Date: Sat, 16 May 2015 12:48:18 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Eliminating preference for avoiding thread pointer? Cost on MIPS? 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 > \u@\h:\w\$ wget http://192.168.2.114:8080/def-bin > Connecting to 192.168.2.114:8080 (192.168.2.114:8080) > \u@\h:\w\$ echo * > def-bin > \u@\h:\w\$ chmod +x def-bin > \u@\h:\w\$ ./def-bin > 0 0.016751000 > \u@\h:\w\$ wget http://192.168.2.114:8080/rd-bin > Connecting to 192.168.2.114:8080 (192.168.2.114:8080) > \u@\h:\w\$ chmod +x rd-bin > \u@\h:\w\$ ./rd-bin > Illegal instruction > > def-bin is withou -DDO_RDHWR, rd-bin is with. > Both compiled static with musl 1.1.6 (because that's the latest musl-cross > toolchain) and stripped. > > free reports 448 kb of 5736 kb free. (In other words, there's a reason it's > that stripped down.) Bleh, it looks like they intentionally broke their kernel to save a few bytes... I don't think it's possible to support such configurations, at least not reasonably. Rich
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