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@sebpop sebpop commented Jun 3, 2025

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.

@sebpop sebpop requested a review from sjoerdmeijer June 3, 2025 22:49
@llvmbot llvmbot added clang Clang issues not falling into any other category clang:driver 'clang' and 'clang++' user-facing binaries. Not 'clang-cl' clang:frontend Language frontend issues, e.g. anything involving "Sema" clang:codegen IR generation bugs: mangling, exceptions, etc. flang:driver flang Flang issues not falling into any other category labels Jun 3, 2025
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llvmbot commented Jun 3, 2025

@llvm/pr-subscribers-flang-driver
@llvm/pr-subscribers-clang-codegen

@llvm/pr-subscribers-clang

Author: Sebastian Pop (sebpop)

Changes

This adds the flag -floop-fuse to the clang and flang drivers.


Full diff: https://github.com/llvm/llvm-project/pull/142686.diff

15 Files Affected:

  • (modified) clang/include/clang/Basic/CodeGenOptions.def (+1)
  • (modified) clang/include/clang/Driver/Options.td (+4)
  • (modified) clang/lib/CodeGen/BackendUtil.cpp (+2)
  • (modified) clang/lib/Driver/ToolChains/Clang.cpp (+1)
  • (modified) clang/lib/Driver/ToolChains/CommonArgs.cpp (+1-1)
  • (modified) clang/lib/Driver/ToolChains/Flang.cpp (+2)
  • (modified) clang/lib/Frontend/CompilerInvocation.cpp (+7)
  • (modified) clang/test/Driver/clang_f_opts.c (+7)
  • (modified) flang/docs/ReleaseNotes.md (+1)
  • (modified) flang/include/flang/Frontend/CodeGenOptions.def (+1)
  • (modified) flang/lib/Frontend/CompilerInvocation.cpp (+3)
  • (modified) flang/lib/Frontend/FrontendActions.cpp (+1)
  • (added) flang/test/Driver/loop-fuse.f90 (+17)
  • (modified) llvm/include/llvm/Passes/PassBuilder.h (+3)
  • (modified) llvm/lib/Passes/PassBuilderPipelines.cpp (+12-1)
diff --git a/clang/include/clang/Basic/CodeGenOptions.def b/clang/include/clang/Basic/CodeGenOptions.def
index aad4e107cbeb3..de40564bc280f 100644
--- a/clang/include/clang/Basic/CodeGenOptions.def
+++ b/clang/include/clang/Basic/CodeGenOptions.def
@@ -326,6 +326,7 @@ CODEGENOPT(TimeTrace         , 1, 0) ///< Set when -ftime-trace is enabled.
 VALUE_CODEGENOPT(TimeTraceGranularity, 32, 500) ///< Minimum time granularity (in microseconds),
                                                ///< traced by time profiler
 CODEGENOPT(InterchangeLoops  , 1, 0) ///< Run loop-interchange.
+CODEGENOPT(FuseLoops         , 1, 0) ///< Run loop-fuse.
 CODEGENOPT(UnrollLoops       , 1, 0) ///< Control whether loops are unrolled.
 CODEGENOPT(RerollLoops       , 1, 0) ///< Control whether loops are rerolled.
 CODEGENOPT(NoUseJumpTables   , 1, 0) ///< Set when -fno-jump-tables is enabled.
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index 5ca31c253ed8f..e5e2c0035e03f 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -4189,6 +4189,10 @@ 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 floop_fuse : Flag<["-"], "floop-fuse">, Group<f_Group>,
+  HelpText<"Enable the loop fuse pass">, Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>;
+def fno_loop_fuse: Flag<["-"], "fno-loop-fuse">, Group<f_Group>,
+  HelpText<"Disable the loop fuse 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>,
diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index cd5fc48c4a22b..539e413e8c86e 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -898,6 +898,7 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
   PipelineTuningOptions PTO;
   PTO.LoopUnrolling = CodeGenOpts.UnrollLoops;
   PTO.LoopInterchange = CodeGenOpts.InterchangeLoops;
+  PTO.LoopFuse = CodeGenOpts.FuseLoops;
   // For historical reasons, loop interleaving is set to mirror setting for loop
   // unrolling.
   PTO.LoopInterleaving = CodeGenOpts.UnrollLoops;
@@ -1339,6 +1340,7 @@ runThinLTOBackend(CompilerInstance &CI, ModuleSummaryIndex *CombinedIndex,
   Conf.SampleProfile = std::move(SampleProfile);
   Conf.PTO.LoopUnrolling = CGOpts.UnrollLoops;
   Conf.PTO.LoopInterchange = CGOpts.InterchangeLoops;
+  Conf.PTO.LoopFuse = 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
index 13842b8cc2870..f3713e12cb5ef 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -7030,6 +7030,7 @@ 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_floop_fuse, options::OPT_fno_loop_fuse);
 
