[llvm] r221960 - IR: Rewrite uniquing and creation of MDString

[David Blaikie]

On Thu, Nov 13, 2014 at 5:17 PM, Duncan P. N. Exon Smith <
dexonsmith at apple.com> wrote:

> Author: dexonsmith
> Date: Thu Nov 13 19:17:09 2014
> New Revision: 221960
>
> URL: http://llvm.org/viewvc/llvm-project?rev=221960&view=rev
> Log:
> IR: Rewrite uniquing and creation of MDString
>
> Stop using `Value::getName()` to get the string behind an `MDString`.
> Switch to `StringMapEntry<MDString>` so that we can find the string by
> its coallocation.
>
> This is part of PR21532.
>
> Modified:
>     llvm/trunk/include/llvm/IR/Metadata.h
>     llvm/trunk/lib/IR/LLVMContextImpl.cpp
>     llvm/trunk/lib/IR/LLVMContextImpl.h
>     llvm/trunk/lib/IR/Metadata.cpp
>
> Modified: llvm/trunk/include/llvm/IR/Metadata.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/Metadata.h?rev=221960&r1=221959&r2=221960&view=diff
>
> ==============================================================================
> --- llvm/trunk/include/llvm/IR/Metadata.h (original)
> +++ llvm/trunk/include/llvm/IR/Metadata.h Thu Nov 13 19:17:09 2014
> @@ -57,14 +57,15 @@ public:
>  ///
>  /// TODO: Inherit from Metadata.
>  class MDString : public Value {
> +  friend class StringMapEntry<MDString>;
>

This seems annoyingly tied to the implementation details of StringMap
(exactly where in the stack the constructor is called, etc)... I wonder if
there's a better answer, but none springs immediately to mind.


> +
>    virtual void anchor();
>    MDString(const MDString &) LLVM_DELETED_FUNCTION;
>
>    explicit MDString(LLVMContext &C);
>
> -private:
>    /// \brief Shadow Value::getName() to prevent its use.
> -  StringRef getName() const { return Value::getName(); }
> +  StringRef getName() const LLVM_DELETED_FUNCTION;
>
>  public:
>    static MDString *get(LLVMContext &Context, StringRef Str);
> @@ -72,17 +73,17 @@ public:
>      return get(Context, Str ? StringRef(Str) : StringRef());
>    }
>
> -  StringRef getString() const { return getName(); }
> +  StringRef getString() const;
>
> -  unsigned getLength() const { return (unsigned)getName().size(); }
> +  unsigned getLength() const { return (unsigned)getString().size(); }
>
>    typedef StringRef::iterator iterator;
>
>    /// \brief Pointer to the first byte of the string.
> -  iterator begin() const { return getName().begin(); }
> +  iterator begin() const { return getString().begin(); }
>
>    /// \brief Pointer to one byte past the end of the string.
> -  iterator end() const { return getName().end(); }
> +  iterator end() const { return getString().end(); }
>
>    /// \brief Methods for support type inquiry through isa, cast, and
> dyn_cast.
>    static bool classof(const Value *V) {
>
> Modified: llvm/trunk/lib/IR/LLVMContextImpl.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/LLVMContextImpl.cpp?rev=221960&r1=221959&r2=221960&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/IR/LLVMContextImpl.cpp (original)
> +++ llvm/trunk/lib/IR/LLVMContextImpl.cpp Thu Nov 13 19:17:09 2014
> @@ -135,7 +135,7 @@ LLVMContextImpl::~LLVMContextImpl() {
>           "Destroying all MDNodes didn't empty the Context's sets.");
>
>    // Destroy MDStrings.
> -  DeleteContainerSeconds(MDStringCache);
> +  MDStringCache.clear();
>  }
>
>  // ConstantsContext anchors
>
> Modified: llvm/trunk/lib/IR/LLVMContextImpl.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/LLVMContextImpl.h?rev=221960&r1=221959&r2=221960&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/IR/LLVMContextImpl.h (original)
> +++ llvm/trunk/lib/IR/LLVMContextImpl.h Thu Nov 13 19:17:09 2014
> @@ -261,7 +261,7 @@ public:
>    FoldingSet<AttributeSetImpl> AttrsLists;
>    FoldingSet<AttributeSetNode> AttrsSetNodes;
>
> -  StringMap<Value*> MDStringCache;
> +  StringMap<MDString> MDStringCache;
>
>    FoldingSet<MDNode> MDNodeSet;
>
>
> Modified: llvm/trunk/lib/IR/Metadata.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/Metadata.cpp?rev=221960&r1=221959&r2=221960&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/IR/Metadata.cpp (original)
> +++ llvm/trunk/lib/IR/Metadata.cpp Thu Nov 13 19:17:09 2014
> @@ -37,13 +37,21 @@ MDString::MDString(LLVMContext &C)
>    : Value(Type::getMetadataTy(C), Value::MDStringVal) {}
>
>  MDString *MDString::get(LLVMContext &Context, StringRef Str) {
> -  LLVMContextImpl *pImpl = Context.pImpl;
> -  StringMapEntry<Value*> &Entry =
> -    pImpl->MDStringCache.GetOrCreateValue(Str);
> -  Value *&S = Entry.getValue();
> -  if (!S) S = new MDString(Context);
> -  S->setValueName(&Entry);
> -  return cast<MDString>(S);
> +  auto &Store = Context.pImpl->MDStringCache;
> +  auto I = Store.find(Str);
> +  if (I != Store.end())
> +    return &I->second;
> +
> +  auto *Entry =
> +      StringMapEntry<MDString>::Create(Str, Store.getAllocator(),
> Context);
>

This seems like it's the wrong API design - forwarding a single argument
and calling an explicit constructor doesn't seem right. APIs (such as
push_back) generally do perfect forwarding of a single argument where the
argument is implicitly convertible to the underlying type, but not for
explicit conversions.

For explicit conversions the idiom seems to be to have an 'emplace'
function.

I think the right API design is to add an 'emplace' to StringMap and use it
something like this:

Store.insert(std::piecewise_construct, std::forward_as_tuple(Str),
std::forward_as_tuple(Context));


> +  bool WasInserted = Store.insert(Entry);
> +  (void)WasInserted;
> +  assert(WasInserted && "Expected entry to be inserted");
> +  return &Entry->second;
> +}
> +
> +StringRef MDString::getString() const {
> +  return
> StringMapEntry<MDString>::GetStringMapEntryFromValue(*this).first();
>  }
>
>
>  //===----------------------------------------------------------------------===//
>
>
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