public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/62156] memcmp doesn't see through memcpy at compile-time
Date: Mon, 18 Aug 2014 14:47:00 -0000	[thread overview]
Message-ID: <bug-62156-4-NGQ7CkaEYq@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-62156-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62156

--- Comment #2 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> What kind of std::string code is this?  That is, can we expect
> memcmp and memcpy to be adjacent (without intermediate memory operations?)

I don't remember the exact code, but it was similar to:
std::string("foo")=="bar"
which, with _GLIBCXX_EXTERN_TEMPLATE=0 (or LTO) gives:

  _29 = _50 + 24;
  __builtin_memcpy (_29, "foo", 3);
  if (_50 != &_S_empty_rep_storage)
    goto <bb 3>;

  <bb 3>:
  MEM[(struct _Rep *)_50].D.20711._M_length = 3;
  MEM[(char_type &)_50 + 27] = 0;
  __r_86 = __builtin_memcmp (_29, "bar", 3);

So it is setting the null character right after the string (could have used
memcpy of size 4?) and the length right before, which requires tight alias
checking to be sure that memcmp is not affected :-(

Probably a bit too specific to be worth it.


  parent reply	other threads:[~2014-08-18 14:47 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-08-15 11:36 [Bug tree-optimization/62156] New: " glisse at gcc dot gnu.org
2014-08-18 14:06 ` [Bug tree-optimization/62156] " rguenth at gcc dot gnu.org
2014-08-18 14:47 ` glisse at gcc dot gnu.org [this message]
2014-08-18 15:19 ` glisse at gcc dot gnu.org
2014-08-19  8:02 ` rguenther at suse dot de
2014-08-19 11:15 ` glisse at gcc dot gnu.org
2014-08-19 11:19 ` rguenther at suse dot de

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=bug-62156-4-NGQ7CkaEYq@http.gcc.gnu.org/bugzilla/ \
    --to=gcc-bugzilla@gcc.gnu.org \
    --cc=gcc-bugs@gcc.gnu.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).