Date: Mon, 18 May 2015 16:16:20 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Eliminating preference for avoiding thread pointer? Cost on MIPS? On Mon, May 18, 2015 at 12:35:55PM -0700, Andre McCurdy wrote: > On Sat, May 16, 2015 at 9:48 AM, Rich Felker <dalias@...c.org> wrote: > > On Sat, May 16, 2015 at 09:33:20AM -0700, Isaac Dunham wrote: > >> 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. > >> > > >> > Unfortunately, there's one arch where thread-pointer access may be > >> > prohibitively costly: old MIPS. On the MIPS o32 ABI, the thread > >> > pointer is accessed via the "rdhwr $3,$29" instruction, which was only > >> > introduced in MIPS32rev2. MIPS-I, MIPS-II, and possibly the original > >> > MIPS32 lack it, and while Linux has a "fast path" trap to emulate it, > >> > I'm not clear on how "fast" it is. > >> > > >> > First, I'd like to find out how slow this trap is. If it's something > >> > like 150 cycles, that's ugly but probably acceptable. If it's more > >> > like 1000 cycles, that's a big problem. If anyone can run the attached > >> > test program on real MIPS-I or MIPS-II hardware and give me the > >> > results, please do! Compile it once with -O3 -DDO_RDHWR and once with > >> > just -O3 and send the (one-line) output of both to the list. It > >> > doesn't matter what libc your MIPS system is using -- any should be > >> > fine, but you might need to link with -lrt on glibc or uclibc. > >> > >> dd-wrt micro on a WRT54Gv8.0: > >> \u@\h:\w\$ cat /proc/version > >> Linux version 2.4.37 (root@...wrt) (gcc version 3.4.6 (OpenWrt-2.0)) #13303 Thu Aug 12 04:47:54 CEST 2010 > > It looks like rdhwr emulation was first added in linux 2.6.15, so > 2.4.37 is likely too old to run this test? Ah yes, that would explain it. Linux 2.4 is pre-NPTL and really doesn't have any of the stuff needed to support threads. I could look and see if LinuxThreads might have had any practical way to do TLS for 2.4 though; this may give us a fallback for accessing TLS quickly on MIPS-I and MIPS-II. Rich
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