[clang] Teardown new PM data structures before running codegen pipeline

[aeubanks]

Summary

Do this by refactoring the optimization and codegen pipelines into separate functions.

This saves a tiny bit of memory in non-LTO builds [1].

[1] https://llvm-compile-time-tracker.com/compare.php?from=fbddf22ef72d3c2e9b14e1501841b03380eef12b&to=cd276df52eb6f2b84a8e1efe5318460c6debf82d&stat=max-rss

[dblaikie]

Any chance of refactoring this code so naturally these objects are lexically scoped to their necessary lifetime, and just cease to exist before the code gen passes are run? (looks like this function is pretty long and might benefit from being broken up into pieces anyway - and one of those pieces could have these managers as locals, and return once that work is done, before the code gen work is started?)

[aeubanks]

refactor

[dblaikie]

Looks great!

clang/lib/CodeGen/BackendUtil.cpp

1441–1442

Looks like this comment got lost - might be worth preserving in RunCodegenPipeline

[aeubanks]

add back a (similar) comment

clang/lib/CodeGen/BackendUtil.cpp

if (!CodeGenOpts.DisableLLVMPasses) {
     }
 
     if (!CodeGenOpts.MemoryProfileOutput.empty()) {
       MPM.addPass(createModuleToFunctionPassAdaptor(MemProfilerPass()));
       MPM.addPass(ModuleMemProfilerPass());
     }
   }
 
   // FIXME: We still use the legacy pass manager to do code generation. We
   // create that pass manager here and use it as needed below.
   legacy::PassManager CodeGenPasses;
   bool NeedCodeGen = false;
   std::unique_ptr<llvm::ToolOutputFile> ThinLinkOS, DwoOS;
 
   // Append any output we need to the pass manager.
   switch (Action) {
   case Backend_EmitNothing:
     break;
#include "llvm/Support/Extension.def"
   // Now that we have all of the passes ready, run them.
   {
     PrettyStackTraceString CrashInfo("Optimizer");
     MPM.run(*TheModule, MAM);
   }
 
   // Now if needed, run the legacy PM for codegen.
   if (NeedCodeGen) {
+    // Save some memory by clearing analyses used in the optimization pipeline.
+    LAM.clear();
+    FAM.clear();
+    CGAM.clear();
+    MAM.clear();
+
     PrettyStackTraceString CrashInfo("Code generation");
     CodeGenPasses.run(*TheModule);
   }
 
   if (ThinLinkOS)
     ThinLinkOS->keep();
   if (DwoOS)
     DwoOS->keep();