   Args.AddLastArg(CmdArgs, options::OPT_fstrict_flex_arrays_EQ);
 
diff --git a/clang/lib/Driver/ToolChains/CommonArgs.cpp b/clang/lib/Driver/ToolChains/CommonArgs.cpp
index 937ee09cac7cc..4217b48a56b05 100644
--- a/clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ b/clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -3157,7 +3157,7 @@ void tools::handleVectorizeSLPArgs(const ArgList &Args,
 
 void tools::handleInterchangeLoopsArgs(const ArgList &Args,
                                        ArgStringList &CmdArgs) {
-  // FIXME: instead of relying on shouldEnableVectorizerAtOLevel, we may want to
+  // FIXME: Instead of relying on shouldEnableVectorizerAtOLevel, we may want to
   // implement a separate function to infer loop interchange from opt level.
   // For now, enable loop-interchange at the same opt levels as loop-vectorize.
   bool EnableInterchange = shouldEnableVectorizerAtOLevel(Args, false);
diff --git a/clang/lib/Driver/ToolChains/Flang.cpp b/clang/lib/Driver/ToolChains/Flang.cpp
index dcc46469df3e9..f34a58cab3ff3 100644
--- a/clang/lib/Driver/ToolChains/Flang.cpp
+++ b/clang/lib/Driver/ToolChains/Flang.cpp
@@ -151,6 +151,8 @@ void Flang::addCodegenOptions(const ArgList &Args,
       !stackArrays->getOption().matches(options::OPT_fno_stack_arrays))
     CmdArgs.push_back("-fstack-arrays");
 
+  Args.AddLastArg(CmdArgs, options::OPT_floop_fuse, options::OPT_fno_loop_fuse);
+
   handleInterchangeLoopsArgs(Args, CmdArgs);
   handleVectorizeLoopsArgs(Args, CmdArgs);
   handleVectorizeSLPArgs(Args, CmdArgs);
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 2c02719121c73..5506e11f51491 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1648,6 +1648,11 @@ void CompilerInvocationBase::GenerateCodeGenArgs(const CodeGenOptions &Opts,
   else
     GenerateArg(Consumer, OPT_fno_loop_interchange);
 
+  if (Opts.FuseLoops)
+    GenerateArg(Consumer, OPT_floop_fuse);
+  else
+    GenerateArg(Consumer, OPT_fno_loop_fuse);
+
   if (!Opts.BinutilsVersion.empty())
     GenerateArg(Consumer, OPT_fbinutils_version_EQ, Opts.BinutilsVersion);
 
@@ -1963,6 +1968,8 @@ 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_floop_fuse, OPT_fno_loop_fuse, 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
index ee7ded265769b..204d5e338190f 100644
--- a/clang/test/Driver/clang_f_opts.c
+++ b/clang/test/Driver/clang_f_opts.c
@@ -52,6 +52,13 @@
 // CHECK-INTERCHANGE-LOOPS: "-floop-interchange"
 // CHECK-NO-INTERCHANGE-LOOPS: "-fno-loop-interchange"
 
+// RUN: %clang -### -S -floop-fuse %s 2>&1 | FileCheck -check-prefix=CHECK-FUSE-LOOPS %s
+// RUN: %clang -### -S -fno-loop-fuse %s 2>&1 | FileCheck -check-prefix=CHECK-NO-FUSE-LOOPS %s
+// RUN: %clang -### -S -fno-loop-fuse -floop-fuse %s 2>&1 | FileCheck -check-prefix=CHECK-FUSE-LOOPS %s
+// RUN: %clang -### -S -floop-fuse -fno-loop-fuse %s 2>&1 | FileCheck -check-prefix=CHECK-NO-FUSE-LOOPS %s
+// CHECK-FUSE-LOOPS: "-floop-fuse"
+// CHECK-NO-FUSE-LOOPS: "-fno-loop-fuse"
+
 // 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
index 36be369595ffd..33795b7d1a55a 100644
--- a/flang/docs/ReleaseNotes.md
+++ b/flang/docs/ReleaseNotes.md
@@ -34,6 +34,7 @@ page](https://llvm.org/releases/).
 
