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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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