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From: Bruno Haible <bruno@clisp.org>
To: Paul Eggert <eggert@cs.ucla.edu>
Cc: Simon Josefsson <simon@josefsson.org>, bug-gnulib@gnu.org
Subject: Re: explicit_bzero and -std=c99
Date: Sun, 27 Nov 2022 19:24:25 +0100	[thread overview]
Message-ID: <6449031.tM3a2QDmDi@nimes> (raw)
In-Reply-To: <4ba22843-7491-b610-bf42-45b99fa302a0@cs.ucla.edu>

Paul Eggert wrote:
> >     2) Is there no other way to implement explicit_bzero without 'asm'?
> >     There is a another fallback code using volatile pointers, but I'm not
> >     sure it really has the same semantics.
> 
> That fallback should work, though it's a bit slower.

I'm afraid that the fallback code

  /* Invoke memset through a volatile function pointer.  This defeats compiler
     optimizations.  */
  void * (* const volatile volatile_memset) (void *, int, size_t) = memset;
  (void) volatile_memset (s, '\0', len);

will stop working, as compilers get "smarter".

The other code based on asm is not so likely to break, since it is documented
that "Using the "memory" clobber effectively forms a read/write memory barrier
for the compiler." [1]

Bruno

[1] https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/Extended-Asm.html





      parent reply	other threads:[~2022-11-27 18:24 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CALAySuJi-gG53FDojZtMwZ9e254o3LbdAxnjZA6esN3724ZXLg@mail.gmail.com>
     [not found] ` <87tu2kps44.fsf@latte>
2022-11-27 11:27   ` explicit_bzero and -std=c99 Simon Josefsson via Gnulib discussion list
2022-11-27 17:48     ` Bruno Haible
2022-11-28 10:32       ` Simon Josefsson via Gnulib discussion list
2022-11-29 14:49         ` Bruno Haible
2022-11-27 18:05     ` Paul Eggert
2022-11-27 18:08       ` Paul Eggert
2022-11-27 18:24       ` Bruno Haible [this message]

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