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From: Bruno Haible <bruno@clisp.org>
To: Paul Eggert <eggert@cs.ucla.edu>
Cc: bug-gnulib@gnu.org
Subject: Re: [PATCH 1/2] explicit_bzero-tests: pacify GCC
Date: Mon, 19 Jul 2021 00:23:04 +0200	[thread overview]
Message-ID: <2568272.mkVSEl9qYJ@omega> (raw)
In-Reply-To: <3ecc5494-65ab-cb9e-19c7-8aaef7b6ff18@cs.ucla.edu>

Paul Eggert wrote:
> I take your point that the patch changes the meaning of the test in an 
> undesirable way. I installed the attached to implement your suggestion.

Thank you.

> This new diagnostic points out a problem with the test, though. If GCC 
> can determine that memcmp reads uninitialized storage, GCC can optimize 
> away the memcmp and act as if memcmp returns 0 (or any other constant). 
> So the test as it stands is problematic given recent advances in 
> practical compilers.

Such compiler optimizations would need to be backed by the standards.
Are there initiatives to "outlaw" references to uninitialized storage
in recent C or C++ standards?

I hope code such as

  int x; /* uninitialized */
  if (!((x & 1) == 0 || (x & 1) == 1))
    abort ();

will never crash. x & 1 can only be 0 or 1. Tertium not datur.

Also, I think the danger is small: GCC does not _know_ that the array
is uninitialized. It's only a "maybe uninitialized". If GCC ever
infers that it is "certainly uninitialized", we could defeat that
through a use of 'volatile', such as

int (* volatile memcmp_func) (const void *, const void *, size_t) = memcmp;

> As an aside, I don't understand the discussion of asynchronous signal 
> invocations in that test's commentary. There is no asynchronous 
> signaling in that code.

In some operating systems, interrupt handling uses the stack of the
currently executing thread. While this is not the case in Linux [1],
it was definitely the case in Atari TOS, and is still probably the
case in some embedded OSes for MMU-less CPUs.

Bruno

[1] https://stackoverflow.com/questions/28759227/which-stack-is-used-by-interrupt-handler-linux



  reply	other threads:[~2021-07-18 22:23 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-18  4:56 [PATCH 1/2] explicit_bzero-tests: pacify GCC Paul Eggert
2021-07-18  4:56 ` [PATCH 2/2] memrchr-tests: " Paul Eggert
2021-07-18  9:12 ` [PATCH 1/2] explicit_bzero-tests: " Bruno Haible
2021-07-18 19:14   ` Paul Eggert
2021-07-18 22:23     ` Bruno Haible [this message]
2021-07-18 23:17       ` Paul Eggert
2021-07-19  0:37         ` Bruno Haible

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