On Monday, 5 January 2015 at 17:44:31 UTC, Daniel Murphy wrote:
I think I've got a handle on this, sort of. I've moved the
declaration of __va_argsave into the glue layer, and added
intrinsic detection for va_start/va_end/va_arg (the two-arg
form).
I've implemented them in the backend for win32 and they have
passed a simple test!
I'd suggest you have a look at Posix x86_64 first before
finalizing the "easy" x86 implementation. The former comes with
two extra niceties compared to the simple "pointer to
stack-allocated arguments" model:
1) You need to copy the registers to the stack on function entry
(in case the arguments are later accessed using va_arg, they are
just regular functions on the caller side), and then be able to
access the address of this area in va_start. This is what
va_argsave is currently used for in DMD.
2) The issue with passing va_list as a parameter (especially
regarding C ABI compatibility) vs. allocating the struct storage
allocation. If you simply make it a pointer on x86_64, it's hard
to implement va_copy correctly. The DMD implementation of the
latter is currently broken, which is the reason for some of the
vararg-related version(X86_64) blocks.
Do you think we can change _all_ the druntime and phobos code
to just use va_arg directly? It would be nice to have it all
portable like that.
Yes. Some parts might need a bit of rework, though. This job
would be quite a bit easier if we could finally ditch the old
vararg-based std.format stuff before.
Be sure to let me know if you have any specific questions.
David
