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

[Nuri Amari via cfe-commits]

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@@ -1558,6 +1562,60 @@ class InProcessThinBackend : public ThinBackendProc {
     return BackendThreadPool.getMaxConcurrency();
   }
 };
+
+/// This Backend will run ThinBackend process but throw away all the output from
+/// the codegen. This class facilitates the first codegen round.
+class NoOutputThinBackend : public InProcessThinBackend {
+public:
+  NoOutputThinBackend(
+      const Config &Conf, ModuleSummaryIndex &CombinedIndex,
+      ThreadPoolStrategy ThinLTOParallelism,
+      const DenseMap<StringRef, GVSummaryMapTy> &ModuleToDefinedGVSummaries,
+      std::unique_ptr<std::vector<llvm::SmallString<0>>> Scratch)
+      : InProcessThinBackend(
+            Conf, CombinedIndex, ThinLTOParallelism, ModuleToDefinedGVSummaries,
+            // Allocate a scratch buffer for each task to write output to.
+            [Allocation = &*Scratch](unsigned Task, const Twine &ModuleName) {
+              return std::make_unique<CachedFileStream>(
+                  std::make_unique<raw_svector_ostream>((*Allocation)[Task]));
+            },
+            FileCache(), nullptr, false, false),
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NuriAmari wrote:

Do you mind labelling these arguments with a comment including the name of the parameter like you've done below: `/*CodeGenOnly=*/true`. Same with the `OptimizedBitcodeThinBackend` constructor.

https://github.com/llvm/llvm-project/pull/90933