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

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be9eb6a
add -floop-fuse to clang and flang
sebpop May 22, 2025
ea7ccc2
rename -floop-fuse / -fexperimental-fuse-loops
sebpop Jun 12, 2025
d1a5ac3
Address review comments
madhur13490 Jul 31, 2025
ae25025
change place of pass in pipeline
madhur13490 Aug 7, 2025
7a9780f
fixup! change place of pass in pipeline
madhur13490 Aug 8, 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 @@ -4268,6 +4268,10 @@ 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]>;
def fexperimental_loop_fusion : Flag<["-"], "fexperimental-loop-fusion">, Group<f_Group>,
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We can use OptInCC1FFlag so that the negative form is not a clang -cc1 option.

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Done

HelpText<"Enable the experimental loop fusion pass">, Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>;
def fno_experimental_loop_fusion: Flag<["-"], "fno-experimental-loop-fusion">, Group<f_Group>,
HelpText<"Disable the experimental loop fusion pass">, Visibility<[ClangOption, CC1Option, 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 @@ -897,6 +897,7 @@ 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 @@ -1332,6 +1333,7 @@ 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 @@ -6870,6 +6870,8 @@ 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.AddLastArg(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.AddLastArg(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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7 changes: 7 additions & 0 deletions clang/lib/Frontend/CompilerInvocation.cpp
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Expand Up @@ -1671,6 +1671,11 @@ void CompilerInvocationBase::GenerateCodeGenArgs(const CodeGenOptions &Opts,
else
GenerateArg(Consumer, OPT_fno_loop_interchange);

if (Opts.FuseLoops)
GenerateArg(Consumer, OPT_fexperimental_loop_fusion);
else
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If OPT_fno_experimental_loop_fusion is not a cc1 option, the else branch will not be needed.

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Thanks for letting me know!

GenerateArg(Consumer, OPT_fno_experimental_loop_fusion);

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

Expand Down Expand Up @@ -1991,6 +1996,8 @@ 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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7 changes: 7 additions & 0 deletions clang/test/Driver/clang_f_opts.c
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Expand Up @@ -52,6 +52,13 @@
// 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
// RUN: %clang -### -S -fno-experimental-loop-fusion %s 2>&1 | FileCheck -check-prefix=CHECK-NO-FUSE-LOOPS %s
// RUN: %clang -### -S -fno-experimental-loop-fusion -fexperimental-loop-fusion %s 2>&1 | FileCheck -check-prefix=CHECK-FUSE-LOOPS %s
// RUN: %clang -### -S -fexperimental-loop-fusion -fno-experimental-loop-fusion %s 2>&1 | FileCheck -check-prefix=CHECK-NO-FUSE-LOOPS %s
// CHECK-FUSE-LOOPS: "-fexperimental-loop-fusion"
// CHECK-NO-FUSE-LOOPS: "-fno-experimental-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 @@ -32,6 +32,8 @@ 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 @@ -275,6 +275,9 @@ 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 @@ -951,6 +951,7 @@ 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;

/// 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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13 changes: 12 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 @@ -204,6 +205,10 @@ static cl::opt<bool>
EnableLoopInterchange("enable-loopinterchange", cl::init(false), cl::Hidden,
cl::desc("Enable the LoopInterchange Pass"));

static cl::opt<bool> EnableLoopFusion("enable-loopfusion", cl::init(false),
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I still don't quite understand why you want to add both a clang flag and an opt flag. But if that's the intention, EnableLoopFusino should be defined in tools/opt/NewPMDriver.cpp, not here. The same applies to EnableLoopInterchange. IIUC, declaring this here allows specifying like clang -mllvm -enable-loopfusion, but it will be overwritten after PipelineTuningOptions's ctor is invoked.

On the other hand, options defined in tools/opt/NewPMDriver.cpp aren't recognized by the clang driver, IIUC.

But again, I'm not sure why clang/flang flags are really needed. Why isn't the opt flag sufficient? IMHO, at the very least, you should clarify the reason.

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As I said, I am taking over this patch from Sebastian. Your comment makes me think.

The primary purpose of this patch is to user-facing option to allow turning fusion on or off. Not everyone is aware of the options provided by developer tools like opt.

I see the precedence in the code. In flang/include/flang/Frontend/CodegenOptions.def I see options for loop-versioning, unrolling etc.
I see clang and flang differ here. clang doesn't seem to have such options except for floop-interchange.

floop-interchange was introduced in #125830

Having fexperimental-loop-fusion is more of syntactic sugar and convenience is based on the same principles of #125830.

I can remove the change from PassBuilderPipelines.cpp i.e. EnableFusion, as it can be redundant.

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I see the precedence in the code. In flang/include/flang/Frontend/CodegenOptions.def I see options for loop-versioning, unrolling etc. I see clang and flang differ here. clang doesn't seem to have such options except for floop-interchange.

I didn't really understand what you meant... There is a clang option -funroll-loops.

I can remove the change from PassBuilderPipelines.cpp i.e. EnableFusion, as it can be redundant.

I didn't mean it is redundant. It might be useful if we want to turn on/off the loop-fusion via opt command. What I meant is, if you also want to add both clang/flang and opt options, then you should move EnableFusion into proper place (maybe it is tools/opt/NewPMDriver.cpp).

At the moment, I don’t have any strong objections to adding experimental flags to clang, although I'm not sure if there's an existing policy on clang side for such cases.

cl::Hidden,
cl::desc("Enable the LoopFuse Pass"));

static cl::opt<bool> EnableUnrollAndJam("enable-unroll-and-jam",
cl::init(false), cl::Hidden,
cl::desc("Enable Unroll And Jam Pass"));
Expand Down Expand Up @@ -313,6 +318,7 @@ PipelineTuningOptions::PipelineTuningOptions() {
SLPVectorization = false;
LoopUnrolling = true;
LoopInterchange = EnableLoopInterchange;
LoopFusion = EnableLoopFusion;
ForgetAllSCEVInLoopUnroll = ForgetSCEVInLoopUnroll;
LicmMssaOptCap = SetLicmMssaOptCap;
LicmMssaNoAccForPromotionCap = SetLicmMssaNoAccForPromotionCap;
Expand Down Expand Up @@ -1548,6 +1554,11 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
if (PTO.LoopInterchange)
LPM.addPass(LoopInterchangePass());

// 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());

OptimizePM.addPass(createFunctionToLoopPassAdaptor(
std::move(LPM), /*UseMemorySSA=*/false, /*UseBlockFrequencyInfo=*/false));

Expand Down Expand Up @@ -2355,4 +2366,4 @@ AAManager PassBuilder::buildDefaultAAPipeline() {
bool PassBuilder::isInstrumentedPGOUse() const {
return (PGOOpt && PGOOpt->Action == PGOOptions::IRUse) ||
!UseCtxProfile.empty();
}
}
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