 * -floop-interchange is now recognized by flang.
 * -floop-interchange is enabled by default at -O2 and above.
+* -floop-fuse is now recognized by flang.
 
 ## Windows Support
 
diff --git a/flang/include/flang/Frontend/CodeGenOptions.def b/flang/include/flang/Frontend/CodeGenOptions.def
index a697872836569..85bec620b54ba 100644
--- a/flang/include/flang/Frontend/CodeGenOptions.def
+++ b/flang/include/flang/Frontend/CodeGenOptions.def
@@ -36,6 +36,7 @@ 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 fuse.
 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
index 90a002929eff0..c2b63819e9f77 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -273,6 +273,9 @@ 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_floop_fuse))
+    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
index 012d0fdfe645f..dc56cbe5c85bc 100644
--- a/flang/lib/Frontend/FrontendActions.cpp
+++ b/flang/lib/Frontend/FrontendActions.cpp
@@ -925,6 +925,7 @@ void CodeGenAction::runOptimizationPipeline(llvm::raw_pwrite_stream &os) {
     si.getTimePasses().setOutStream(ci.getTimingStreamLLVM());
   pto.LoopUnrolling = opts.UnrollLoops;
   pto.LoopInterchange = opts.InterchangeLoops;
+  pto.LoopFuse = 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
new file mode 100644
index 0000000000000..240d00fdb62d7
--- /dev/null
+++ b/flang/test/Driver/loop-fuse.f90
@@ -0,0 +1,17 @@
+! RUN: %flang -### -S -floop-fuse %s 2>&1 | FileCheck -check-prefix=CHECK-LOOP-FUSE %s
+! RUN: %flang -### -S -fno-loop-fuse %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: "-floop-fuse"
+! CHECK-NO-LOOP-FUSE-NOT: "-floop-fuse"
+! RUN: %flang_fc1 -emit-llvm -O2 -floop-fuse -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-loop-fuse -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
index 51ccaa53447d7..287a57c7d4a7d 100644
--- a/llvm/include/llvm/Passes/PassBuilder.h
+++ b/llvm/include/llvm/Passes/PassBuilder.h
@@ -64,6 +64,9 @@ class PipelineTuningOptions {
   /// false.
   bool LoopInterchange;
 
+  /// Tuning option to enable/disable loop fuse. Its default value is false.
+  bool LoopFuse;
+
   /// 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
index f3654600c5abb..aaf43a9b535e9 100644
--- 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"
@@ -205,6 +206,10 @@ static cl::opt<bool>
     EnableLoopInterchange("enable-loopinterchange", cl::init(false), cl::Hidden,
                           cl::desc("Enable the LoopInterchange Pass"));
 
+static cl::opt<bool> EnableLoopFuse("enable-loopfuse", cl::init(false),
+                                    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"));
@@ -314,6 +319,7 @@ PipelineTuningOptions::PipelineTuningOptions() {
   SLPVectorization = false;
   LoopUnrolling = true;
   LoopInterchange = EnableLoopInterchange;
+  LoopFuse = EnableLoopFuse;
   ForgetAllSCEVInLoopUnroll = ForgetSCEVInLoopUnroll;
   LicmMssaOptCap = SetLicmMssaOptCap;
   LicmMssaNoAccForPromotionCap = SetLicmMssaNoAccForPromotionCap;
@@ -518,6 +524,9 @@ PassBuilder::buildO1FunctionSimplificationPipeline(OptimizationLevel Level,
 
   invokeLoopOptimizerEndEPCallbacks(LPM2, Level);
 
+  if (PTO.LoopFuse)
+    FPM.addPass(LoopFusePass());
+
   FPM.addPass(createFunctionToLoopPassAdaptor(std::move(LPM1),
                                               /*UseMemorySSA=*/true,
                                               /*UseBlockFrequencyInfo=*/true));
@@ -709,6 +718,9 @@ PassBuilder::buildFunctionSimplificationPipeline(OptimizationLevel Level,
 
   invokeLoopOptimizerEndEPCallbacks(LPM2, Level);
 
