[LegacyPM] Reduce number of calls to getName

[aengelke]

Summary

Repeatedly calling getName adds some overhead, which can be easily
avoided by querying the name just once per function. The improvements
are rather small (~0.5% back-end time in a compile-time optimized
setting), but also very easy to achieve.

Note that getting the name should be entirely avoidable in the common
case, but would require more substantial changes.

[MaskRay]

llvm/lib/IR/LegacyPassManager.cpp

1412

Since the users are far away, consider const StringRef Name = ...

[MaskRay]

LGTM, the performance overhead is likely due to hash table lookups in Value::getValueName

[aengelke]

Make Name const.

llvm/lib/IR/LegacyPassManager.cpp

bool FPPassManager::runOnFunction(Function &F) {
   StringMap<std::pair<unsigned, unsigned>> FunctionToInstrCount;
   bool EmitICRemark = M.shouldEmitInstrCountChangedRemark();
   // Collect the initial size of the module.
   if (EmitICRemark) {
     InstrCount = initSizeRemarkInfo(M, FunctionToInstrCount);
     FunctionSize = F.getInstructionCount();
   }
 
-  llvm::TimeTraceScope FunctionScope("OptFunction", F.getName());
+  // Store name outside of loop to avoid redundant calls.
+  const StringRef Name = F.getName();
+  llvm::TimeTraceScope FunctionScope("OptFunction", Name);
 
   for (unsigned Index = 0; Index < getNumContainedPasses(); ++Index) {
     FunctionPass *FP = getContainedPass(Index);
     bool LocalChanged = false;
 
     // 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());
+    dumpPassInfo(FP, EXECUTION_MSG, ON_FUNCTION_MSG, Name);
     dumpRequiredSet(FP);
 
     initializeAnalysisImpl(FP);
 
     {
       PassManagerPrettyStackEntry X(FP, F);
       TimeRegion PassTimer(getPassTimer(FP));
 #ifdef EXPENSIVE_CHECKS
#endif
           InstrCount = static_cast<int64_t>(InstrCount) + Delta;
           FunctionSize = NewSize;
         }
       }
     }
 
     Changed |= LocalChanged;
     if (LocalChanged)
-      dumpPassInfo(FP, MODIFICATION_MSG, ON_FUNCTION_MSG, F.getName());
+      dumpPassInfo(FP, MODIFICATION_MSG, ON_FUNCTION_MSG, Name);
     dumpPreservedSet(FP);
     dumpUsedSet(FP);
 
     verifyPreservedAnalysis(FP);
     if (LocalChanged)
       removeNotPreservedAnalysis(FP);
     recordAvailableAnalysis(FP);
-    removeDeadPasses(FP, F.getName(), ON_FUNCTION_MSG);
+    removeDeadPasses(FP, Name, ON_FUNCTION_MSG);
   }
 
   return Changed;
 }
 
 bool FPPassManager::runOnModule(Module &M) {
   bool Changed = false;