[clang] [llvm] [fatlto] Use the ThinLTO default pipeline for FatLTO (PR #134434)

[Paul Kirth via llvm-commits]

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@@ -1692,6 +1692,19 @@ PassBuilder::buildFatLTODefaultPipeline(OptimizationLevel Level, bool ThinLTO,
   if (ThinLTO && PGOOpt && PGOOpt->Action == PGOOptions::SampleUse)
     MPM.addPass(buildThinLTODefaultPipeline(Level, /*ImportSummary=*/nullptr));
   else {
+    // ModuleSimplification does not run the coroutine passes for
+    // ThinLTOPreLink, so we need the coroutine passes to run for ThinLTO
+    // builds, otherwise they will miscompile.
+    if (ThinLTO) {
+      // TODO: replace w/ buildCoroWrapper() when it takes phase and level into
+      // consideration.
+      CGSCCPassManager CGPM;
+      CGPM.addPass(CoroSplitPass(Level != OptimizationLevel::O0));
+      CGPM.addPass(CoroAnnotationElidePass());
+      MPM.addPass(createModuleToPostOrderCGSCCPassAdaptor(std::move(CGPM)));
+      MPM.addPass(CoroCleanupPass());
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ilovepi wrote:

Also possible to do `MPM.addPass(buildModuleSimplificationPipeline(Level, ThinOrFullLTOPhase::ThinLTOPostLink));`

That seems to generate better code here, since it allows the tail call in the test to get removed. But that seems a bit expensive, just to give the inliner a second chance.

https://github.com/llvm/llvm-project/pull/134434