+  if (PTO.LoopFuse)
+    FPM.addPass(LoopFusePass());
+
   FPM.addPass(createFunctionToLoopPassAdaptor(std::move(LPM1),
                                               /*UseMemorySSA=*/true,
                                               /*UseBlockFrequencyInfo=*/true));
@@ -2112,7 +2124,6 @@ PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
     LPM.addPass(LoopFlattenPass());
   LPM.addPass(IndVarSimplifyPass());
   LPM.addPass(LoopDeletionPass());
-  // FIXME: Add loop interchange.
 
   // Unroll small loops and perform peeling.
   LPM.addPass(LoopFullUnrollPass(Level.getSpeedupLevel(),

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llvmbot commented Jun 3, 2025

@llvm/pr-subscribers-clang-driver

Author: Sebastian Pop (sebpop)

Changes

This adds the flag -floop-fuse to the clang and flang drivers.


Full diff: https://github.com/llvm/llvm-project/pull/142686.diff

15 Files Affected:

  • (modified) clang/include/clang/Basic/CodeGenOptions.def (+1)
  • (modified) clang/include/clang/Driver/Options.td (+4)
  • (modified) clang/lib/CodeGen/BackendUtil.cpp (+2)
  • (modified) clang/lib/Driver/ToolChains/Clang.cpp (+1)
  • (modified) clang/lib/Driver/ToolChains/CommonArgs.cpp (+1-1)
  • (modified) clang/lib/Driver/ToolChains/Flang.cpp (+2)
  • (modified) clang/lib/Frontend/CompilerInvocation.cpp (+7)
  • (modified) clang/test/Driver/clang_f_opts.c (+7)
  • (modified) flang/docs/ReleaseNotes.md (+1)
  • (modified) flang/include/flang/Frontend/CodeGenOptions.def (+1)
  • (modified) flang/lib/Frontend/CompilerInvocation.cpp (+3)
  • (modified) flang/lib/Frontend/FrontendActions.cpp (+1)
  • (added) flang/test/Driver/loop-fuse.f90 (+17)
  • (modified) llvm/include/llvm/Passes/PassBuilder.h (+3)
  • (modified) llvm/lib/Passes/PassBuilderPipelines.cpp (+12-1)
diff --git a/clang/include/clang/Basic/CodeGenOptions.def b/clang/include/clang/Basic/CodeGenOptions.def
index aad4e107cbeb3..de40564bc280f 100644
--- a/clang/include/clang/Basic/CodeGenOptions.def
+++ b/clang/include/clang/Basic/CodeGenOptions.def
@@ -326,6 +326,7 @@ CODEGENOPT(TimeTrace         , 1, 0) ///< Set when -ftime-trace is enabled.
 VALUE_CODEGENOPT(TimeTraceGranularity, 32, 500) ///< Minimum time granularity (in microseconds),
                                                ///< traced by time profiler
 CODEGENOPT(InterchangeLoops  , 1, 0) ///< Run loop-interchange.
+CODEGENOPT(FuseLoops         , 1, 0) ///< Run loop-fuse.
 CODEGENOPT(UnrollLoops       , 1, 0) ///< Control whether loops are unrolled.
 CODEGENOPT(RerollLoops       , 1, 0) ///< Control whether loops are rerolled.
 CODEGENOPT(NoUseJumpTables   , 1, 0) ///< Set when -fno-jump-tables is enabled.
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index 5ca31c253ed8f..e5e2c0035e03f 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -4189,6 +4189,10 @@ 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 floop_fuse : Flag<["-"], "floop-fuse">, Group<f_Group>,
+  HelpText<"Enable the loop fuse pass">, Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>;
+def fno_loop_fuse: Flag<["-"], "fno-loop-fuse">, Group<f_Group>,
+  HelpText<"Disable the loop fuse 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>,
diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index cd5fc48c4a22b..539e413e8c86e 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -898,6 +898,7 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
   PipelineTuningOptions PTO;
   PTO.LoopUnrolling = CodeGenOpts.UnrollLoops;
   PTO.LoopInterchange = CodeGenOpts.InterchangeLoops;
+  PTO.LoopFuse = CodeGenOpts.FuseLoops;
   // For historical reasons, loop interleaving is set to mirror setting for loop
   // unrolling.
   PTO.LoopInterleaving = CodeGenOpts.UnrollLoops;
@@ -1339,6 +1340,7 @@ runThinLTOBackend(CompilerInstance &CI, ModuleSummaryIndex *CombinedIndex,
   Conf.SampleProfile = std::move(SampleProfile);
   Conf.PTO.LoopUnrolling = CGOpts.UnrollLoops;
   Conf.PTO.LoopInterchange = CGOpts.InterchangeLoops;
+  Conf.PTO.LoopFuse = 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
index 13842b8cc2870..f3713e12cb5ef 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -7030,6 +7030,7 @@ 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_floop_fuse, options::OPT_fno_loop_fuse);
 
