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Merged
merged 11 commits into from
Sep 15, 2025
Merged

Introduce -fexperimental-loop-fuse to clang and flang #142686

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e2f5344
add -floop-fuse to clang and flang
sebpop May 22, 2025
7b4fdc4
rename -floop-fuse / -fexperimental-fuse-loops
sebpop Jun 12, 2025
fe53e39
Address review comments
madhur13490 Jul 31, 2025
ab202f0
change place of pass in pipeline
madhur13490 Aug 7, 2025
3f88802
fixup! change place of pass in pipeline
madhur13490 Aug 8, 2025
9f264b4
fixup! address review comments about optInFlag
madhur13490 Aug 28, 2025
76780b0
fixup! addOptInFlag for Flang.cpp too.
madhur13490 Aug 28, 2025
00d5ca5
fixup! alignments
madhur13490 Aug 29, 2025
551813b
fixup! add check for EnableFusion flag
madhur13490 Aug 29, 2025
a007cea
fixup! move enable-fusion to NewPMDriver
madhur13490 Sep 1, 2025
4c72de9
fixup! bring back lost change in LI
madhur13490 Sep 15, 2025
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1 change: 1 addition & 0 deletions clang/include/clang/Basic/CodeGenOptions.def
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Expand Up @@ -322,6 +322,7 @@ CODEGENOPT(TimeTrace , 1, 0, Benign) ///< Set when -ftime-trace is enabl
VALUE_CODEGENOPT(TimeTraceGranularity, 32, 500, Benign) ///< Minimum time granularity (in microseconds),
///< traced by time profiler
CODEGENOPT(InterchangeLoops , 1, 0, Benign) ///< Run loop-interchange.
CODEGENOPT(FuseLoops , 1, 0, Benign) ///< Run loop-fusion.
CODEGENOPT(UnrollLoops , 1, 0, Benign) ///< Control whether loops are unrolled.
CODEGENOPT(RerollLoops , 1, 0, Benign) ///< Control whether loops are rerolled.
CODEGENOPT(NoUseJumpTables , 1, 0, Benign) ///< Set when -fno-jump-tables is enabled.
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4 changes: 4 additions & 0 deletions clang/include/clang/Driver/Options.td
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Expand Up @@ -4304,6 +4304,10 @@ def floop_interchange : Flag<["-"], "floop-interchange">, Group<f_Group>,
HelpText<"Enable the loop interchange pass">, Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>;
def fno_loop_interchange: Flag<["-"], "fno-loop-interchange">, Group<f_Group>,
HelpText<"Disable the loop interchange pass">, Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>;
defm experimental_loop_fusion
: OptInCC1FFlag<"experimental-loop-fusion", "Enable", "Disable",
"Enable the loop fusion pass",
[ClangOption, FlangOption, FC1Option]>;
def funroll_loops : Flag<["-"], "funroll-loops">, Group<f_Group>,
HelpText<"Turn on loop unroller">, Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>;
def fno_unroll_loops : Flag<["-"], "fno-unroll-loops">, Group<f_Group>,
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2 changes: 2 additions & 0 deletions clang/lib/CodeGen/BackendUtil.cpp
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Expand Up @@ -896,6 +896,7 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
PipelineTuningOptions PTO;
PTO.LoopUnrolling = CodeGenOpts.UnrollLoops;
PTO.LoopInterchange = CodeGenOpts.InterchangeLoops;
PTO.LoopFusion = CodeGenOpts.FuseLoops;
// For historical reasons, loop interleaving is set to mirror setting for loop
// unrolling.
PTO.LoopInterleaving = CodeGenOpts.UnrollLoops;
Expand Down Expand Up @@ -1331,6 +1332,7 @@ runThinLTOBackend(CompilerInstance &CI, ModuleSummaryIndex *CombinedIndex,
Conf.SampleProfile = std::move(SampleProfile);
Conf.PTO.LoopUnrolling = CGOpts.UnrollLoops;
Conf.PTO.LoopInterchange = CGOpts.InterchangeLoops;
Conf.PTO.LoopFusion = CGOpts.FuseLoops;
// For historical reasons, loop interleaving is set to mirror setting for loop
// unrolling.
Conf.PTO.LoopInterleaving = CGOpts.UnrollLoops;
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2 changes: 2 additions & 0 deletions clang/lib/Driver/ToolChains/Clang.cpp
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Expand Up @@ -6854,6 +6854,8 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
options::OPT_fno_unroll_loops);
Args.AddLastArg(CmdArgs, options::OPT_floop_interchange,
options::OPT_fno_loop_interchange);
Args.addOptInFlag(CmdArgs, options::OPT_fexperimental_loop_fusion,
options::OPT_fno_experimental_loop_fusion);

