[llvm] [SCEV] Rewrite more SCEVAddExpr when applying guards. (PR #159942)
Florian Hahn via llvm-commits
llvm-commits at lists.llvm.org
Sat Sep 20 13:27:24 PDT 2025
https://github.com/fhahn updated https://github.com/llvm/llvm-project/pull/159942
>From 74447bb177900f054f84cdda7b5b90d4985bcc3c Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Fri, 19 Sep 2025 10:22:23 +0100
Subject: [PATCH] [SCEV] Rewrite more SCEVAddExpr when applying guards.
When re-writing SCEVAddExprs to apply information from guards, check if
we have information for the expression itself. If so, apply it.
When we have an expression of the form (Const + A), check if we have
have guard info for (Const + 1 + A) and use it. This is needed to avoid
regressions in a few cases, where we have BTCs with a subtracted
constant.
Rewriting expressions could cause regressions, e.g. when comparing 2
SCEV expressions where we are only able to rewrite one side, but I could
not find any cases where this happens more with this patch in practice.
https://alive2.llvm.org/ce/z/cre6rr
---
llvm/lib/Analysis/ScalarEvolution.cpp | 43 +++++++++++++------
.../IndVarSimplify/canonicalize-cmp.ll | 6 +--
.../runtime-checks-difference.ll | 7 +--
3 files changed, 32 insertions(+), 24 deletions(-)
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index b08399b381f34..00ebae32b0cd5 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -16059,29 +16059,44 @@ const SCEV *ScalarEvolution::LoopGuards::rewrite(const SCEV *Expr) const {
}
const SCEV *visitAddExpr(const SCEVAddExpr *Expr) {
+ SmallVector<const SCEV *, 2> Operands;
+ bool Changed = false;
+ for (const auto *Op : Expr->operands()) {
+ Operands.push_back(
+ SCEVRewriteVisitor<SCEVLoopGuardRewriter>::visit(Op));
+ Changed |= Op != Operands.back();
+ }
+ if (Changed) {
+ // We are only replacing operands with equivalent values, so transfer
+ // the flags from the original expression.
+ return SE.getAddExpr(Operands, ScalarEvolution::maskFlags(
+ Expr->getNoWrapFlags(), FlagMask));
+ }
+ if (const SCEV *S = Map.lookup(Expr))
+ return S;
+ if (!isa<SCEVConstant>(Expr->getOperand(0)))
+ return Expr;
// Trip count expressions sometimes consist of adding 3 operands, i.e.
// (Const + A + B). There may be guard info for A + B, and if so, apply
// it.
// TODO: Could more generally apply guards to Add sub-expressions.
- if (isa<SCEVConstant>(Expr->getOperand(0)) &&
- Expr->getNumOperands() == 3) {
+ if (Expr->getNumOperands() == 3) {
if (const SCEV *S = Map.lookup(
SE.getAddExpr(Expr->getOperand(1), Expr->getOperand(2))))
return SE.getAddExpr(Expr->getOperand(0), S);
}
- SmallVector<const SCEV *, 2> Operands;
- bool Changed = false;
- for (const auto *Op : Expr->operands()) {
- Operands.push_back(
- SCEVRewriteVisitor<SCEVLoopGuardRewriter>::visit(Op));
- Changed |= Op != Operands.back();
+
+ // For expressions of the form (Const + A), check if we have guard info
+ // for (Const + 1 + A), and rewrite to ((Const + 1 + A) - 1). This makes
+ // sure we don't loose information when rewriting expressions based on
+ // back-edge taken counts in some cases..
+ if (Expr->getNumOperands() == 2) {
+ auto *NewC =
+ SE.getAddExpr(Expr->getOperand(0), SE.getOne(Expr->getType()));
+ if (const SCEV *S = Map.lookup(SE.getAddExpr(NewC, Expr->getOperand(1))))
+ return SE.getMinusSCEV(S, SE.getOne(Expr->getType()));
}
- // We are only replacing operands with equivalent values, so transfer the
- // flags from the original expression.