   Args.AddLastArg(CmdArgs, options::OPT_fstrict_flex_arrays_EQ);
 
diff --git a/clang/lib/Driver/ToolChains/CommonArgs.cpp b/clang/lib/Driver/ToolChains/CommonArgs.cpp
index 937ee09cac7cc..4217b48a56b05 100644
--- a/clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ b/clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -3157,7 +3157,7 @@ void tools::handleVectorizeSLPArgs(const ArgList &Args,
 
 void tools::handleInterchangeLoopsArgs(const ArgList &Args,
                                        ArgStringList &CmdArgs) {
-  // FIXME: instead of relying on shouldEnableVectorizerAtOLevel, we may want to
+  // FIXME: Instead of relying on shouldEnableVectorizerAtOLevel, we may want to
   // implement a separate function to infer loop interchange from opt level.
   // For now, enable loop-interchange at the same opt levels as loop-vectorize.
   bool EnableInterchange = shouldEnableVectorizerAtOLevel(Args, false);
diff --git a/clang/lib/Driver/ToolChains/Flang.cpp b/clang/lib/Driver/ToolChains/Flang.cpp
index dcc46469df3e9..f34a58cab3ff3 100644
--- a/clang/lib/Driver/ToolChains/Flang.cpp
+++ b/clang/lib/Driver/ToolChains/Flang.cpp
@@ -151,6 +151,8 @@ void Flang::addCodegenOptions(const ArgList &Args,
       !stackArrays->getOption().matches(options::OPT_fno_stack_arrays))
     CmdArgs.push_back("-fstack-arrays");
 
+  Args.AddLastArg(CmdArgs, options::OPT_floop_fuse, options::OPT_fno_loop_fuse);
+
   handleInterchangeLoopsArgs(Args, CmdArgs);
   handleVectorizeLoopsArgs(Args, CmdArgs);
   handleVectorizeSLPArgs(Args, CmdArgs);
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 2c02719121c73..5506e11f51491 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1648,6 +1648,11 @@ void CompilerInvocationBase::GenerateCodeGenArgs(const CodeGenOptions &Opts,
   else
     GenerateArg(Consumer, OPT_fno_loop_interchange);
 
+  if (Opts.FuseLoops)
+    GenerateArg(Consumer, OPT_floop_fuse);
+  else
+    GenerateArg(Consumer, OPT_fno_loop_fuse);
+
   if (!Opts.BinutilsVersion.empty())
     GenerateArg(Consumer, OPT_fbinutils_version_EQ, Opts.BinutilsVersion);
 
@@ -1963,6 +1968,8 @@ 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_floop_fuse, OPT_fno_loop_fuse, 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
index ee7ded265769b..204d5e338190f 100644
--- a/clang/test/Driver/clang_f_opts.c
+++ b/clang/test/Driver/clang_f_opts.c
@@ -52,6 +52,13 @@
 // CHECK-INTERCHANGE-LOOPS: "-floop-interchange"
 // CHECK-NO-INTERCHANGE-LOOPS: "-fno-loop-interchange"
 
