[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

bool StackProtector::HasAddressTaken(const Instruction *AI,
   return false;
 }
 
 /// Search for the first call to the llvm.stackprotector intrinsic and return it
 /// if present.
 static const CallInst *findStackProtectorIntrinsic(Function &F) {
   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))
-          return CI;
+      if (const auto *II = dyn_cast<IntrinsicInst>(&I))
+        if (II->getIntrinsicID() == Intrinsic::stackprotector)
+          return II;
   return nullptr;
 }
 
 /// Check whether or not this function needs a stack protector based
 /// upon the stack protector level.
 ///
 /// We use two heuristics: a standard (ssp) and strong (sspstrong).
 /// The standard heuristic which will add a guard variable to functions that
 /// call alloca with a either a variable size or a size >= SSPBufferSize,