = icmp slt i64 [[SUB]], [[B]]
-; CHECK-NEXT: ret i1 [[T]]
+; CHECK-NEXT: [[SUB:%.*]] = add nsw i64 [[X]], -1
+; CHECK-NEXT: ret i1 true
;
%x.non.neg = icmp sge i64 %x, 0
call void @llvm.assume(i1 %x.non.neg)
@@ -943,9 +942,8 @@ define i1 @add_neg_1_nonneg_from_ult(i64 %x, i64 %c) {
; CHECK-NEXT: call void @llvm.assume(i1 [[X_NON_NEG]])
; CHECK-NEXT: [[B:%.*]] = icmp ult i64 [[X:%.*]], [[C]]
; CHECK-NEXT: call void @llvm.assume(i1 [[B]])
-; CHECK-NEXT: [[SUB:%.*]] = add i64 [[X]], -1
-; CHECK-NEXT: [[T:%.*]] = icmp slt i64 [[SUB]], [[C]]
-; CHECK-NEXT: ret i1 [[T]]
+; CHECK-NEXT: [[SUB:%.*]] = add nsw i64 [[X]], -1
+; CHECK-NEXT: ret i1 true
;
%x.non.neg = icmp sge i64 %c, 0
call void @llvm.assume(i1 %x.non.neg)
diff --git a/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit-latch-counted.ll b/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit-latch-counted.ll
index 31ae62c564dd6..9d60fc7270482 100644
--- a/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit-latch-counted.ll
+++ b/llvm/test/Transforms/ConstraintElimination/induction-condition-in-loop-exit-latch-counted.ll
@@ -8,7 +8,7 @@ define i1 @latch_counted_header_check_removable(ptr %p, i64 %n, i64 %lim) {
; CHECK-NEXT: [[POS:%.*]] = icmp sgt i64 [[N]], 0
; CHECK-NEXT: br i1 [[POS]], label %[[PH:.*]], label %[[EXIT_0:.*]]
; CHECK: [[PH]]:
-; CHECK-NEXT: [[NM1:%.*]] = add i64 [[N]], -1
+; CHECK-NEXT: [[NM1:%.*]] = add nsw i64 [[N]], -1
; CHECK-NEXT: [[G4:%.*]] = shl nuw nsw i64 [[NM1]], 2
; CHECK-NEXT: [[G:%.*]] = icmp ult i64 [[G4]], [[LIM]]
; CHECK-NEXT: br i1 [[G]], label %[[LOOP_HEADER:.*]], label %[[EXIT_1:.*]]
diff --git a/llvm/test/Transforms/ConstraintElimination/large-constant-ints.ll b/llvm/test/Transforms/ConstraintElimination/large-constant-ints.ll
index 8ee55f98f0e36..70c3a6480d3f1 100644
--- a/llvm/test/Transforms/ConstraintElimination/large-constant-ints.ll
+++ b/llvm/test/Transforms/ConstraintElimination/large-constant-ints.ll
@@ -432,4 +432,28 @@ define i1 @pr68751(i128 %arg) {
ret i1 %cmp
}
+; The fact %c.2 is implied by the single row added for %c.1. Multiplying
+; coefficients would overflow during Fourier-Motzkin elimination.
