[llvm] [SCEV] Prove umin(Ops) u< RHS by checking individual operands (PR #207729)
via llvm-commits
llvm-commits at lists.llvm.org
Mon Jul 6 06:08:44 PDT 2026
llvmorg-github-actions[bot] wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-analysis
Author: Aleksandr Popov (aleks-tmb)
<details>
<summary>Changes</summary>
Teach IsKnownPredicateViaMinOrMax to handle ICMP_ULT/UGT for SCEVUMinExpr. Since umin(Ops) u<= each Op by definition, proving Op u< RHS for any single operand is sufficient to establish umin(Ops) u< RHS by transitivity.
This unblocks loop vectorization for early-exit loops whose trip count is a umin of a buffer length and an iteration bound (e.g. searching a bounded slice for a sentinel value).
Fixes https://github.com/llvm/llvm-project/issues/196935
Alive2 proof: https://alive2.llvm.org/ce/z/9DjA2f
---
Full diff: https://github.com/llvm/llvm-project/pull/207729.diff
2 Files Affected:
- (modified) llvm/lib/Analysis/ScalarEvolution.cpp (+24)
- (modified) llvm/test/Transforms/LoopVectorize/early-exit-umin-trip-count.ll (+37-6)
``````````diff
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index c0cdce982e623..a168e4e8e1c12 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -12924,6 +12924,30 @@ static bool IsKnownPredicateViaMinOrMax(ScalarEvolution &SE, CmpPredicate Pred,
IsMinMaxConsistingOf<SCEVUMinExpr>(LHS, RHS) ||
// A <= max(A, ...)
IsMinMaxConsistingOf<SCEVUMaxExpr>(RHS, LHS);
+
+ case ICmpInst::ICMP_UGT:
+ std::swap(LHS, RHS);
+ [[fallthrough]];
+ case ICmpInst::ICMP_ULT:
+ // umin(Ops) u<= each Op, so proving Op u< RHS for any Op proves
+ // umin(Ops) u< RHS.
+ //
+ // Use computeConstantDifference instead of the more powerful
+ // isKnownPredicate to keep this check cheap: isKnownPredicateViaMinOrMax
+ // is called from isKnownViaNonRecursiveReasoning, so recursing into
+ // the full predicate prover would be expensive.
+ if (const auto *Min = dyn_cast<SCEVUMinExpr>(LHS)) {
+ for (SCEVUse Op : Min->operands()) {
+ std::optional<APInt> Diff = SE.computeConstantDifference(RHS, Op);
+ // When Op and RHS share a common base differing by a
+ // constant offset D (RHS - Op = D), Op u< RHS holds iff D != 0 and
+ // RHS >= D (unsigned), i.e. the subtraction doesn't underflow.
+ if (Diff && !Diff->isZero() &&
+ SE.getUnsignedRangeMin(RHS).uge(*Diff))
+ return true;
+ }
+ }
+ return false;
}
llvm_unreachable("covered switch fell through?!");
diff --git a/llvm/test/Transforms/LoopVectorize/early-exit-umin-trip-count.ll b/llvm/test/Transforms/LoopVectorize/early-exit-umin-trip-count.ll
index 36936ec9832a4..762a118130a46 100644
--- a/llvm/test/Transforms/LoopVectorize/early-exit-umin-trip-count.ll
+++ b/llvm/test/Transforms/LoopVectorize/early-exit-umin-trip-count.ll
@@ -1,7 +1,7 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
; RUN: opt -S -passes=loop-vectorize %s | FileCheck %s
-; TODO: The loop should be vectorized. The vectorizer needs to prove
+; The loop should be vectorized. The vectorizer needs to prove
; (A umin B) u<= A to establish memory safety from the dereferenceable
; assumption.
