[LegacyPM] Call getPassName only when needed

[aengelke]

Summary

Even when time tracing is disabled, getPassName is currently still
called. This adds an avoidable virtual function call for each pass.
Fetching the pass name only when required slightly improves
compile-time (particularly when LLVM is built without LTO).

[aengelke]

Format changes + re-trigger CI

[nikic]

LGTM, though the improvement here is barely measurable.

[MaskRay]

Ideally give some numbers. How costly is it? Is the cost mainly due to PassInfoMap.lookup(TI)?

[aengelke]

In D148022#4259195, @MaskRay wrote:

Ideally give some numbers. How costly is it?

I measured an improvement between 0.5% and 1% in our (fairly compile-time optimized) setting.

Is the cost mainly due to PassInfoMap.lookup(TI)?

Mostly yes, but also the mutex in PassRegistry::getPassInfo and the virtual function call itself contribute overhead.

llvm/lib/IR/LegacyPassManager.cpp

bool FPPassManager::runOnFunction(Function &F) {
   }
 
   llvm::TimeTraceScope FunctionScope("OptFunction", F.getName());
 
   for (unsigned Index = 0; Index < getNumContainedPasses(); ++Index) {
     FunctionPass *FP = getContainedPass(Index);
     bool LocalChanged = false;
 
-    llvm::TimeTraceScope PassScope("RunPass", FP->getPassName());
+    // Call getPassName only when required. The call itself is fairly cheap, but
+    // still virtual and repeated calling adds unnecessary overhead.
+    llvm::TimeTraceScope PassScope(
+        "RunPass", [FP]() { return std::string(FP->getPassName()); });
 
     dumpPassInfo(FP, EXECUTION_MSG, ON_FUNCTION_MSG, F.getName());
     dumpRequiredSet(FP);
 
     initializeAnalysisImpl(FP);
 
     {
       PassManagerPrettyStackEntry X(FP, F);