[CGData][ThinLTO] Global Outlining with Two-CodeGen Rounds (#90933)

This feature is enabled by `-codegen-data-thinlto-two-rounds`, which
effectively runs the `-codegen-data-generate` and `-codegen-data-use` in
two rounds to enable global outlining with ThinLTO.
1. The first round: Run both optimization + codegen with a scratch
output.
Before running codegen, we serialize the optimized bitcode modules to a
temporary path.
 2. From the scratch object files, we merge them into the codegen data.
3. The second round: Read the optimized bitcode modules and start the
codegen only this time.
Using the codegen data, the machine outliner effectively performs the
global outlining.
 
Depends on #90934, #110461 and  #110463.
This is a patch for
https://discourse.llvm.org/t/rfc-enhanced-machine-outliner-part-2-thinlto-nolto/78753.

1 files changed, 11 insertions, 1 deletions

diff --git a/llvm/lib/LTO/LTOBackend.cpp b/llvm/lib/LTO/LTOBackend.cpp
index 06eeed3..ad332d2 100644
--- a/llvm/lib/LTO/LTOBackend.cpp
+++ b/llvm/lib/LTO/LTOBackend.cpp
@@ -20,6 +20,7 @@
 #include "llvm/Analysis/TargetLibraryInfo.h"
 #include "llvm/Bitcode/BitcodeReader.h"
 #include "llvm/Bitcode/BitcodeWriter.h"
+#include "llvm/CGData/CodeGenData.h"
 #include "llvm/IR/LLVMRemarkStreamer.h"
 #include "llvm/IR/LegacyPassManager.h"
 #include "llvm/IR/PassManager.h"
@@ -565,7 +566,8 @@ Error lto::thinBackend(const Config &Conf, unsigned Task, AddStreamFn AddStream,
                        const FunctionImporter::ImportMapTy &ImportList,
                        const GVSummaryMapTy &DefinedGlobals,
                        MapVector<StringRef, BitcodeModule> *ModuleMap,
-                       bool CodeGenOnly, const std::vector<uint8_t> &CmdArgs) {
+                       bool CodeGenOnly, AddStreamFn IRAddStream,
+                       const std::vector<uint8_t> &CmdArgs) {
   Expected<const Target *> TOrErr = initAndLookupTarget(Conf, Mod);
   if (!TOrErr)
     return TOrErr.takeError();
@@ -599,11 +601,19 @@ Error lto::thinBackend(const Config &Conf, unsigned Task, AddStreamFn AddStream,
   auto OptimizeAndCodegen =
       [&](Module &Mod, TargetMachine *TM,
           std::unique_ptr<ToolOutputFile> DiagnosticOutputFile) {
+        // Perform optimization and code generation for ThinLTO.
         if (!opt(Conf, TM, Task, Mod, /*IsThinLTO=*/true,
                  /*ExportSummary=*/nullptr, /*ImportSummary=*/&CombinedIndex,
                  CmdArgs))
           return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 
+        // Save the current module before the first codegen round.
+        // Note that the second codegen round runs only `codegen()` without
+        // running `opt()`. We're not reaching here as it's bailed out earlier
+        // with `CodeGenOnly` which has been set in `SecondRoundThinBackend`.
+        if (IRAddStream)
+          cgdata::saveModuleForTwoRounds(Mod, Task, IRAddStream);
+
         codegen(Conf, TM, AddStream, Task, Mod, CombinedIndex);
         return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
       };