Date: Mon, 18 May 2015 12:35:55 -0700 From: Andre McCurdy <armccurdy@...il.com> To: musl@...ts.openwall.com Subject: Re: Eliminating preference for avoiding thread pointer? Cost on MIPS? 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? >> \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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