Args.AddLastArg(CmdArgs, options::OPT_fstrict_flex_arrays_EQ);

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3 changes: 3 additions & 0 deletions clang/lib/Driver/ToolChains/Flang.cpp
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Expand Up @@ -151,6 +151,9 @@ void Flang::addCodegenOptions(const ArgList &Args,
!stackArrays->getOption().matches(options::OPT_fno_stack_arrays))
CmdArgs.push_back("-fstack-arrays");

Args.addOptInFlag(CmdArgs, options::OPT_fexperimental_loop_fusion,
options::OPT_fno_experimental_loop_fusion);

handleInterchangeLoopsArgs(Args, CmdArgs);
handleVectorizeLoopsArgs(Args, CmdArgs);
handleVectorizeSLPArgs(Args, CmdArgs);
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5 changes: 5 additions & 0 deletions clang/lib/Frontend/CompilerInvocation.cpp
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Expand Up @@ -1680,6 +1680,9 @@ void CompilerInvocationBase::GenerateCodeGenArgs(const CodeGenOptions &Opts,
else
GenerateArg(Consumer, OPT_fno_loop_interchange);

if (Opts.FuseLoops)
GenerateArg(Consumer, OPT_fexperimental_loop_fusion);

if (!Opts.BinutilsVersion.empty())
GenerateArg(Consumer, OPT_fbinutils_version_EQ, Opts.BinutilsVersion);

Expand Down Expand Up @@ -2001,6 +2004,8 @@ bool CompilerInvocation::ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args,
(Opts.OptimizationLevel > 1));
Opts.InterchangeLoops =
Args.hasFlag(OPT_floop_interchange, OPT_fno_loop_interchange, false);
Opts.FuseLoops = Args.hasFlag(OPT_fexperimental_loop_fusion,
OPT_fno_experimental_loop_fusion, false);
Opts.BinutilsVersion =
std::string(Args.getLastArgValue(OPT_fbinutils_version_EQ));

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9 changes: 9 additions & 0 deletions clang/test/Driver/clang_f_opts.c
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Expand Up @@ -52,6 +52,15 @@
// CHECK-INTERCHANGE-LOOPS: "-floop-interchange"
// CHECK-NO-INTERCHANGE-LOOPS: "-fno-loop-interchange"

// RUN: %clang -### -S -fexperimental-loop-fusion %s 2>&1 | FileCheck -check-prefix=CHECK-FUSE-LOOPS %s
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This should check that -fno-experimental-loop-fusion is also recognized by clang.

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I don't think this is usually done or necessary. I checked other options, which are introduced by addOptInFlag, namely funique_internal_linkage_names, fseparate_named_sections, fatomic_ignore_denormal_mode, fsplit_stack, and many others, and I don't see any precedence of having a test which checks if the negative options are recognized.

Do we want to make an exception for this?

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I think it's fine not to test -fno-experimental-loop-fusion. I also don't mind if the author adds it and adds a CHECK-NOT.

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I was partly basing this on the fact that the loop-interchange above does check for both, but if I have not checked if that uses addOptInFlag. If there is precedence for not testing it, that's fine too.

// CHECK-FUSE-LOOPS: "-fexperimental-loop-fusion"
//
// RUN: %clang -c -fexperimental-loop-fusion -mllvm -print-pipeline-passes -O3 %s 2>&1 | FileCheck --check-prefixes=LOOP-FUSION-ON %s
// RUN: %clang -c -mllvm -print-pipeline-passes -O3 %s 2>&1 | FileCheck --check-prefixes=LOOP-FUSION-OFF %s

// LOOP-FUSION-ON: loop-fusion
// LOOP-FUSION-OFF-NOT: loop-fusion

// RUN: %clang -### -S -fprofile-sample-accurate %s 2>&1 | FileCheck -check-prefix=CHECK-PROFILE-SAMPLE-ACCURATE %s
// CHECK-PROFILE-SAMPLE-ACCURATE: "-fprofile-sample-accurate"

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2 changes: 2 additions & 0 deletions flang/docs/ReleaseNotes.md
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Expand Up @@ -35,6 +35,8 @@ page](https://llvm.org/releases/).

## New Compiler Flags

* -fexperimental-loop-fusion is now recognized by flang.

