Date: Sat, 16 May 2015 02:19:33 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Eliminating preference for avoiding thread pointer? Cost on MIPS? On Fri, May 15, 2015 at 11:55:44PM -0400, Rich Felker wrote: > Traditionally, musl has gone to pretty great lengths to avoid > depending on the thread pointer. The original reason was that it was > not always initialized, and when it was, the init was lazy. This > resulted in a lot of cruft, where we would have lots of constructs of > the form: > > bar = some_predicate ? __pthread_self()->foo : global_foo > > or similar. Being that these predicates depend(ed) on globals, they > were/are rather expensive in position-independent code on most archs. > Now that the thread pointer is always initialized at startup (since > 1.1.0) and assumed to have succeeded (since 1.1.9; musl now performs > HCF if it fails), this seems to be an unnecessary cost. Not only does > it cost cycles; it also has a complexity cost in terms of code to > maintain the state of the predicates (e.g. the atomics for locale > state) and in terms of libc-internal assumptions. So I'd like to just > use the thread pointer directly wherever it makes sense, and take > advantage of the fact that we have it. > > [...] > > So I think, whatever the performance results end up being, we have an > acceptable path forward to use the (possibly virtual) thread pointer > unconditionally throughout musl. Just committed the first stage of this work: http://git.musl-libc.org/cgit/musl/commit/?id=68630b55c0c7219fe9df70dc28ffbf9efc8021d8 If there are old-MIPS performance regressions we'll fix them at the arch level rather than keeping this cruft all over the source. Rich
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