[llvm] [SCEV] Avoid erase+insert in constant folding (NFC) (PR #101642)
Nikita Popov via llvm-commits
llvm-commits at lists.llvm.org
Fri Aug 2 02:05:12 PDT 2024
https://github.com/nikic created https://github.com/llvm/llvm-project/pull/101642
Based on Philip's suggestion in #101473, this changes the constant folding implementation to keep the SCEVConstant as the first element in the Ops, instead of erasing it and then later reinserting.
This gives a small compile-time improvement:
http://llvm-compile-time-tracker.com/compare.php?from=85c5265feae82d8e26869adf4505b448b3a17534&to=67271fa74cb49ff958865bc56e3eea79a934bafc&stat=instructions:u
>From 919a6b8dc0f2838f5f0f251994d1e2cf41b3466d Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Fri, 2 Aug 2024 10:09:15 +0200
Subject: [PATCH] [SCEV] Avoid erase+insert in constant folding (NFC)
Based on Philip's suggestion in #101473, this changes the constant
folding implementation to keep the SCEVConstant as the first
element in the Ops, instead of erasing it and then later reinserting.
This gives a small compile-time improvement:
http://llvm-compile-time-tracker.com/compare.php?from=85c5265feae82d8e26869adf4505b448b3a17534&to=67271fa74cb49ff958865bc56e3eea79a934bafc&stat=instructions:u
---
llvm/lib/Analysis/ScalarEvolution.cpp | 43 +++++++++++++++------------
1 file changed, 24 insertions(+), 19 deletions(-)
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 06dd17dd3648b..f4c4dbd252dd7 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -777,7 +777,7 @@ CompareSCEVComplexity(EquivalenceClasses<const SCEV *> &EqCacheSCEV,
/// results from this routine. In other words, we don't want the results of
/// this to depend on where the addresses of various SCEV objects happened to
/// land in memory.
-static void GroupByComplexity(SmallVectorImpl<const SCEV *> &Ops,
+static void GroupByComplexity(MutableArrayRef<const SCEV *> Ops,
LoopInfo *LI, DominatorTree &DT) {
if (Ops.size() < 2) return; // Noop
@@ -844,34 +844,39 @@ static const SCEV *
constantFoldAndGroupOps(ScalarEvolution &SE, LoopInfo &LI, DominatorTree &DT,
SmallVectorImpl<const SCEV *> &Ops, FoldT Fold,
IsIdentityT IsIdentity, IsAbsorberT IsAbsorber) {
- const SCEVConstant *Folded = nullptr;
+ bool HasConst = false;
for (unsigned Idx = 0; Idx < Ops.size();) {
const SCEV *Op = Ops[Idx];
if (const auto *C = dyn_cast<SCEVConstant>(Op)) {
- if (!Folded)
- Folded = C;
- else
- Folded = cast<SCEVConstant>(
- SE.getConstant(Fold(Folded->getAPInt(), C->getAPInt())));
- Ops.erase(Ops.begin() + Idx);
- continue;
+ if (!HasConst) {
+ // Move the constant to the start.
+ std::swap(Ops[0], Ops[Idx]);
+ HasConst = true;
+ } else {
+ Ops[0] = SE.getConstant(
+ Fold(cast<SCEVConstant>(Ops[0])->getAPInt(), C->getAPInt()));
+ Ops.erase(Ops.begin() + Idx);
+ // Revisit this index.
+ continue;
+ }
}
++Idx;
}
- if (Ops.empty()) {
- assert(Folded && "Must have folded value");
- return Folded;
- }
+ if (Ops.size() == 1)
+ return Ops[0];
- if (Folded && IsAbsorber(Folded->getAPInt()))
- return Folded;
+ if (HasConst && IsIdentity(cast<SCEVConstant>(Ops[0])->getAPInt())) {
+ Ops.erase(Ops.begin());
+ HasConst = false;
+ }
- GroupByComplexity(Ops, &LI, DT);
- if (Folded && !IsIdentity(Folded->getAPInt()))
- Ops.insert(Ops.begin(), Folded);
+ if (Ops.size() == 1 ||
+ (HasConst && IsAbsorber(cast<SCEVConstant>(Ops[0])->getAPInt())))
+ return Ops[0];
- return Ops.size() == 1 ? Ops[0] : nullptr;
+ GroupByComplexity(MutableArrayRef(Ops).drop_front(HasConst), &LI, DT);
+ return nullptr;
}
//===----------------------------------------------------------------------===//
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