;
@@ -22,17 +22,42 @@ define ptr @main(i32 %length, ptr %ptr, i64 %0) #0 {
; CHECK-NEXT: [[NULL_CHECK:%.*]] = icmp eq i32 [[LENGTH]], 0
; CHECK-NEXT: br i1 [[NULL_CHECK]], label %[[DEOPT]], label %[[SCALAR_PH:.*]]
; CHECK: [[SCALAR_PH]]:
+; CHECK-NEXT: [[MIN_ITERS_CHECK:%.*]] = icmp ult i64 [[EXIT]], 4
+; CHECK-NEXT: br i1 [[MIN_ITERS_CHECK]], label %[[SCALAR_PH1:.*]], label %[[VECTOR_PH:.*]]
+; CHECK: [[VECTOR_PH]]:
+; CHECK-NEXT: [[N_MOD_VF:%.*]] = urem i64 [[EXIT]], 4
+; CHECK-NEXT: [[N_VEC:%.*]] = sub i64 [[EXIT]], [[N_MOD_VF]]
; CHECK-NEXT: br label %[[LOOP:.*]]
; CHECK: [[LOOP]]:
-; CHECK-NEXT: [[IV:%.*]] = phi i64 [ [[IV_NEXT:%.*]], %[[LATCH:.*]] ], [ 0, %[[SCALAR_PH]] ]
+; CHECK-NEXT: [[IV:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[VECTOR_BODY_INTERIM:.*]] ]
; CHECK-NEXT: [[ELEMENT_GEP:%.*]] = getelementptr i8, ptr [[PTR]], i64 [[IV]]
-; CHECK-NEXT: [[ELEMENT:%.*]] = load i8, ptr [[ELEMENT_GEP]], align 1
+; CHECK-NEXT: [[WIDE_LOAD:%.*]] = load <4 x i8>, ptr [[ELEMENT_GEP]], align 1
+; CHECK-NEXT: [[TMP2:%.*]] = icmp eq <4 x i8> [[WIDE_LOAD]], zeroinitializer
+; CHECK-NEXT: [[TMP3:%.*]] = freeze <4 x i1> [[TMP2]]
+; CHECK-NEXT: [[TMP4:%.*]] = call i1 @llvm.vector.reduce.or.v4i1(<4 x i1> [[TMP3]])
+; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i64 [[IV]], 4
+; CHECK-NEXT: [[TMP5:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
+; CHECK-NEXT: br i1 [[TMP4]], label %[[VECTOR_EARLY_EXIT:.*]], label %[[VECTOR_BODY_INTERIM]]
+; CHECK: [[VECTOR_BODY_INTERIM]]:
+; CHECK-NEXT: br i1 [[TMP5]], label %[[MIDDLE_BLOCK:.*]], label %[[LOOP]], !llvm.loop [[LOOP0:![0-9]+]]
+; CHECK: [[MIDDLE_BLOCK]]:
+; CHECK-NEXT: [[CMP_N:%.*]] = icmp eq i64 [[EXIT]], [[N_VEC]]
+; CHECK-NEXT: br i1 [[CMP_N]], label %[[DEOPT_LOOPEXIT:.*]], label %[[SCALAR_PH1]]
+; CHECK: [[VECTOR_EARLY_EXIT]]:
+; CHECK-NEXT: br label %[[FOUND:.*]]
+; CHECK: [[SCALAR_PH1]]:
+; CHECK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i64 [ [[N_VEC]], %[[MIDDLE_BLOCK]] ], [ 0, %[[SCALAR_PH]] ]
+; CHECK-NEXT: br label %[[LOOP1:.*]]
+; CHECK: [[LOOP1]]:
+; CHECK-NEXT: [[IV1:%.*]] = phi i64 [ [[IV_NEXT:%.*]], %[[LATCH:.*]] ], [ [[BC_RESUME_VAL]], %[[SCALAR_PH1]] ]
+; CHECK-NEXT: [[ELEMENT_GEP1:%.*]] = getelementptr i8, ptr [[PTR]], i64 [[IV1]]
+; CHECK-NEXT: [[ELEMENT:%.*]] = load i8, ptr [[ELEMENT_GEP1]], align 1
; CHECK-NEXT: [[FOUND_CHECK:%.*]] = icmp eq i8 [[ELEMENT]], 0
-; CHECK-NEXT: br i1 [[FOUND_CHECK]], label %[[FOUND:.*]], label %[[LATCH]]
+; CHECK-NEXT: br i1 [[FOUND_CHECK]], label %[[FOUND]], label %[[LATCH]]
; CHECK: [[LATCH]]:
-; CHECK-NEXT: [[IV_NEXT]] = add i64 [[IV]], 1
+; CHECK-NEXT: [[IV_NEXT]] = add i64 [[IV1]], 1
; CHECK-NEXT: [[RANGE_CHECK:%.*]] = icmp ult i64 [[IV_NEXT]], [[EXIT]]
-; CHECK-NEXT: br i1 [[RANGE_CHECK]], label %[[LOOP]], label %[[DEOPT_LOOPEXIT:.*]]
+; CHECK-NEXT: br i1 [[RANGE_CHECK]], label %[[LOOP1]], label %[[DEOPT_LOOPEXIT]], !llvm.loop [[LOOP3:![0-9]+]]
; CHECK: [[FOUND]]:
; CHECK-NEXT: ret ptr null
; CHECK: [[DEOPT_LOOPEXIT]]:
@@ -75,3 +100,9 @@ declare i64 @llvm.umin.i64(i64, i64)
declare void @llvm.assume(i1 noundef)
attributes #0 = { nofree nosync }
+;.
+; CHECK: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]], [[META2:![0-9]+]]}
+; CHECK: [[META1]] = !{!"llvm.loop.isvectorized", i32 1}
+; CHECK: [[META2]] = !{!"llvm.loop.unroll.runtime.disable"}
+; CHECK: [[LOOP3]] = distinct !{[[LOOP3]], [[META2]], [[META1]]}
+;.
\ No newline at end of file
``````````
</details>
https://github.com/llvm/llvm-project/pull/207729
More information about the llvm-commits
mailing list