[PATCH] D31057: Make Argument::getArgNo() constant time, not O(#args)

[Reid Kleckner via Phabricator via llvm-commits]

rnk created this revision.
Herald added a subscriber: mehdi_amini.

getArgNo is actually hot in LLVM, because its how we check for
attributes on arguments:

  bool Argument::hasNonNullAttr() const {
    if (!getType()->isPointerTy()) return false;
    if (getParent()->getAttributes().
          hasAttribute(getArgNo()+1, Attribute::NonNull))
      return true;

It actually shows up as the 23rd hottest leaf function in a 13s sample
of LTO of llc.

This grows Argument by four bytes, but I have another pending patch to
shrink it by removing its ilist_node base.


https://reviews.llvm.org/D31057

Files:
  include/llvm/IR/Argument.h
  lib/IR/Function.cpp


Index: lib/IR/Function.cpp
===================================================================
--- lib/IR/Function.cpp
+++ lib/IR/Function.cpp
@@ -39,27 +39,15 @@
 
 void Argument::anchor() { }
 
-Argument::Argument(Type *Ty, const Twine &Name)
-    : Value(Ty, Value::ArgumentVal), Parent(nullptr) {
+Argument::Argument(Type *Ty, const Twine &Name, unsigned ArgNo)
+    : Value(Ty, Value::ArgumentVal), Parent(nullptr), ArgNo(ArgNo) {
   setName(Name);
 }
 
 void Argument::setParent(Function *parent) {
   Parent = parent;
 }
 
-unsigned Argument::getArgNo() const {
-  const Function *F = getParent();
-  assert(F && "Argument is not in a function");
-
-  Function::const_arg_iterator AI = F->arg_begin();
-  unsigned ArgIdx = 0;
-  for (; &*AI != this; ++AI)
-    ++ArgIdx;
-
-  return ArgIdx;
-}
-
 bool Argument::hasNonNullAttr() const {
   if (!getType()->isPointerTy()) return false;
   if (getParent()->getAttributes().
@@ -240,7 +228,8 @@
   for (unsigned i = 0, e = FT->getNumParams(); i != e; ++i) {
     assert(!FT->getParamType(i)->isVoidTy() &&
            "Cannot have void typed arguments!");
-    ArgumentList.push_back(new Argument(FT->getParamType(i)));
+    ArgumentList.push_back(
+        new Argument(FT->getParamType(i), "", i));
   }
 
   // Clear the lazy arguments bit.
Index: include/llvm/IR/Argument.h
===================================================================
--- include/llvm/IR/Argument.h
+++ include/llvm/IR/Argument.h
@@ -32,21 +32,22 @@
 class Argument : public Value, public ilist_node<Argument> {
   virtual void anchor();
   Function *Parent;
+  unsigned ArgNo;
 
   friend class SymbolTableListTraits<Argument>;
   void setParent(Function *parent);
 
 public:
   /// Argument constructor.
-  explicit Argument(Type *Ty, const Twine &Name = "");
+  explicit Argument(Type *Ty, const Twine &Name = "", unsigned ArgNo = 0);
 
   inline const Function *getParent() const { return Parent; }
   inline       Function *getParent()       { return Parent; }
 
   /// Return the index of this formal argument in its containing function.
   ///
   /// For example in "void foo(int a, float b)" a is 0 and b is 1.
-  unsigned getArgNo() const;
+  unsigned getArgNo() const { return ArgNo; }
 
   /// Return true if this argument has the nonnull attribute. Also returns true
   /// if at least one byte is known to be dereferenceable and the pointer is in


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