+// RUN: %clang -### -S -floop-fuse %s 2>&1 | FileCheck -check-prefix=CHECK-FUSE-LOOPS %s
+// RUN: %clang -### -S -fno-loop-fuse %s 2>&1 | FileCheck -check-prefix=CHECK-NO-FUSE-LOOPS %s
+// RUN: %clang -### -S -fno-loop-fuse -floop-fuse %s 2>&1 | FileCheck -check-prefix=CHECK-FUSE-LOOPS %s
+// RUN: %clang -### -S -floop-fuse -fno-loop-fuse %s 2>&1 | FileCheck -check-prefix=CHECK-NO-FUSE-LOOPS %s
+// CHECK-FUSE-LOOPS: "-floop-fuse"
+// CHECK-NO-FUSE-LOOPS: "-fno-loop-fuse"
+
 // 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
index 36be369595ffd..33795b7d1a55a 100644
--- a/flang/docs/ReleaseNotes.md
+++ b/flang/docs/ReleaseNotes.md
@@ -34,6 +34,7 @@ page](https://llvm.org/releases/).
 
 * -floop-interchange is now recognized by flang.
 * -floop-interchange is enabled by default at -O2 and above.
+* -floop-fuse is now recognized by flang.
 
 ## Windows Support
 
diff --git a/flang/include/flang/Frontend/CodeGenOptions.def b/flang/include/flang/Frontend/CodeGenOptions.def
index a697872836569..85bec620b54ba 100644
--- a/flang/include/flang/Frontend/CodeGenOptions.def
+++ b/flang/include/flang/Frontend/CodeGenOptions.def
@@ -36,6 +36,7 @@ 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 fuse.
 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
index 90a002929eff0..c2b63819e9f77 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -273,6 +273,9 @@ 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_floop_fuse))
+    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
index 012d0fdfe645f..dc56cbe5c85bc 100644
--- a/flang/lib/Frontend/FrontendActions.cpp
+++ b/flang/lib/Frontend/FrontendActions.cpp
@@ -925,6 +925,7 @@ void CodeGenAction::runOptimizationPipeline(llvm::raw_pwrite_stream &os) {
     si.getTimePasses().setOutStream(ci.getTimingStreamLLVM());
   pto.LoopUnrolling = opts.UnrollLoops;
   pto.LoopInterchange = opts.InterchangeLoops;
+  pto.LoopFuse = 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
new file mode 100644
index 0000000000000..240d00fdb62d7
--- /dev/null
+++ b/flang/test/Driver/loop-fuse.f90
@@ -0,0 +1,17 @@
+! RUN: %flang -### -S -floop-fuse %s 2>&1 | FileCheck -check-prefix=CHECK-LOOP-FUSE %s
+! RUN: %flang -### -S -fno-loop-fuse %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: "-floop-fuse"
+! CHECK-NO-LOOP-FUSE-NOT: "-floop-fuse"
+! RUN: %flang_fc1 -emit-llvm -O2 -floop-fuse -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-loop-fuse -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
index 51ccaa53447d7..287a57c7d4a7d 100644
--- a/llvm/include/llvm/Passes/PassBuilder.h
+++ b/llvm/include/llvm/Passes/PassBuilder.h
@@ -64,6 +64,9 @@ class PipelineTuningOptions {
   /// false.
   bool LoopInterchange;
 
+  /// Tuning option to enable/disable loop fuse. Its default value is false.
+  bool LoopFuse;
+
   /// 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
index f3654600c5abb..aaf43a9b535e9 100644
--- 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"
@@ -205,6 +206,10 @@ static cl::opt<bool>
     EnableLoopInterchange("enable-loopinterchange", cl::init(false), cl::Hidden,
                           cl::desc("Enable the LoopInterchange Pass"));
 
+static cl::opt<bool> EnableLoopFuse("enable-loopfuse", cl::init(false),
+                                    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"));
@@ -314,6 +319,7 @@ PipelineTuningOptions::PipelineTuningOptions() {
   SLPVectorization = false;
   LoopUnrolling = true;
   LoopInterchange = EnableLoopInterchange;
+  LoopFuse = EnableLoopFuse;
   ForgetAllSCEVInLoopUnroll = ForgetSCEVInLoopUnroll;
   LicmMssaOptCap = SetLicmMssaOptCap;
   LicmMssaNoAccForPromotionCap = SetLicmMssaNoAccForPromotionCap;
@@ -518,6 +524,9 @@ PassBuilder::buildO1FunctionSimplificationPipeline(OptimizationLevel Level,
 
   invokeLoopOptimizerEndEPCallbacks(LPM2, Level);
 
