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[Passes][LoopRotate] Move minsize handling fully into pass (#189956)

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Make this dependent only on the minsize attribute and drop the pipeline
handling.

Rename the enable-loop-header-duplication option to
enable-loop-header-duplication-at-minsize to clarify that it controls
header duplication at minsize only (in other cases it is enabled by
default, independently of this option).
This commit is contained in:
Nikita Popov 2026-04-02 09:32:56 +02:00 committed by GitHub
parent 40e7fa632d
commit 1662c200a5
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GPG key ID: B5690EEEBB952194
4 changed files with 58 additions and 44 deletions
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26
llvm/lib/Passes/PassBuilderPipelines.cpp
View file

@ -219,15 +219,6 @@ static cl::opt<bool> EnableLoopFlatten("enable-loop-flatten", cl::init(false),
cl::Hidden,
cl::desc("Enable the LoopFlatten Pass"));
// Experimentally allow loop header duplication. This should allow for better
// optimization at Oz, since loop-idiom recognition can then recognize things
// like memcpy. If this ends up being useful for many targets, we should drop
// this flag and make a code generation option that can be controlled
// independent of the opt level and exposed through the frontend.
static cl::opt<bool> EnableLoopHeaderDuplication(
"enable-loop-header-duplication", cl::init(false), cl::Hidden,
cl::desc("Enable loop header duplication at any optimization level"));
static cl::opt<bool>
EnableDFAJumpThreading("enable-dfa-jump-thread",
cl::desc("Enable DFA jump threading"),
@ -688,10 +679,8 @@ PassBuilder::buildFunctionSimplificationPipeline(OptimizationLevel Level,
LPM1.addPass(LICMPass(PTO.LicmMssaOptCap, PTO.LicmMssaNoAccForPromotionCap,
/*AllowSpeculation=*/false));
// Disable header duplication in loop rotation at -Oz.
LPM1.addPass(LoopRotatePass(EnableLoopHeaderDuplication ||
Level != OptimizationLevel::Oz,
isLTOPreLink(Phase)));
LPM1.addPass(
LoopRotatePass(/*EnableHeaderDuplication=*/true, isLTOPreLink(Phase)));
// TODO: Investigate promotion cap for O1.
LPM1.addPass(LICMPass(PTO.LicmMssaOptCap, PTO.LicmMssaNoAccForPromotionCap,
/*AllowSpeculation=*/true));
@ -855,10 +844,8 @@ void PassBuilder::addPostPGOLoopRotation(ModulePassManager &MPM,
if (EnablePostPGOLoopRotation) {
// Disable header duplication in loop rotation at -Oz.
MPM.addPass(createModuleToFunctionPassAdaptor(
createFunctionToLoopPassAdaptor(
LoopRotatePass(EnableLoopHeaderDuplication ||
Level != OptimizationLevel::Oz),
/*UseMemorySSA=*/false),
createFunctionToLoopPassAdaptor(LoopRotatePass(),
/*UseMemorySSA=*/false),
PTO.EagerlyInvalidateAnalyses));
}
}
@ -1571,9 +1558,8 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
LoopPassManager LPM;
// First rotate loops that may have been un-rotated by prior passes.
// Disable header duplication at -Oz.
LPM.addPass(LoopRotatePass(EnableLoopHeaderDuplication ||
Level != OptimizationLevel::Oz,
LTOPreLink, /*CheckExitCount=*/true));
LPM.addPass(LoopRotatePass(/*EnableLoopHeaderDuplication=*/true, LTOPreLink,
/*CheckExitCount=*/true));
// Some loops may have become dead by now. Try to delete them.
// FIXME: see discussion in https://reviews.llvm.org/D112851,
// this may need to be revisited once we run GVN before loop deletion

20
llvm/lib/Transforms/Scalar/LoopRotation.cpp
View file

@ -38,6 +38,15 @@ static cl::opt<bool> PrepareForLTOOption(
cl::desc("Run loop-rotation in the prepare-for-lto stage. This option "
"should be used for testing only."));
// Experimentally allow loop header duplication. This should allow for better
// optimization at Oz, since loop-idiom recognition can then recognize things
// like memcpy. If this ends up being useful for many targets, we should drop
// this flag and make a code generation option that can be controlled
// independent of the opt level and exposed through the frontend.
static cl::opt<bool> EnableLoopHeaderDuplicationAtMinSize(
"enable-loop-header-duplication-at-minsize", cl::init(false), cl::Hidden,
cl::desc("Enable loop header duplication even for minsize"));
LoopRotatePass::LoopRotatePass(bool EnableHeaderDuplication, bool PrepareForLTO,
bool CheckExitCount)
: EnableHeaderDuplication(EnableHeaderDuplication),
@ -68,11 +77,12 @@ PreservedAnalyses LoopRotatePass::run(Loop &L, LoopAnalysisManager &AM,
// Vectorization requires loop-rotation. Use default threshold for loops the
// user explicitly marked for vectorization, even when header duplication is
// disabled.
int Threshold =
(EnableHeaderDuplication && !L.getHeader()->getParent()->hasMinSize()) ||
hasVectorizeTransformation(&L) == TM_ForcedByUser
? DefaultRotationThreshold
: 0;
int Threshold = EnableHeaderDuplication &&
(!L.getHeader()->getParent()->hasMinSize() ||
EnableLoopHeaderDuplicationAtMinSize ||
hasVectorizeTransformation(&L) == TM_ForcedByUser)
? DefaultRotationThreshold
: 0;
const DataLayout &DL = L.getHeader()->getDataLayout();
const SimplifyQuery SQ = getBestSimplifyQuery(AR, DL);

