[flang-commits] [clang] [flang] [Flang][Driver] Support for -fsplit-lto-unit option in flang driver (PR #202858)

[via flang-commits]

https://github.com/shivaramaarao updated https://github.com/llvm/llvm-project/pull/202858

>From b3cb46767631c13a87c393a000627bd4b0e75017 Mon Sep 17 00:00:00 2001
From: Rahul Kumar <Rahul.Kumar3 at amd.com>
Date: Wed, 10 Jun 2026 11:09:03 +0530
Subject: [PATCH] [Flang][Driver] Adding -fsplit-lto-unit option to enable
 combined LTO build

When mixing Fortran objects from Flang with C/C++ objects compiled by Clang during a combined LTO build,
it is necessary to ensure that all files use the same setting for split-lto-unit. This requires the support
for -fsplit-lto-unit option in the flang driver. This support is added as part of this commit
---
 clang/include/clang/Options/Options.td          | 4 ++--
 clang/lib/Driver/ToolChains/Flang.cpp           | 8 ++++++++
 flang/include/flang/Frontend/CodeGenOptions.def | 1 +
 flang/lib/Frontend/CompilerInvocation.cpp       | 4 ++++
 flang/lib/Frontend/FrontendActions.cpp          | 6 ++++++
 5 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/clang/include/clang/Options/Options.td b/clang/include/clang/Options/Options.td
index 8f5ed945a40fe..ed0354786c43e 100644
--- a/clang/include/clang/Options/Options.td
+++ b/clang/include/clang/Options/Options.td
@@ -4804,9 +4804,9 @@ defm whole_program_vtables : BoolFOption<"whole-program-vtables",
   NegFlag<SetFalse>, BothFlags<[], [ClangOption, CLOption]>>;
 defm split_lto_unit : BoolFOption<"split-lto-unit",
   CodeGenOpts<"EnableSplitLTOUnit">, DefaultFalse,
-  PosFlag<SetTrue, [], [ClangOption, CC1Option],
+  PosFlag<SetTrue, [], [ClangOption, CC1Option, FlangOption, FC1Option],
           "Enables splitting of the LTO unit">,
-  NegFlag<SetFalse>, BothFlags<[], [ClangOption, CLOption]>>;
+  NegFlag<SetFalse>, BothFlags<[], [ClangOption, CLOption, FlangOption, FC1Option]>>;
 defm force_emit_vtables : BoolFOption<"force-emit-vtables",
   CodeGenOpts<"ForceEmitVTables">, DefaultFalse,
   PosFlag<SetTrue, [], [ClangOption, CC1Option],
diff --git a/clang/lib/Driver/ToolChains/Flang.cpp b/clang/lib/Driver/ToolChains/Flang.cpp
index 224ece3239efd..d98ab694f401a 100644
--- a/clang/lib/Driver/ToolChains/Flang.cpp
+++ b/clang/lib/Driver/ToolChains/Flang.cpp
@@ -282,6 +282,14 @@ void Flang::addLTOOptions(const ArgList &Args, ArgStringList &CmdArgs) const {
     CmdArgs.push_back("-flto=full");
   else if (LTOMode == LTOK_Thin)
     CmdArgs.push_back("-flto=thin");
+
+  if (Arg *splitLTOArg = Args.getLastArg(options::OPT_fsplit_lto_unit,
+                                         options::OPT_fno_split_lto_unit)) {
+    if (splitLTOArg->getOption().matches(options::OPT_fsplit_lto_unit)) {
+      CmdArgs.push_back("-fsplit-lto-unit");
+    }
+  }
+
   Args.addAllArgs(CmdArgs, {options::OPT_ffat_lto_objects,
                             options::OPT_fno_fat_lto_objects});
 }
diff --git a/flang/include/flang/Frontend/CodeGenOptions.def b/flang/include/flang/Frontend/CodeGenOptions.def
index d8bbb94bd8cde..37931c0ffecc1 100644
--- a/flang/include/flang/Frontend/CodeGenOptions.def
+++ b/flang/include/flang/Frontend/CodeGenOptions.def
@@ -45,6 +45,7 @@ CODEGENOPT(PrepareForFullLTO , 1, 0) ///< Set when -flto is enabled on the
                                      ///< compile step.
 CODEGENOPT(PrepareForThinLTO , 1, 0) ///< Set when -flto=thin is enabled on the
                                      ///< compile step.
+CODEGENOPT(EnableSplitLTOUnit, 1, 0) ///< Set when -fsplit-lto-unit is enabled.
 CODEGENOPT(ProtectParens, 1, 1) ///< -fprotect-parens (enable parenthesis protection)
 CODEGENOPT(StackArrays, 1, 0) ///< -fstack-arrays (enable the stack-arrays pass)
 CODEGENOPT(EnableSafeTrampoline, 1, 0) ///< -fsafe-trampoline (W^X compliant trampoline pool)
diff --git a/flang/lib/Frontend/CompilerInvocation.cpp b/flang/lib/Frontend/CompilerInvocation.cpp
index a3335fc9a250f..bf10ef346b1fc 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -1551,6 +1551,10 @@ static bool parseLinkerOptionsArgs(CompilerInvocation &invoc,
       opts.PrepareForThinLTO = true;
   }
 
+  // -fsplit-lto-unit option
+  if (args.hasArg(clang::options::OPT_fsplit_lto_unit))
+    opts.EnableSplitLTOUnit = true;
+
   // -ffat-lto-objects
   if (const llvm::opt::Arg *arg =
           args.getLastArg(clang::options::OPT_ffat_lto_objects,
diff --git a/flang/lib/Frontend/FrontendActions.cpp b/flang/lib/Frontend/FrontendActions.cpp
index 66602ed52f6cd..e484e857dbe43 100644
--- a/flang/lib/Frontend/FrontendActions.cpp
+++ b/flang/lib/Frontend/FrontendActions.cpp
@@ -1065,6 +1065,12 @@ void CodeGenAction::runOptimizationPipeline(llvm::raw_pwrite_stream &os) {
       llvmModule->addModuleFlag(llvm::Module::Error, "ThinLTO", uint32_t(0));
     }
 
+    if (opts.EnableSplitLTOUnit && emitSummary &&
+        !llvmModule->getModuleFlag("EnableSplitLTOUnit")) {
+      llvmModule->addModuleFlag(llvm::Module::Error, "EnableSplitLTOUnit",
+                                uint32_t(1));
+    }
+
     if (action == BackendActionTy::Backend_EmitBC) {
       if (opts.PrepareForThinLTO) {
         mpm.addPass(llvm::ThinLTOBitcodeWriterPass(os, nullptr));