Date: Thu, 11 Mar 2004 05:19:07 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> To: Tim Robbins <tjr@freebsd.org> Cc: John Birrell <jb@cimlogic.com.au> Subject: Re: cvs commit: src/lib/libc/stdio _flock_stub.c local.h Message-ID: <20040311044539.S2105@gamplex.bde.org> In-Reply-To: <20040310131348.GA95975@cat.robbins.dropbear.id.au> References: <200403090245.i292j0a6035728@repoman.freebsd.org> <20040309032248.GA88649@cat.robbins.dropbear.id.au> <20040309035532.GA88825@cat.robbins.dropbear.id.au> <20040309043207.GA65153@kanpc.gte.com> <20040310035912.GQ56622@elvis.mu.org> <20040310131348.GA95975@cat.robbins.dropbear.id.au>
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On Thu, 11 Mar 2004, Tim Robbins wrote: > On Tue, Mar 09, 2004 at 07:59:12PM -0800, Alfred Perlstein wrote: > > > * Bruce Evans <bde@zeta.org.au> [040309 07:50] wrote: > > > This would pessimize even getc_unlocked() and putc_unlocked(). getc() > > > and putc() are now extern functions, but the old macro/inline versions > > > are still available as getc_unlocked() and putc_unlocked(). Simple > > > benchmarks for reading a 100MB file on an Athlon XP1600 overclocked > > > show that the function versions are up to 9 times slower: > > > ... > > > > Hmm, can't we use macros that do this: > > > > #define getc() (__isthreaded ? old_unlocked_code : getc_unlocked()) > > > > Where __isthreaded is a global that's set by threading libraries > > to 1 and 0 by non-threaded libc, this should get rid of a lot of > > the function call overhead. > > Sounds like a good idea to me. In my testing, this approach was about 5% > slower than calling getc_unlocked() directly (due to the conditional jump), > but roughly 3 times faster than a call to the getc() function. You must not have tested the dynamic linkage case :-). > If there aren't any objections, I think we should implement getc()/putc() > this way (and all the other stdio functions that have traditionally had > macro equivalents) before 5-stable to try to recoup some of the performance > losses caused by the removal of the macros. Is __isthreaded always set early enough? What about if the application is dynamically linked and loads thread support later (is this supported)? The 5% cost of checking on every call can be avoided by pushing the check into a fucntion. E.g.: for getc(): % #define __sgetc(p) (--(p)->_r < 0 ? __srget(p) : (int)(*(p)->_p++)) It can be arranged that --(p)->_r < 0 is always true for the threaded case (by keeping only a flag in it and keeping the real count elsewhere). The threaded case would become slightly slower since it would always have the dummy count check plus a dummy count fixup. Actually it shouldn't need the fixup. The above is hand-optimized for PDP11's and would probably be no slower on current hardware with current compilers written as: #define getc(p) ((p)->_r <= 0 ? __srget(p) : (--(p)->_r, (int)(*(p)->_p++))) I once wrote a version of stdio that optimized the usual case putc() similarly by arranging that the write count is always 0 except for the fully buffered case, so that other cases get handled by a function (this pessimizes the line buffered case relative to the FreeBSD putc_unlocked()). Bruce
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