[StackProtector] Speed up RequiresStackProtector

[nadav]

Summary

Speed up the method RequiresStackProtector by checking the intrinsic
value of the call. The original code calls getName() that returns an
allocating std::string on each check. This change removes about 96072 std::string
instances when compiling sqlite3.c; The function was discovered with a
Facebook-internal performance tool.

[craig.topper]

llvm/lib/CodeGen/StackProtector.cpp

257

Can this just be CI->getIntrinsicID() == Intrinsic::stackprotector?

[nadav]

@craig.topper Good catch. Thank you. I've updated the diff.

[craig.topper]

LGTM

llvm/lib/CodeGen/StackProtector.cpp

 /// Search for the first call to the llvm.stackprotector intrinsic and return it
 /// if present.
 static const CallInst *findStackProtectorIntrinsic(Function &F) {
+  Function *StackProtectorDecl =
+      Intrinsic::getDeclaration(F.getParent(), Intrinsic::stackprotector);
   for (const BasicBlock &BB : F)
     for (const Instruction &I : BB)
       if (const CallInst *CI = dyn_cast<CallInst>(&I))
-        if (CI->getCalledFunction() ==
-            Intrinsic::getDeclaration(F.getParent(), Intrinsic::stackprotector))
+        if (CI->getCalledFunction() == StackProtectorDecl)
           return CI;
   return nullptr;
 }