Date: Mon, 18 May 2015 01:24:53 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Potential cleanup work post-1.1.9 On Sat, May 02, 2015 at 06:48:18PM -0400, Rich Felker wrote: > Just some ideas to jot down somewhere public: > > - Changing pthread_cancel signal handler to use mcontext_t members by > name (with the name provided by pthread_arch.h) instead of offset > macros and a fake overlaid array. Not yet started. > - Re-unifying x86_64 and x32 sigsetjmp.s by moving both saved values > to after the signal mask. Not yet started. Should be trivial to do. > - Providing inline atomics instead of runtime-switched for sh4a. Patch under review; looks ok. More related cleanup work may follow. > - Cleaning out cruft from struct __pthread, possibly condensing > multiple locks, packing small/boolean values into chars or > bitfields, etc. Not yet started. > - Evaluating whether it still makes sense to have struct __libc, or > all the members in it. I got rid of the locale atomics in __libc that had no purpose but avoiding TLS access. Nothing else there looks immediately removable, but here's some analysis: Presently, the following members are unconditionally needed no matter what you link; there's no size benefit to factoring them out: - can_do_threads - secure - auxv - tls_size - page_size - global_locale Of these, page_size could be made optional; it only needs to be stored on archs that have variable PAGE_SIZE. Of these, global_locale is pretty large and it would be nice if we could avoid it when not using functions that depend on locale, but the main thread's __pthread struct needs a pointer to its active locale and that's always initialized at startup. And can_do_threads may be useless; it's only probed correctly on i386 now anyway. As for other members, here's a summary of where they're used (read, not updated): - threaded - sigaction, all functions which create FILEs, synccall - threads_minus_1 - dynlink, malloc, synccall, __lock - ofl_head and ofl_lock - fclose, all functions which create FILEs, stdio exit code Of these, only the ofl stuff seems practical to move, and it would be a small savings at best. So for this whole item, I think it makes more sense to try to gradually phase out members that aren't useful rather than doing any immediately overhaul or refactoring. > - Eliminating float code in malloc on soft-float archs. Moving forward with this requires a way to actually _know_ whether the target is soft-float. Or we could require the arch to expose a preferred function for this computation which could theoretically be independent of whether it's hard- or soft-float. > - Deduplicating atomic.h emulations of a_* in terms of a_cas, so that > archs where everything is emulated in terms of a_cas can just define > a_cas (much like how syscall.h and syscall_arch.h work now). Prototype sent to list already. > - Default atomics implementation for new archs to use (at least > initially) based on __sync_* builtins. Prototype sent to list already. > - Bits deduplication. Still needs to be done. Big task. > - Possibly eliminating preference of globals over TLS, at least when > it uglifies/enlarges/complexifies the code, for example use of > libc.threads_minus_1 to determine whether to skip locks and > libc.uselocale_cnt (instead of __pthread_self()->locale) in the > CURRENT_LOCALE macro. The worst offender, locale stuff, is already fixed. There are probably some places still checking libc.threads_minus_1 or libc.threaded in ways that don't actually benefit performance and produce uglier code, but most places doing this actually have good reason to. Rich
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