## Windows Support

## Fortran Language Changes in Flang
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1 change: 1 addition & 0 deletions flang/include/flang/Frontend/CodeGenOptions.def
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Expand Up @@ -43,6 +43,7 @@ CODEGENOPT(StackArrays, 1, 0) ///< -fstack-arrays (enable the stack-arrays pass)
CODEGENOPT(VectorizeLoop, 1, 0) ///< Enable loop vectorization.
CODEGENOPT(VectorizeSLP, 1, 0) ///< Enable SLP vectorization.
CODEGENOPT(InterchangeLoops, 1, 0) ///< Enable loop interchange.
CODEGENOPT(FuseLoops, 1, 0) ///< Enable loop fusion.
CODEGENOPT(LoopVersioning, 1, 0) ///< Enable loop versioning.
CODEGENOPT(UnrollLoops, 1, 0) ///< Enable loop unrolling
CODEGENOPT(AliasAnalysis, 1, 0) ///< Enable alias analysis pass
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3 changes: 3 additions & 0 deletions flang/lib/Frontend/CompilerInvocation.cpp
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Expand Up @@ -276,6 +276,9 @@ static void parseCodeGenArgs(Fortran::frontend::CodeGenOptions &opts,
if (args.getLastArg(clang::driver::options::OPT_floop_interchange))
opts.InterchangeLoops = 1;

if (args.getLastArg(clang::driver::options::OPT_fexperimental_loop_fusion))
opts.FuseLoops = 1;

if (args.getLastArg(clang::driver::options::OPT_vectorize_loops))
opts.VectorizeLoop = 1;

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1 change: 1 addition & 0 deletions flang/lib/Frontend/FrontendActions.cpp
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Expand Up @@ -958,6 +958,7 @@ void CodeGenAction::runOptimizationPipeline(llvm::raw_pwrite_stream &os) {
si.getTimePasses().setOutStream(ci.getTimingStreamLLVM());
pto.LoopUnrolling = opts.UnrollLoops;
pto.LoopInterchange = opts.InterchangeLoops;
pto.LoopFusion = opts.FuseLoops;
pto.LoopInterleaving = opts.UnrollLoops;
pto.LoopVectorization = opts.VectorizeLoop;
pto.SLPVectorization = opts.VectorizeSLP;
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17 changes: 17 additions & 0 deletions flang/test/Driver/loop-fuse.f90
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@@ -0,0 +1,17 @@
! RUN: %flang -### -S -fexperimental-loop-fusion %s 2>&1 | FileCheck -check-prefix=CHECK-LOOP-FUSE %s
! RUN: %flang -### -S -fno-experimental-loop-fusion %s 2>&1 | FileCheck -check-prefix=CHECK-NO-LOOP-FUSE %s
! RUN: %flang -### -S -O0 %s 2>&1 | FileCheck -check-prefix=CHECK-NO-LOOP-FUSE %s
! RUN: %flang -### -S -O1 %s 2>&1 | FileCheck -check-prefix=CHECK-NO-LOOP-FUSE %s
! RUN: %flang -### -S -O2 %s 2>&1 | FileCheck -check-prefix=CHECK-NO-LOOP-FUSE %s
! RUN: %flang -### -S -O3 %s 2>&1 | FileCheck -check-prefix=CHECK-NO-LOOP-FUSE %s
! RUN: %flang -### -S -Os %s 2>&1 | FileCheck -check-prefix=CHECK-NO-LOOP-FUSE %s
! RUN: %flang -### -S -Oz %s 2>&1 | FileCheck -check-prefix=CHECK-NO-LOOP-FUSE %s
! CHECK-LOOP-FUSE: "-fexperimental-loop-fusion"
! CHECK-NO-LOOP-FUSE-NOT: "-fexperimental-loop-fusion"
! RUN: %flang_fc1 -emit-llvm -O2 -fexperimental-loop-fusion -mllvm -print-pipeline-passes -o /dev/null %s 2>&1 | FileCheck -check-prefix=CHECK-LOOP-FUSE-PASS %s
! RUN: %flang_fc1 -emit-llvm -O2 -fno-experimental-loop-fusion -mllvm -print-pipeline-passes -o /dev/null %s 2>&1 | FileCheck -check-prefix=CHECK-NO-LOOP-FUSE-PASS %s
! CHECK-LOOP-FUSE-PASS: loop-fusion
! CHECK-NO-LOOP-FUSE-PASS-NOT: loop-fusion

program test
end program
3 changes: 3 additions & 0 deletions llvm/include/llvm/Passes/PassBuilder.h
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Expand Up @@ -65,6 +65,9 @@ class PipelineTuningOptions {
/// false.
bool LoopInterchange;

/// Tuning option to enable/disable loop fusion. Its default value is false.
bool LoopFusion;
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As far as I can tell, this default is never actually set anywhere. There is no assignment in the PipelineTuningOptions ctor. (Probably that ctor should be replaced with direct struct member initialization...)

