public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/52080] Stores to bitfields introduce a store-data-race on adjacent data
Date: Wed, 01 Feb 2012 11:13:00 -0000	[thread overview]
Message-ID: <bug-52080-4-Cd4w8LFuCE@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-52080-4@http.gcc.gnu.org/bugzilla/>

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52080

--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-02-01 11:12:41 UTC ---
Btw,

  offset = bitnum / unit;
  bitpos = bitnum % unit;
  byte_offset = (bitnum % BITS_PER_WORD) / BITS_PER_UNIT
                + (offset * UNITS_PER_WORD);

byte_offset is bollocks (or has a really poor name).  On SPARC64 I see

(gdb) p unit
$11 = 8
(gdb) p offset
$12 = 12
(gdb) p bitpos
$13 = 0
(gdb) p byte_offset
$14 = 100

Other than that we are falling into the generic store_fixed_bit_field
routine which at the top does

  unsigned int total_bits = BITS_PER_WORD;

and

      mode = GET_MODE (op0);
      if (GET_MODE_BITSIZE (mode) == 0
          || GET_MODE_BITSIZE (mode) > GET_MODE_BITSIZE (word_mode))
        mode = word_mode;

(now, why we use a BLKmode mem here and not a QImode mem may be
surprising)

get_best_mode still returns DImode because that's the largest mode
the given alignment supports.  After that we're lost.  So it seems
that to fix this case we'd need to figure out some other largest
mode we can pass to get_best_mode.  The only hint would be from
providing a different mode for the initial MEM we create, like with

Index: gcc/expr.c
===================================================================
--- gcc/expr.c  (revision 183791)
+++ gcc/expr.c  (working copy)
@@ -4705,6 +4705,12 @@ expand_assignment (tree to, tree from, b
            to_rtx = adjust_address (to_rtx, mode1, 0);
          else if (GET_MODE (to_rtx) == VOIDmode)
            to_rtx = adjust_address (to_rtx, BLKmode, 0);
+         else if (TREE_CODE (to) == COMPONENT_REF
+                  && DECL_BIT_FIELD (TREE_OPERAND (to, 1))
+                  && DECL_MODE (TREE_OPERAND (to, 1)) != BLKmode)
+           to_rtx = adjust_address (to_rtx,
+                                    TYPE_MODE (DECL_BIT_FIELD_TYPE
+                                       (TREE_OPERAND (to, 1))), 0);
        }

       if (offset != 0)

That avoids the use of QImode we have on the field-decl but also
adjusts MEM_SIZE ...


  parent reply	other threads:[~2012-02-01 11:13 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-02-01 10:07 [Bug target/52080] New: " rguenth at gcc dot gnu.org
2012-02-01 10:23 ` [Bug target/52080] " rguenth at gcc dot gnu.org
2012-02-01 10:25 ` rguenth at gcc dot gnu.org
2012-02-01 10:39 ` rguenth at gcc dot gnu.org
2012-02-01 11:13 ` rguenth at gcc dot gnu.org [this message]
2012-02-01 17:12 ` ebotcazou at gcc dot gnu.org
2012-02-01 17:45 ` ebotcazou at gcc dot gnu.org
2012-02-01 17:50 ` matz at gcc dot gnu.org
2012-02-01 18:53 ` bergner at gcc dot gnu.org
2012-02-02 11:01 ` gcc-bugs at tesarici dot cz
2012-02-02 11:11 ` rguenth at gcc dot gnu.org
2012-02-02 12:40 ` gcc-bugs at tesarici dot cz
2012-02-21 12:04 ` rguenth at gcc dot gnu.org
2012-03-14 11:00 ` rguenth at gcc dot gnu.org
2012-03-14 11:01 ` rguenth at gcc dot gnu.org
2012-06-04  8:44 ` rguenth at gcc dot gnu.org
2013-02-19 20:37 ` LpSolit at netscape dot net
2014-02-16 13:17 ` jackie.rosen at hushmail dot com

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-52080-4-Cd4w8LFuCE@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).