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

sebpop · madhur13490 · web-flow · commit 895cda70a955 · 2025-09-15T18:48:32.000+05:30
This patch adds the flag -fexperimental-loop-fuse to the clang and flang
drivers. This is primarily useful for experiments as we envision to
enable the pass one day.

The options are based on the same principles and reason on which we have
`floop-interchange`.

---------

Co-authored-by: Madhur Amilkanthwar &lt;madhura@nvidia.com&gt;
diff --git a/clang/include/clang/Basic/CodeGenOptions.def b/clang/include/clang/Basic/CodeGenOptions.def
@@ -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.
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
@@ -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>,
diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
@@ -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;
@@ -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;
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -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);
 
diff --git a/clang/lib/Driver/ToolChains/Flang.cpp b/clang/lib/Driver/ToolChains/Flang.cpp
@@ -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);
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -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);
 
@@ -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));
 
diff --git a/clang/test/Driver/clang_f_opts.c b/clang/test/Driver/clang_f_opts.c
@@ -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
+// 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"
 
diff --git a/flang/docs/ReleaseNotes.md b/flang/docs/ReleaseNotes.md
@@ -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
diff --git a/flang/include/flang/Frontend/CodeGenOptions.def b/flang/include/flang/Frontend/CodeGenOptions.def
@@ -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
diff --git a/flang/lib/Frontend/CompilerInvocation.cpp b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -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;
 
diff --git a/flang/lib/Frontend/FrontendActions.cpp b/flang/lib/Frontend/FrontendActions.cpp
@@ -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;
diff --git a/flang/test/Driver/loop-fuse.f90 b/flang/test/Driver/loop-fuse.f90
@@ -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
diff --git a/llvm/include/llvm/Passes/PassBuilder.h b/llvm/include/llvm/Passes/PassBuilder.h
@@ -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;
diff --git a/llvm/lib/Passes/PassBuilderPipelines.cpp b/llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -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"
@@ -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
@@ -2355,4 +2361,4 @@ AAManager PassBuilder::buildDefaultAAPipeline() {
 bool PassBuilder::isInstrumentedPGOUse() const {
   return (PGOOpt && PGOOpt->Action == PGOOptions::IRUse) ||
          !UseCtxProfile.empty();
-}
+}
diff --git a/llvm/tools/opt/NewPMDriver.cpp b/llvm/tools/opt/NewPMDriver.cpp
@@ -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 "
@@ -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);
 
