Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: anton@mips.complang.tuwien.ac.at (Anton Ertl)
Newsgroups: comp.arch
Subject: Re: Memory ordering
Date: Tue, 03 Dec 2024 09:01:44 GMT
Organization: Institut fuer Computersprachen, Technische Universitaet Wien
Lines: 27
Message-ID: <2024Dec3.100144@mips.complang.tuwien.ac.at>
References: <vfono1$14l9r$1@dont-email.me> <vh4530$2mar5$1@dont-email.me> <-rKdnTO4LdoWXKj6nZ2dnZfqnPWdnZ2d@supernews.com> <vh5t5b$312cl$2@dont-email.me> <5yqdnU9eL_Y_GKv6nZ2dnZfqn_GdnZ2d@supernews.com> <2024Nov15.082512@mips.complang.tuwien.ac.at> <vh7ak1$3cm56$1@dont-email.me> <20241115152459.00004c86@yahoo.com> <vh8bn7$3j6ql$1@dont-email.me> <vhb2dc$73fe$1@dont-email.me> <vhct2q$lk1b$2@dont-email.me> <2024Nov17.161752@mips.complang.tuwien.ac.at> <vhh16e$1lp5h$1@dont-email.me>
Injection-Date: Tue, 03 Dec 2024 10:36:12 +0100 (CET)
Injection-Info: dont-email.me; posting-host="4f224cff8eacfd56eb8a0f66fcc6d139";
	logging-data="4187762"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/2O/4KI1+nT2UipSnOrQk6"
Cancel-Lock: sha1:7YJFpz1e4Wz78nUdx91vz+idByQ=
X-newsreader: xrn 10.11
Bytes: 2541

"Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> writes:
>On 11/17/2024 7:17 AM, Anton Ertl wrote:
>> jseigh <jseigh_es00@xemaps.com> writes:
>>> Or maybe disable reordering or optimization altogether
>>> for those target architectures.
>> 
>> So you want to throw out the baby with the bathwater.
>
>No, keep the weak order systems and not throw them out wrt a system that 
>is 100% seq_cst? Perhaps? What am I missing here?

Disabling optimization altogether costs a lot; e.g., look at
<http://www.complang.tuwien.ac.at/anton/bentley.pdf>: if you compare
the lines for clang-3.5 -O0 with clang-3.5 -O3, you see a factor >2.5
for the tsp9 program.  For gcc-5.2.0 the difference is even bigger.

That's why jseigh and people like him (I have read that suggestion
several times before) love to suggest disabling optimization
altogether.  It's a straw man that does not even need beating up.  Of
course they usually don't show results for the supposed benefits of
the particular "optimization" they advocate (or the drawbacks of
disabling it), and jseigh follows this pattern nicely.

- anton
-- 
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>