+  if (PTO.LoopFuse)
+    FPM.addPass(LoopFusePass());
+
   FPM.addPass(createFunctionToLoopPassAdaptor(std::move(LPM1),
                                               /*UseMemorySSA=*/true,
                                               /*UseBlockFrequencyInfo=*/true));
@@ -709,6 +718,9 @@ PassBuilder::buildFunctionSimplificationPipeline(OptimizationLevel Level,
 
   invokeLoopOptimizerEndEPCallbacks(LPM2, Level);
 
+  if (PTO.LoopFuse)
+    FPM.addPass(LoopFusePass());
+
   FPM.addPass(createFunctionToLoopPassAdaptor(std::move(LPM1),
                                               /*UseMemorySSA=*/true,
                                               /*UseBlockFrequencyInfo=*/true));
@@ -2112,7 +2124,6 @@ PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
     LPM.addPass(LoopFlattenPass());
   LPM.addPass(IndVarSimplifyPass());
   LPM.addPass(LoopDeletionPass());
-  // FIXME: Add loop interchange.
 
   // Unroll small loops and perform peeling.
   LPM.addPass(LoopFullUnrollPass(Level.getSpeedupLevel(),

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github-actions bot commented Jun 3, 2025

✅ With the latest revision this PR passed the C/C++ code formatter.

@efriedma-quic
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If the goal here is gcc compatibility, I'd suggest not hooking up the flag to anything; the existing LoopFusePass isn't used by anything and hasn't been touched in years, so it's very likely to have issues.

@sebpop
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sebpop commented Jun 10, 2025

The goal is to make it easier to turn on loop fusion and to collect and fix bug reports against loop fusion. Once we are happy with the stability of the pass the goal is to enable by default in flang at -O2 and above.

GCC does not have an equivalent for -floop-fuse.

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Flang changes LGTM.

nit: I am no clang expert but I wonder if you could use MarshallingInfoFlag in Options.td to avoid having to set CGOpts.FuseLoops manually for clang (this is not supported for flang unfortunately). If people more familiar with clang are happy with it how it is then that is fine by me.

Please wait for somebody else to review this from a clang/ and llvm/ perspective.

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I don't think we should expose clang driver options for passes that are known to have significant issues.

If you want to add a cl::opt flag to allow scheduling this in the pipeline and accessible for early testing via -mllvm, that would be a different matter.

If we really want a clang driver option, then it should be named -fexperimental-fuse-loops.

@tarunprabhu
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If we really want a clang driver option, then it should be named -fexperimental-fuse-loops.

I think this is reasonable even for flang.

@@ -34,6 +34,7 @@ page](https://llvm.org/releases/).

* -floop-interchange is now recognized by flang.
* -floop-interchange is enabled by default at -O2 and above.
* -fexperimental-fuse-loops is now recognized by flang.
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We should make a similar entry in clang/docs/ReleaseNotes.md.

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I can confirm that this is useful for testing purposes.
If folks are happy with the 'experimental' prefix, let's do that, LGTM.

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So, why wouldn't adding a cl::opt flag work in the end? In fact, you've added enable-loopfuse flag in this patch. If it's enough, I personally think it might be better to avoid introducing additional flags unnecessarily.

@@ -2112,7 +2124,6 @@ PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
LPM.addPass(LoopFlattenPass());
LPM.addPass(IndVarSimplifyPass());
LPM.addPass(LoopDeletionPass());
// FIXME: Add loop interchange.
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I don't think this patch resolves this comment.

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Done

Comment on lines 527 to 529
if (PTO.LoopFuse)
FPM.addPass(LoopFusePass());

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Is this an appropriate place? I think this is an optimization pass rather than simplification one.

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I have moved it to the function simplification pipeline. LoopFusion is a function pass. I am open to change the place if this doesn't feel alright.

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LoopFusion is a function pass

I don't think this implies that the pass should be part of the function simplification pipeline. Its position should be determined by what the pass actually does. The appropriate placement likely depends on what you're trying to achieve with it.

// FIXME: instead of relying on shouldEnableVectorizerAtOLevel, we may want to
// FIXME: Instead of relying on shouldEnableVectorizerAtOLevel, we may want to
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nit: unrelated change.

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Done

@madhur13490
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I am taking over this change. Trying to address review comments.

@kasuga-fj
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You probably made a mistake with the rebase...