+define i1 @fact_implied_by_single_row_coefficient_overflow(i64 %x, i64 %y) {
+; CHECK-LABEL: @fact_implied_by_single_row_coefficient_overflow(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: [[S:%.*]] = shl nuw i64 [[X:%.*]], 33
+; CHECK-NEXT: [[T:%.*]] = shl nuw i64 [[Y:%.*]], 33
+; CHECK-NEXT: [[C_1:%.*]] = icmp ult i64 [[S]], [[T]]
+; CHECK-NEXT: call void @llvm.assume(i1 [[C_1]])
+; CHECK-NEXT: [[C_2:%.*]] = icmp ule i64 [[S]], [[T]]
+; CHECK-NEXT: call void @llvm.assume(i1 [[C_2]])
+; CHECK-NEXT: ret i1 true
+;
+entry:
+ %s = shl nuw i64 %x, 33
+ %t = shl nuw i64 %y, 33
+ %c.1 = icmp ult i64 %s, %t
+ call void @llvm.assume(i1 %c.1)
+ %c.2 = icmp ule i64 %s, %t
+ call void @llvm.assume(i1 %c.2)
+ %c.3 = icmp ule i64 %s, %t
+ ret i1 %c.3
+}
+
declare void @llvm.assume(i1)
diff --git a/llvm/test/Transforms/PhaseOrdering/X86/pr38280.ll b/llvm/test/Transforms/PhaseOrdering/X86/pr38280.ll
index 43f6cf7e51d09..a0c79a30ac9d7 100644
--- a/llvm/test/Transforms/PhaseOrdering/X86/pr38280.ll
+++ b/llvm/test/Transforms/PhaseOrdering/X86/pr38280.ll
@@ -32,7 +32,7 @@ define void @apply_delta(ptr nocapture noundef %dst, ptr nocapture noundef reado
; CHECK-NEXT: [[DST_ADDR_130:%.*]] = phi ptr [ [[INCDEC_PTR:%.*]], [[WHILE_BODY4]] ], [ [[DST_ADDR_0_LCSSA]], [[WHILE_COND3_PREHEADER]] ]
; CHECK-NEXT: [[SRC_ADDR_129:%.*]] = phi ptr [ [[INCDEC_PTR8:%.*]], [[WHILE_BODY4]] ], [ [[SRC_ADDR_0_LCSSA]], [[WHILE_COND3_PREHEADER]] ]
; CHECK-NEXT: [[COUNT_ADDR_128:%.*]] = phi i64 [ [[DEC:%.*]], [[WHILE_BODY4]] ], [ [[COUNT_ADDR_0_LCSSA]], [[WHILE_COND3_PREHEADER]] ]
-; CHECK-NEXT: [[DEC]] = add i64 [[COUNT_ADDR_128]], -1
+; CHECK-NEXT: [[DEC]] = add nsw i64 [[COUNT_ADDR_128]], -1
; CHECK-NEXT: [[TMP2:%.*]] = load i8, ptr [[SRC_ADDR_129]], align 1
; CHECK-NEXT: [[ARRAYIDX:%.*]] = getelementptr inbounds i8, ptr [[DST_ADDR_130]], i64 [[NEG_OFFS]]
; CHECK-NEXT: [[TMP3:%.*]] = load i8, ptr [[ARRAYIDX]], align 1
diff --git a/llvm/unittests/Analysis/ConstraintSystemTest.cpp b/llvm/unittests/Analysis/ConstraintSystemTest.cpp
index edd7fdd89aac3..50abf0023b7c4 100644
--- a/llvm/unittests/Analysis/ConstraintSystemTest.cpp
+++ b/llvm/unittests/Analysis/ConstraintSystemTest.cpp
@@ -197,6 +197,9 @@ TEST(ConstraintSolverTest, IsConditionImpliedOverflow) {
// Make sure isConditionImplied returns false when there is an overflow.
int64_t Limit = std::numeric_limits<int64_t>::max();
addVariableRow(CS, {Limit - 1, Limit - 2, Limit - 3});
- EXPECT_FALSE(isConditionImplied(CS, {Limit - 1, Limit - 2, Limit - 3}));
+ EXPECT_FALSE(isConditionImplied(CS, {Limit - 1, Limit - 2, Limit - 4}));
+ // The same row is implied by the single row of the system, without
+ // Fourier-Motzkin elimination.
+ EXPECT_TRUE(isConditionImplied(CS, {Limit - 1, Limit - 2, Limit - 3}));
}
} // namespace
``````````
</details>
https://github.com/llvm/llvm-project/pull/228369
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