[LegacyPM] Reduce number of calls to getName

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.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D148145

llvm/lib/IR/LegacyPassManager.cpp

@@ -1408,7 +1408,9 @@ bool FPPassManager::runOnFunction(Function &F) {
     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);
@@ -1419,7 +1421,7 @@ bool FPPassManager::runOnFunction(Function &F) {
     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);
@@ -1458,15 +1460,15 @@ bool FPPassManager::runOnFunction(Function &F) {
 
     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;