- return !Changed ? Expr
- : SE.getAddExpr(Operands,
- ScalarEvolution::maskFlags(
- Expr->getNoWrapFlags(), FlagMask));
+ return Expr;
}
const SCEV *visitMulExpr(const SCEVMulExpr *Expr) {
diff --git a/llvm/test/Transforms/IndVarSimplify/canonicalize-cmp.ll b/llvm/test/Transforms/IndVarSimplify/canonicalize-cmp.ll
index 4b52479fc6c4d..40e3c63cbe04a 100644
--- a/llvm/test/Transforms/IndVarSimplify/canonicalize-cmp.ll
+++ b/llvm/test/Transforms/IndVarSimplify/canonicalize-cmp.ll
@@ -343,14 +343,13 @@ define void @slt_no_smax_needed(i64 %n, ptr %dst) {
; CHECK-NEXT: [[PRE:%.*]] = icmp ult i32 [[ADD_1]], 8
; CHECK-NEXT: br i1 [[PRE]], label [[EXIT:%.*]], label [[LOOP_PREHEADER:%.*]]
; CHECK: loop.preheader:
-; CHECK-NEXT: [[SMAX:%.*]] = call i32 @llvm.smax.i32(i32 [[SHR]], i32 1)
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ [[IV_NEXT:%.*]], [[LOOP]] ], [ 0, [[LOOP_PREHEADER]] ]
; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds i8, ptr [[DST:%.*]], i32 [[IV]]
; CHECK-NEXT: store i8 0, ptr [[GEP]], align 1
; CHECK-NEXT: [[IV_NEXT]] = add nuw nsw i32 [[IV]], 1
-; CHECK-NEXT: [[EXITCOND:%.*]] = icmp ne i32 [[IV_NEXT]], [[SMAX]]
+; CHECK-NEXT: [[EXITCOND:%.*]] = icmp ne i32 [[IV_NEXT]], [[SHR]]
; CHECK-NEXT: br i1 [[EXITCOND]], label [[LOOP]], label [[EXIT_LOOPEXIT:%.*]]
; CHECK: exit.loopexit:
; CHECK-NEXT: br label [[EXIT]]
@@ -385,14 +384,13 @@ define void @ult_no_umax_needed(i64 %n, ptr %dst) {
; CHECK-NEXT: [[PRE:%.*]] = icmp ult i32 [[ADD_1]], 8
; CHECK-NEXT: br i1 [[PRE]], label [[EXIT:%.*]], label [[LOOP_PREHEADER:%.*]]
; CHECK: loop.preheader:
-; CHECK-NEXT: [[UMAX:%.*]] = call i32 @llvm.umax.i32(i32 [[SHR]], i32 1)
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ [[IV_NEXT:%.*]], [[LOOP]] ], [ 0, [[LOOP_PREHEADER]] ]
; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds i8, ptr [[DST:%.*]], i32 [[IV]]
; CHECK-NEXT: store i8 0, ptr [[GEP]], align 1
; CHECK-NEXT: [[IV_NEXT]] = add nuw nsw i32 [[IV]], 1
-; CHECK-NEXT: [[EXITCOND:%.*]] = icmp ne i32 [[IV_NEXT]], [[UMAX]]
+; CHECK-NEXT: [[EXITCOND:%.*]] = icmp ne i32 [[IV_NEXT]], [[SHR]]
; CHECK-NEXT: br i1 [[EXITCOND]], label [[LOOP]], label [[EXIT_LOOPEXIT:%.*]]
; CHECK: exit.loopexit:
; CHECK-NEXT: br label [[EXIT]]
diff --git a/llvm/test/Transforms/LoopVectorize/runtime-checks-difference.ll b/llvm/test/Transforms/LoopVectorize/runtime-checks-difference.ll
index 648ebc7e6c3a5..a556b15adbefc 100644
--- a/llvm/test/Transforms/LoopVectorize/runtime-checks-difference.ll
+++ b/llvm/test/Transforms/LoopVectorize/runtime-checks-difference.ll
@@ -465,12 +465,7 @@ define void @remove_diff_checks_via_guards(i32 %x, i32 %y, ptr %A) {
; CHECK-NEXT: [[TMP13:%.*]] = icmp ugt i64 [[SMAX]], 4294967295
; CHECK-NEXT: [[TMP14:%.*]] = or i1 [[TMP12]], [[TMP13]]
; CHECK-NEXT: [[TMP15:%.*]] = or i1 [[TMP9]], [[TMP14]]
-; CHECK-NEXT: br i1 [[TMP15]], [[SCALAR_PH]], label %[[VECTOR_MEMCHECK:.*]]
-; CHECK: [[VECTOR_MEMCHECK]]:
-; CHECK-NEXT: [[TMP16:%.*]] = sext i32 [[OFFSET]] to i64
-; CHECK-NEXT: [[TMP17:%.*]] = shl nsw i64 [[TMP16]], 2
-; CHECK-NEXT: [[DIFF_CHECK:%.*]] = icmp ult i64 [[TMP17]], 16
-; CHECK-NEXT: br i1 [[DIFF_CHECK]], [[SCALAR_PH]], [[VECTOR_PH1:label %.*]]
+; CHECK-NEXT: br i1 [[TMP15]], [[SCALAR_PH]], [[VECTOR_PH:label %.*]]
;
entry:
%offset = sub i32 %x, %y
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