[cfe-dev] RFC: Clang driver redesign

[David Chisnall]

Hi James,

It would be worth linking to this somewhere in your draft, as a lot of the issues have already been discussed:

http://clang.llvm.org/UniversalDriver.html

On 4 Nov 2011, at 11:11, James Molloy wrote:

> The first draft of this document is attached in HTML form and inline below
> in ReST form. I've documented the obvious use cases that I can think of, but
> I am certain to have missed some out. I've also started a strawman proposal
> solution, with the intention of stimulating discussion by providing
> something concrete.

Since we're throwing up use-cases, here are the ones I have where the current driver is not the best:

Driving plugins that produce some non-LLVM output.  I have written one that generates JavaScript from an ObjC AST and there are others that generate different compiler IRs for codegen by something other than LLVM.  Invoking these is pretty painful with the current driver.  It would be great to be able to just provide a config file that would remove the LLVM CodeGen and substitute something else.  Currently I have to do something like -fsyntax-only (which seems nonsense, because I actually do want code generated, just not LLVM code), -load full/path/to/plugin.so and a few more options.  

Shipping a cross-compile toolchain: Most of these include gcc, binutils, headers and libraries.  Including clang with the toolchain wouldn't make sense, because it would be the same clang as the version for the host system, unlike GCC where you need one specifically configured for each target.  Ideally, you'd just provide a config file that would tell clang where to find everything and it would just work.

Driving extra LLVM optimisations.  I have a set for Objective-C that will automatically add themselves to the optimisation toolchain if the library is loaded.  Currently this is done by adding -Xclang -load -Xclang path/to/plugin.so to the command line.  This is far from ideal, I'd like to be able to provide a config file that said something like 'when compiling Objective-C, load this library'.  I imagine other libraries may eventually want to add some library-specific optimisations.  For example, GObject or Qt's signals and slots mechanism may both benefit from running some extra LLVM passes that optimise their specific uses.  

Finally, an idea that's been floated before and may or may not require changes to the driver is that of a compile daemon. This would start once, parse all prefix headers, and then spawn threads to handle the parsing and compiling of each new source file.  It could more easily limit resource usage than a parallel make (for example, C source files typically take under 100MB to compile, C++ ones can take 500MB easily if they're full of templates - being able to limit clangd (clanged?) to using 1GB of RAM would be better than running make -j2 on a quad-core system).  Ideally, it could also be used from libclang so that files could be parsed frequently and sent off for codegen when clangd has spare resources and the file has been unmodified for a while (i.e. not when it's being frequently queried by the IDE for code completion, or autocorrection).  

>From a user perspective, the command line should be simple.  Being able to easily add command line arguments that simply map to combinations of others would be a huge win for usability.  As a user, I want to be able to just say something like -target=touchpad and have it load touchpad.conf, which specifies all of the arguments for that specific cross-compile toolchain.  I want it to load my user config and add a -fexperimental-stuff option, which expands to -Xclang -load -Xclang ~/my/buggy/experimental/optimisations.so 

A lot of the things I want are possible with the current driver, but they require command lines so complicated that people generally just don't bother with them.

David

-- Sent from my brain