[LLVMdev] GSOC - Use more StringRef in clang.

[Benjamin Kramer]

On 17.03.2014, at 18:22, Kevin Cox <kevincox at kevincox.ca> wrote:

> Thanks for the feedback David.
> 
> I have created a quick draft of my proposal and would appreciate any
> feedback.

I don't enjoy to be the conveyer of bad news, but I don't think that this is a
very useful GSoC project.

1. It's an utterly boring and rewardless task ;)

2. StringRef'ization is mostly done, the remaining cases may be just pieces
of code where StringRef is not obviously the best way to pass a string. Think
of passing a std::string by reference because it's modified in the callee. I highly
doubt that there are many cases in LLVM where someone just forgot to add
a "const".

StringRef'ization may be a good task if you just want to kill a couple of hours
and get a deeper understanding of the relationships between LLVM/Clang
components, for GSoC you can/should pick something cooler. :)

You can take a look at the projects we had in the last couple of years (LLVM
participated every year since 2006) to get an idea of what a project that is likely
to get accepted looks like.

- Ben

> 
> 
> GSOC Proposal -- StringRef'ize APIs
> ===================================
> 
> Background
> ----------
> 
> LLVM provides a StringRef class that quite simply references a string
> (arbitrary byte buffer).  Using llvm::StringRef copies can be avoided
> when a string passed into a function is only going to be read.  While
> this class is used in Clang there are still places where std::string are
> used.  By replacing these with llvm::StringRef copies can be avoided,
> improving performance.  Furthermore there are places where a const char*
> is used, in these situations llvm::StringRef can be used to improve
> safety and convenience through use of llvm::StringRef's already written
> functions and bounds checks when assertions are enabled.
> 
> Project Goals
> -------------
> 
> The goal of this project is to replaces these uses of const std::string
> and const char* inside Clang with llvm::StringRef.  Furthermore
> std::string and char* that are not marked const will also be considered
> to see if they do not need to be modified, in which case a StringRef
> will be used as well.
> 
> This project entails changing both the headers and implementation to use
> llvm:StringRefs as well as updating the documentation.  Other goals of
> the project include no API breakage (unless discussed specifically) and
> maintaining good performance.  As this project is largely performance
> focused if other performance improvements are possible while changing
> the code they will likely be explored as well.
> 
> Another important part of the project is allowing llvm::StringRefs to be
> used in more places rather than being converted everywhere.  This is
> where the real benefits of StringRefs are, allowing them to be passed
> far down the call stack.
> 
> Who am I
> --------
> 
> Hello, I am Kevin Cox, a Canadian student from Carleton University.  By
> this summer I will be in a Third Year Software Engineering standing.
> 
> I have been using Linux for over 6 years and compiling my C and C++ code
> with clang for nearly that long.  I love working with low level code and
> really enjoy squeezing all of the performance out of it.  I think that
> LLVM and Clang are great projects and am glad to have an opportunity to
> help out.
> 
> Contact
> -------
> 
> Communication is vitally important to success, especially when working
> with a new code base.  To facilitate quick communication I will be
> reading email constantly and idling in IRC whenever I am working on my
> project (and most likely more often than that).  I also have a good
> quality microphone so voice and video chats are a viable option for live
> communication.
> 
> Email: kevincox at kevincox.ca
> XMPP:  kevincox at kevincox.ca
> PGP:   E394 3366 624E 7449 B9B4 85AE C075 8A3B 34D5 2E74
> IRC:   kevincox
> Phone: <omitted>
> 
> In addition to regular communication I propose weekly or bi-weekly
> meetings with my mentor to keep in touch and ensure the project is
> moving forward.
> 
> Goals
> -----
> 
> Google Summer of Code is a three month program.  Over the course of
> three months I will be working to convert as many APIs as possible to
> use llvm::StringRef.  While I have identified a number of APIs that can
> be converted to use StringRefs listing them would be a waste of time and
> energy.  Instead, I have used some incredible high tech methods to count
> the uses of std::string& and const char* in Clang and hope to reduce
> that number.  Please note that not all of these matches can be converted
> based on quick analysis about 1/8 to 1/4 of the functions can be converted.
> 
> % grep 'const\s*char\s*\\*' **/*.{h,cpp} | wc -l
> 3272
> % grep 'std::string\s*&' **/*.{h,cpp} | wc -l
> 506
> 
> Throughout the summer I expect to significantly reduce these numbers.  I
> am also going to create and maintain a document of APIs that can and
> can't be converted (think of a tri-color collector) and work through
> this document throughout the summer.  Idealy, by the end of the summer
> all APIs that can be converted have been and call sites have been
> updated to take advantage of the new APIs.
> 
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