[clang] [llvm] [Clang]: Enable speculative devirtualization (PR #159685)

[Hassnaa Hamdi via llvm-commits]

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@@ -1641,6 +1642,24 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
   if (!LTOPreLink)
     MPM.addPass(RelLookupTableConverterPass());
 
+  if (PTO.DevirtualizeSpeculatively && LTOPhase == ThinOrFullLTOPhase::None) {
+    MPM.addPass(WholeProgramDevirtPass(
+        /*ExportSummary*/ nullptr,
+        /*ImportSummary*/ nullptr,
+        /*DevirtSpeculatively*/ PTO.DevirtualizeSpeculatively));
+    MPM.addPass(LowerTypeTestsPass(nullptr, nullptr,
+                                   lowertypetests::DropTestKind::Assume));
+    if (EnableModuleInliner) {
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hassnaaHamdi wrote:

> Can you clarify what vtable GV optimization is needed to expose these opportunities?
I mean optimisation passes like GlobalOpt.cpp pass which eliminates the unused GVs, and then when devirtualization works, it finds a single GV that is refering to the virtual function, so it can devirtualize.

https://github.com/llvm/llvm-project/pull/159685