Re: [PATCH] libgccjit.so: an embeddable JIT-compiler based on GCC

["Joseph S. Myers" <joseph at codesourcery dot com>]

On Wed, 2 Oct 2013, David Malcolm wrote:

> The idea is that GCC is configured with a special --enable-host-shared
> option, which leads to it being built as position-independent code. You
> would configure it with host==target, given that the generated machine
> code will be executed within the same process (the whole point of JIT).

Configuring with a different target also makes sense - consider JIT 
generation of code for a GPU (for example).  (Of course lots of other 
cleanups would be needed to be able to generate code for multiple targets 
within the same process / same copy of libgccjit.so, but the idea seems 
useful.  There are probably tricks available involving building multiple 
copies with different SONAMEs / symbol names.  Anyway, it may make sense 
now to consider defining the interfaces in a way that would support 
choosing the target for which you get a context.)

>   * There are some grotesque kludges in internal-api.c, especially in
> how we go from .s assembler files to a DSO (grep for "gross hack" ;) )

Apart from making the assembler into a shared library itself, it would 
also be nice to avoid needing those files at all (via an API for writing 
assembler text that doesn't depend on a FILE * pointer, although on GNU 
hosts you can always use in-memory files via open_memstream, which would 
make the internal changes much smaller).  But in the absence of such a 
cleanup, running the assembler via the driver should work, although 
inefficient.

What do you do about errors and warnings (other than hope that they don't 
get generated)?  Again, one step is to avoid anything that directly uses 
stderr / stdout (explicitly, or implicitly through functions such as 
printf), so allowing FILE * pointers to instead be something opened with 
open_memstream, while a larger cleanup would narrow the interface to 
writing such text as far as possible (to well-defined diagnostic and 
dumping interfaces that don't directly take a FILE *, eliminating all ad 
hoc calls to fprintf etc.).

-- 
Joseph S. Myers
joseph@codesourcery.com