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In ThinLTOCodeGenerator.cpp, I need to do PTO.LoopFusion = False. Such change is also needed in mlir/lib/ExecutionEngine/OptUtils.cpp.

This is required at each site which is using PipelineTuningOptions. For LoopInterchange and others, initialization occurs in PassBuilderPipelines.cpp, but for LoopFusion, I can't do this because the definition of the LoopFusion variable is in NewPMDriver.cpp, as suggested by @kasuga-fj.

I am inclined to follow the way LoopInterchange is initialized (and thus move the definition of LoopFusion from NewPMDriver.cpp to PassBuilderPipelines.cpp)

What do you think?

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This is required at each site which is using PipelineTuningOptions. For LoopInterchange and others, initialization occurs in PassBuilderPipelines.cpp, but for LoopFusion, I can't do this because the definition of the LoopFusion variable is in NewPMDriver.cpp, as suggested by @kasuga-fj.

Why can't you do the initialization to false in PassBuilderPipelines? Isn't this just the default value that later gets overwritten by the code in NewPMDriver?

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Yes, that is another option. #158844. Can you please have a quick look?


/// Tuning option to forget all SCEV loops in LoopUnroll. Its default value
/// is that of the flag: `-forget-scev-loop-unroll`.
bool ForgetAllSCEVInLoopUnroll;
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8 changes: 7 additions & 1 deletion llvm/lib/Passes/PassBuilderPipelines.cpp
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Expand Up @@ -104,6 +104,7 @@
#include "llvm/Transforms/Scalar/LoopDeletion.h"
#include "llvm/Transforms/Scalar/LoopDistribute.h"
#include "llvm/Transforms/Scalar/LoopFlatten.h"
#include "llvm/Transforms/Scalar/LoopFuse.h"
#include "llvm/Transforms/Scalar/LoopIdiomRecognize.h"
#include "llvm/Transforms/Scalar/LoopInstSimplify.h"
#include "llvm/Transforms/Scalar/LoopInterchange.h"
Expand Down Expand Up @@ -1551,6 +1552,11 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
OptimizePM.addPass(createFunctionToLoopPassAdaptor(
std::move(LPM), /*UseMemorySSA=*/false, /*UseBlockFrequencyInfo=*/false));

// FIXME: This may not be the right place in the pipeline.
// We need to have the data to support the right place.
if (PTO.LoopFusion)
OptimizePM.addPass(LoopFusePass());

// Distribute loops to allow partial vectorization. I.e. isolate dependences
// into separate loop that would otherwise inhibit vectorization. This is
// currently only performed for loops marked with the metadata
Expand Down Expand Up @@ -2355,4 +2361,4 @@ AAManager PassBuilder::buildDefaultAAPipeline() {
bool PassBuilder::isInstrumentedPGOUse() const {
return (PGOOpt && PGOOpt->Action == PGOOptions::IRUse) ||
!UseCtxProfile.empty();
}
}
4 changes: 4 additions & 0 deletions llvm/tools/opt/NewPMDriver.cpp
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Expand Up @@ -60,6 +60,9 @@ cl::opt<bool> VerifyEachDebugInfoPreserve(
cl::desc("Start each pass with collecting and end it with checking of "
"debug info preservation."));

static cl::opt<bool> EnableLoopFusion("enable-loopfusion", cl::init(false),
cl::Hidden,
cl::desc("Enable the LoopFuse Pass"));
cl::opt<std::string>
VerifyDIPreserveExport("verify-di-preserve-export",
cl::desc("Export debug info preservation failures into "
Expand Down Expand Up @@ -446,6 +449,7 @@ bool llvm::runPassPipeline(
// option has been enabled.
PTO.LoopUnrolling = !DisableLoopUnrolling;
PTO.UnifiedLTO = UnifiedLTO;
PTO.LoopFusion = EnableLoopFusion;
PassBuilder PB(TM, PTO, P, &PIC);
registerEPCallbacks(PB);

Expand Down
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