Move whole-program virtual call optimization pass after function attribute inference in LTO pipeline.

[pcc]

Summary

As a result of D18634 we no longer infer certain attributes on linkonce_odr
functions at compile time, and may only infer them at LTO time. The readnone
attribute in particular is required for virtual constant propagation (part
of whole-program virtual call optimization) to work correctly.

This change moves the whole-program virtual call optimization pass after
the function attribute inference passes, and enables the attribute inference
passes at opt level 1, so that virtual constant propagation has a chance to
work correctly for linkonce_odr functions.

[pcc]

• Remove an unused declaration

[mehdi_amini]

lib/Transforms/IPO/PassManagerBuilder.cpp

590

Why is this enable now at O1?

[pcc]

lib/Transforms/IPO/PassManagerBuilder.cpp

590

My initial thought was that if the user provided profile information, we should probably use it. Although now that I think about it more, it isn't clear that we can make meaningful use of it with our current O1 pipeline, and in any case, that's orthogonal to this change. Disabled.

[pcc]

• Disable the ICP pass at O1

[mehdi_amini]

LGTM

include/llvm/Transforms/IPO/PassManagerBuilder.h

public:
   static void addGlobalExtension(ExtensionPointTy Ty, ExtensionFn Fn);
   void addExtension(ExtensionPointTy Ty, ExtensionFn Fn);
 
 private:
   void addExtensionsToPM(ExtensionPointTy ETy,
                          legacy::PassManagerBase &PM) const;
   void addInitialAliasAnalysisPasses(legacy::PassManagerBase &PM) const;
   void addLTOOptimizationPasses(legacy::PassManagerBase &PM);
-  void addEarlyLTOOptimizationPasses(legacy::PassManagerBase &PM);
   void addLateLTOOptimizationPasses(legacy::PassManagerBase &PM);
   void addPGOInstrPasses(legacy::PassManagerBase &MPM);
   void addFunctionSimplificationPasses(legacy::PassManagerBase &MPM);
   void addInstructionCombiningPass(legacy::PassManagerBase &MPM) const;
 
 public:
   /// populateFunctionPassManager - This fills in the function pass manager,
   /// which is expected to be run on each function immediately as it is

lib/Transforms/IPO/PassManagerBuilder.cpp

void PassManagerBuilder::populateModulePassManager(
 
   if (MergeFunctions)
     MPM.add(createMergeFunctionsPass());
 
   addExtensionsToPM(EP_OptimizerLast, MPM);
 }
 
 void PassManagerBuilder::addLTOOptimizationPasses(legacy::PassManagerBase &PM) {
+  // Remove unused virtual tables to improve the quality of code generated by
+  // whole-program devirtualization and bitset lowering.
+  PM.add(createGlobalDCEPass());
+
   // Provide AliasAnalysis services for optimizations.
   addInitialAliasAnalysisPasses(PM);
 
   if (ModuleSummary)
     PM.add(createFunctionImportPass(ModuleSummary));
 
   // Allow forcing function attributes as a debugging and tuning aid.
   PM.add(createForceFunctionAttrsLegacyPass());
 
   // Infer attributes about declarations if possible.
   PM.add(createInferFunctionAttrsLegacyPass());
 
   // Indirect call promotion. This should promote all the targets that are left
   // by the earlier promotion pass that promotes intra-module targets.
   // This two-step promotion is to save the compile time. For LTO, it should
   // produce the same result as if we only do promotion here.
   PM.add(createPGOIndirectCallPromotionLegacyPass(true));
 
+  if (OptLevel > 1) {
     // Propagate constants at call sites into the functions they call.  This
     // opens opportunities for globalopt (and inlining) by substituting function
     // pointers passed as arguments to direct uses of functions.
     PM.add(createIPSCCPPass());
+  }
 
-  // Now that we internalized some globals, see if we can hack on them!
+  // Infer attributes about definitions. The readnone attribute in particular is
+  // required for virtual constant propagation.
   PM.add(createPostOrderFunctionAttrsLegacyPass());
   PM.add(createReversePostOrderFunctionAttrsPass());
+
+  // Apply whole-program devirtualization and virtual constant propagation.
+  PM.add(createWholeProgramDevirtPass());
+
+  // That's all we need at opt level 1.
+  if (OptLevel == 1)
+    return;
+
+  // Now that we internalized some globals, see if we can hack on them!
   PM.add(createGlobalOptimizerPass());
   // Promote any localized global vars.
   PM.add(createPromoteMemoryToRegisterPass());
 
   // Linking modules together can lead to duplicated global constants, only
   // keep one copy of each constant.
   PM.add(createConstantMergePass());
 
void PassManagerBuilder::addLTOOptimizationPasses(legacy::PassManagerBase &PM) {
 
   // Cleanup and simplify the code after the scalar optimizations.
   addInstructionCombiningPass(PM);
   addExtensionsToPM(EP_Peephole, PM);
 
   PM.add(createJumpThreadingPass());
 }
 
-void PassManagerBuilder::addEarlyLTOOptimizationPasses(
-    legacy::PassManagerBase &PM) {
-  // Remove unused virtual tables to improve the quality of code generated by
-  // whole-program devirtualization and bitset lowering.
-  PM.add(createGlobalDCEPass());
-
-  // Apply whole-program devirtualization and virtual constant propagation.
-  PM.add(createWholeProgramDevirtPass());
-}
-
 void PassManagerBuilder::addLateLTOOptimizationPasses(
     legacy::PassManagerBase &PM) {
   // Delete basic blocks, which optimization passes may have killed.
   PM.add(createCFGSimplificationPass());
 
   // Drop bodies of available externally objects to improve GlobalDCE.
   PM.add(createEliminateAvailableExternallyPass());
 

 void PassManagerBuilder::populateLTOPassManager(legacy::PassManagerBase &PM) {
   if (LibraryInfo)
     PM.add(new TargetLibraryInfoWrapperPass(*LibraryInfo));
 
   if (VerifyInput)
     PM.add(createVerifierPass());
 
   if (OptLevel != 0)
-    addEarlyLTOOptimizationPasses(PM);
-
-  if (OptLevel > 1)
     addLTOOptimizationPasses(PM);
 
   // Create a function that performs CFI checks for cross-DSO calls with targets
   // in the current module.
   PM.add(createCrossDSOCFIPass());
 
   // Lower bit sets to globals. This pass supports Clang's control flow
   // integrity mechanisms (-fsanitize=cfi*) and needs to run at link time if CFI