@tarunprabhu
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What do you think is the right place in pass pipeline?
I don't know, but just a gut feeling, somewhere in buildModuleOptimizationPipeline looks reasonable to me rather than buildFunctionSimplificationPipeline.
Yeah, I don't have enough data to support the right place in the pipeline.

In that case. it may be better to move it to buildModuleOptimizationPipeline. This is undoubtedly an optimization, and a non-trivial one at that. There are still open questions about the exact placement of the pass within the optimization pipeline, but moving it into that function, with a (perhaps slightly tweaked) FIXME comment is probably better than leaving it in buildFunctionSimplificationPipeline.

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In that case. it may be better to move it to buildModuleOptimizationPipeline. This is undoubtedly an optimization, and a non-trivial one at that. There are still open questions about the exact placement of the pass within the optimization pipeline, but moving it into that function, with a (perhaps slightly tweaked) FIXME comment is probably better than leaving it in buildFunctionSimplificationPipeline.

+1 on this.

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In that case. it may be better to move it to buildModuleOptimizationPipeline. This is undoubtedly an optimization, and a non-trivial one at that. There are still open questions about the exact placement of the pass within the optimization pipeline, but moving it into that function, with a (perhaps slightly tweaked) FIXME comment is probably better than leaving it in buildFunctionSimplificationPipeline.

+1 on this.

Done. I have placed in the suggested function, just after LoopInterchange (with FIXME)

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Just in case: it's not "just after LoopInterchange", it's "before LoopDeletion". If you want to run it just after LoopInterchange, please add LoopFuse after adding LPM to OptimizePM. It might be better adding some test to check the position.

Also, you may want to avoid rebasing and force pushing when possible (see Updating Pull Requests).

@@ -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

@@ -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!

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madhur13490 commented Aug 11, 2025

Just in case: it's not "just after LoopInterchange", it's "before LoopDeletion". If you want to run it just after LoopInterchange, please add LoopFuse after adding LPM to OptimizePM. It might be better adding some test to check the position.

Yes, realized this after I pushed. Regarding the test, do you mean clang/flang test or a opt test with -enable-loopfusion?
In the latest revision, I have added a clang test.

Also, you may want to avoid rebasing and force pushing when possible (see Updating Pull Requests).

I prefer to test my changes with the trunk each time I push. Testing with trunk ensures reviewers are looking at the working change with the trunk. I try to minimize force pushes, but for a long-running PR, I prefer rebasing. However, in the latest revision, I had to do rebase as I did git commit --amend after I pushed a fixup.

also add a RUN line for testing position in pass pipeline
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Gentle ping!

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madhur13490 commented Aug 28, 2025

Ping! Can we please proceed on this?

defm experimental_loop_fusion
: OptInCC1FFlag<"experimental-loop-fusion", "Enable", "Disable",
"Enable the loop fusion pass",
[ClangOption, CC1Option, FlangOption, FC1Option]>;
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CC1Option should be dropped. OptInCC1FFlag adds CC1Option for the positive form.

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Done.

@@ -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,
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addOptInFlag to ensure the negative option does not get forwarded to cc1

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Done in the latest revision. This eliminated negative option. Is it possible to have similar thing on Flang side?

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It should be possible to do this for flang as well. I think in your last commit, you changed addLastArg to addOptInFlag in Clang.cpp. You might only need to do the same in Flang.cpp as well.

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Yes, I spoke too soon. That works. Done.

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MaskRay commented Aug 28, 2025

You need to click "Re-request review" from folks who have requested changes.

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madhur13490 commented Aug 28, 2025

You need to click "Re-request review" from folks who have requested changes.

Thanks for that! But I am following the below from "Contributing to LLVM"

If you have received no comments on your patch for a week, you can request a review by ‘ping’ing the GitHub PR with “Ping” in a comment. The common courtesy ‘ping’ rate is once a week. Please remember that you are asking for valuable time from other professional developers.

If notifying via GitHub's mechanism is also expected, we should update the guideline?

@@ -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.
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NIT: The trailing comment does not need to be aligned with the previous trailing comment. I assume that this was done manually.

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Done.

@@ -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.

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Thanks for seeing this through

@@ -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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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.

@tarunprabhu
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@nikic

Could you take a look again and see if your concerns have been addressed.

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10 participants