[clang] [llvm] [WPD]: Apply speculative WPD in non-lto mode. (PR #145031)

[Teresa Johnson via llvm-commits]

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@@ -794,12 +815,29 @@ PreservedAnalyses WholeProgramDevirtPass::run(Module &M,
     return FAM.getResult<DominatorTreeAnalysis>(F);
   };
   if (UseCommandLine) {
-    if (!DevirtModule::runForTesting(M, AARGetter, OREGetter, LookupDomTree))
+    if (TestNoLTOMode)
+      // we are outside LTO mode. enable speculative devirtualization:
+      DevirtCheckMode = WPDCheckMode::Fallback;
+    if (!DevirtModule::runForTesting(M, AARGetter, OREGetter, LookupDomTree,
+                                     !TestNoLTOMode))
       return PreservedAnalyses::all();
     return PreservedAnalyses::none();
   }
+  std::optional<ModuleSummaryIndex> Index;
+  // Force Fallback mode as it's safe in case it's non-LTO mode where
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teresajohnson wrote:

Under the proposed option, we could do this speculative devirtualization in LTO mode where there is public visibility, but it looks like that would take some additional code restructuring/tracking. For now maybe just guard this by the proposed more general cl::opt, with a TODO to support using the speculative fallback mode just for vtables with public visibility. In fact, non-speculative devirt can kick in for hidden vis classes even in non-LTO mode. 

https://github.com/llvm/llvm-project/pull/145031