Add IR Profile-Guided Optimization (IR PGO) support to the Flang compiler (#136098)

This patch implements IR-based Profile-Guided Optimization support in
Flang through the following flags:

- `-fprofile-generate` for instrumentation-based profile generation

- `-fprofile-use=<dir>/file` for profile-guided optimization

Resolves #74216 (implements IR PGO support phase)

**Key changes:**

- Frontend flag handling aligned with Clang/GCC semantics

- Instrumentation hooks into LLVM PGO infrastructure

- LIT tests verifying:

    - Instrumentation metadata generation

    - Profile loading from specified path

    - Branch weight attribution (IR checks)

**Tests:**

- Added gcc-flag-compatibility.f90 test module verifying:

    -  Flag parsing boundary conditions

    -  IR-level profile annotation consistency

    -  Profile input path normalization rules

- SPEC2006 benchmark results will be shared in comments

For details on LLVM's PGO framework, refer to [Clang PGO
Documentation](https://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization).

This implementation was developed by [XSCC Compiler
Team](https://github.com/orgs/OpenXiangShan/teams/xscc).

---------

Co-authored-by: ict-ql <168183727+ict-ql@users.noreply.github.com>
Co-authored-by: Tom Eccles <t@freedommail.info>

2 files changed, 36 insertions, 0 deletions

diff --git a/flang/lib/Frontend/CompilerInvocation.cpp b/flang/lib/Frontend/CompilerInvocation.cpp
index 90a0029..0571aea 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -30,6 +30,7 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/Frontend/Debug/Options.h"
+#include "llvm/Frontend/Driver/CodeGenOptions.h"
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Option/OptTable.h"
@@ -452,6 +453,15 @@ static void parseCodeGenArgs(Fortran::frontend::CodeGenOptions &opts,
       opts.IsPIE = 1;
   }
 
+  if (args.hasArg(clang::driver::options::OPT_fprofile_generate)) {
+    opts.setProfileInstr(llvm::driver::ProfileInstrKind::ProfileIRInstr);
+  }
+
+  if (auto A = args.getLastArg(clang::driver::options::OPT_fprofile_use_EQ)) {
+    opts.setProfileUse(llvm::driver::ProfileInstrKind::ProfileIRInstr);
+    opts.ProfileInstrumentUsePath = A->getValue();
+  }
+
   // -mcmodel option.
   if (const llvm::opt::Arg *a =
           args.getLastArg(clang::driver::options::OPT_mcmodel_EQ)) {
diff --git a/flang/lib/Frontend/FrontendActions.cpp b/flang/lib/Frontend/FrontendActions.cpp
index 012d0fd..da8fa51 100644
--- a/flang/lib/Frontend/FrontendActions.cpp
+++ b/flang/lib/Frontend/FrontendActions.cpp
@@ -56,10 +56,12 @@
 #include "llvm/Passes/PassBuilder.h"
 #include "llvm/Passes/PassPlugin.h"
 #include "llvm/Passes/StandardInstrumentations.h"
+#include "llvm/ProfileData/InstrProfCorrelator.h"
 #include "llvm/Support/AMDGPUAddrSpace.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/FileSystem.h"
+#include "llvm/Support/PGOOptions.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/ToolOutputFile.h"
@@ -67,6 +69,7 @@
 #include "llvm/TargetParser/RISCVISAInfo.h"
 #include "llvm/TargetParser/RISCVTargetParser.h"
 #include "llvm/Transforms/IPO/Internalize.h"
+#include "llvm/Transforms/Instrumentation/InstrProfiling.h"
 #include "llvm/Transforms/Utils/ModuleUtils.h"
 #include <memory>
 #include <system_error>
@@ -918,6 +921,29 @@ void CodeGenAction::runOptimizationPipeline(llvm::raw_pwrite_stream &os) {
   llvm::PassInstrumentationCallbacks pic;
   llvm::PipelineTuningOptions pto;
   std::optional<llvm::PGOOptions> pgoOpt;
+
+  if (opts.hasProfileIRInstr()) {
+    // -fprofile-generate.
+    pgoOpt = llvm::PGOOptions(opts.InstrProfileOutput.empty()
+                                  ? llvm::driver::getDefaultProfileGenName()
+                                  : opts.InstrProfileOutput,
+                              "", "", opts.MemoryProfileUsePath, nullptr,
+                              llvm::PGOOptions::IRInstr,
+                              llvm::PGOOptions::NoCSAction,
+                              llvm::PGOOptions::ColdFuncOpt::Default, false,
+                              /*PseudoProbeForProfiling=*/false, false);
+  } else if (opts.hasProfileIRUse()) {
+    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS =
+        llvm::vfs::getRealFileSystem();
+    // -fprofile-use.
+    auto CSAction = opts.hasProfileCSIRUse() ? llvm::PGOOptions::CSIRUse
+                                             : llvm::PGOOptions::NoCSAction;
+    pgoOpt = llvm::PGOOptions(
+        opts.ProfileInstrumentUsePath, "", opts.ProfileRemappingFile,
+        opts.MemoryProfileUsePath, VFS, llvm::PGOOptions::IRUse, CSAction,
+        llvm::PGOOptions::ColdFuncOpt::Default, false);
+  }
+
   llvm::StandardInstrumentations si(llvmModule->getContext(),
                                     opts.DebugPassManager);
   si.registerCallbacks(pic, &mam);