39
llvm/test/Transforms/LoopRotate/oz-disable.ll
View file

@ -1,21 +1,40 @@
; REQUIRES: asserts
; RUN: opt < %s -S -Os -debug -debug-only=loop-rotate 2>&1 | FileCheck %s -check-prefix=OS
; RUN: opt < %s -S -Oz -debug -debug-only=loop-rotate 2>&1 | FileCheck %s -check-prefix=OZ
; RUN: opt < %s -S -passes='default<Os>' -debug -debug-only=loop-rotate 2>&1 | FileCheck %s -check-prefix=OS
; RUN: opt < %s -S -passes='default<Oz>' -debug -debug-only=loop-rotate 2>&1 | FileCheck %s -check-prefix=OZ
; RUN: opt < %s -S -O2 -debug -debug-only=loop-rotate 2>&1 | FileCheck %s --check-prefixes=CHECK,DEFAULT
; RUN: opt < %s -S -passes='default<O2>' -debug -debug-only=loop-rotate 2>&1 | FileCheck %s --check-prefixes=CHECK,DEFAULT
;; Make sure -allow-loop-header-duplication overrides the default behavior at Oz
; RUN: opt < %s -S -passes='default<Oz>' -enable-loop-header-duplication -debug -debug-only=loop-rotate 2>&1 | FileCheck %s -check-prefix=OS
;; Make sure -enable-loop-header-duplication-at-minsize overrides the default behavior at Oz
; RUN: opt < %s -S -passes='default<O2>' -enable-loop-header-duplication-at-minsize -debug -debug-only=loop-rotate 2>&1 | FileCheck %s -check-prefixes=CHECK,ALLOW
; Loop should be rotated for -Os but not for -Oz.
; OS: rotating Loop at depth 1
; OZ-NOT: rotating Loop at depth 1
; optsize loop should always be rotated.
; CHECK: rotating Loop at depth 1
; minsize loop should only be rotated under the option.
; DEFAULT-NOT: rotating Loop at depth 1
; ALLOW: rotating Loop at depth 1
@e = global i32 10
declare void @use(i32)
define void @test() {
define void @test_optsize() optsize {
entry:
%end = load i32, ptr @e
br label %loop
loop:
%n.phi = phi i32 [ %n, %loop.fin ], [ 0, %entry ]
%cond = icmp eq i32 %n.phi, %end
br i1 %cond, label %exit, label %loop.fin
loop.fin:
%n = add i32 %n.phi, 1
call void @use(i32 %n)
br label %loop
exit:
ret void
}
define void @test_minsize() minsize {
entry:
%end = load i32, ptr @e
br label %loop

17
llvm/test/Transforms/PhaseOrdering/enable-loop-header-duplication-oz.ll
View file

@ -1,17 +1,16 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 3
;; Check that -enable-loop-header-duplication at Oz enables certain types of
;; optimizations, for example replacing the loop body w/ a call to memset. If
;; loop idiom recognition begins to recognize unrotated loops, this test will
;; need to be updated.
;; Check that -enable-loop-header-duplication-at-minsize at Oz enables certain
;; types of optimizations, for example replacing the loop body w/ a call to
;; memset. If loop idiom recognition begins to recognize unrotated loops, this
;; test will need to be updated.
; RUN: opt -passes='default<Oz>' -S < %s | FileCheck %s --check-prefix=NOROTATION
; RUN: opt -passes='default<Oz>' -S -enable-loop-header-duplication < %s | FileCheck %s --check-prefix=ROTATION
; RUN: opt -passes='default<O2>' -S < %s | FileCheck %s --check-prefix=ROTATION
; RUN: opt -passes='default<Oz>' -S -enable-loop-header-duplication-at-minsize < %s | FileCheck %s --check-prefix=ROTATION
define void @test(i8* noalias nonnull align 1 %start, i8* %end) unnamed_addr {
define void @test(i8* noalias nonnull align 1 %start, i8* %end) minsize {
; NOROTATION-LABEL: define void @test(
; NOROTATION-SAME: ptr noalias nonnull writeonly align 1 captures(address) [[START:%.*]], ptr readnone captures(address) [[END:%.*]]) unnamed_addr #[[ATTR0:[0-9]+]] {
; NOROTATION-SAME: ptr noalias nonnull writeonly align 1 captures(address) [[START:%.*]], ptr readnone captures(address) [[END:%.*]]) local_unnamed_addr #[[ATTR0:[0-9]+]] {
; NOROTATION-NEXT: entry:
; NOROTATION-NEXT: br label [[LOOP_HEADER:%.*]]
; NOROTATION: loop.header:
@ -26,7 +25,7 @@ define void @test(i8* noalias nonnull align 1 %start, i8* %end) unnamed_addr {
; NOROTATION-NEXT: ret void
;
; ROTATION-LABEL: define void @test(
; ROTATION-SAME: ptr noalias nonnull writeonly align 1 captures(address) [[START:%.*]], ptr readnone captures(address) [[END:%.*]]) unnamed_addr #[[ATTR0:[0-9]+]] {
; ROTATION-SAME: ptr noalias nonnull writeonly align 1 captures(address) [[START:%.*]], ptr readnone captures(address) [[END:%.*]]) local_unnamed_addr #[[ATTR0:[0-9]+]] {
; ROTATION-NEXT: entry:
; ROTATION-NEXT: [[_12_I1:%.*]] = icmp eq ptr [[START]], [[END]]
; ROTATION-NEXT: br i1 [[_12_I1]], label [[EXIT:%.*]], label [[LOOP_LATCH_PREHEADER:%.*]]