public inbox for libc-alpha@sourceware.org
 help / color / mirror / Atom feed
From: "Carlos O'Donell" <carlos@redhat.com>
To: John David Anglin <dave.anglin@bell.net>,
	Mike Frysinger <vapier@gentoo.org>
Cc: libc-alpha@sourceware.org, carlos@systemhalted.org
Subject: Re: [PATCH] hppa: fix __O_SYNC to match the kernel
Date: Sun, 08 Mar 2015 19:52:00 -0000	[thread overview]
Message-ID: <54FCA873.3030008@redhat.com> (raw)
In-Reply-To: <BLU436-SMTP175B7131A983C54990446FB97130@phx.gbl>

On 03/01/2015 02:55 PM, John David Anglin wrote:
> In fesetenv.c, the post increment of bufptr was retained in the first asm but the constraint for it
> does not indicate that bufptr is modified.  As a result, GCC miscompiled fesetenv.  We get better
> code by not modifying bufptr as GCC doesn't have to reload bufptr.

It uses "+r" which means the operand, the register, is read and modified.

The fact that we get better code, or work around a gcc bug, is a good reason
to make the change, but I don't see what's wrong with the original code.

> The main bug in feholdexcept is the second set of bufptr.  This existed to the restore the exception
> registers in reverse order.  This statement should have been removed when the code was changed
> to only update the status and first exception registers.  The offset used in the statement is also off
> by a factor two, so it probably never worked correctly.  With the current patch, the code loads zero to
> the status and exception register.  As a result, the T bit is not set properly.

This doesn't sound right either.

fstd,ma %%fr0,8(%1)

Results in a store to bufptr, with bubptr-=8 *after* the store.

fldd,mb -8(%0),%%fr0

Results in a bufptr-=8, followed by a load from bufptr to fr0.

Thus while the operations are left-over from previous iterations of
the code where we did save/load all of the registers, the above code
is idempotent with respect to your changes.

The one problem is that bufptr is not marked as changed in the *second*
assembly contstraint which just lists bufptr as input (which is wrong)
and an optimization might reorder things such that this breaks.

Cheers,
Carlos.

  parent reply	other threads:[~2015-03-08 19:52 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-02-24  5:19 Mike Frysinger
2015-02-24 14:50 ` John David Anglin
2015-02-24 16:20   ` Joseph Myers
2015-02-27  5:06     ` Carlos O'Donell
2015-02-27  6:53   ` Mike Frysinger
2015-03-01 19:55     ` John David Anglin
2015-03-01 22:24       ` Joseph Myers
2015-03-02 13:18         ` John David Anglin
2015-03-02 14:09           ` Joseph Myers
2015-03-11  7:33           ` Mike Frysinger
2015-03-08 19:52       ` Carlos O'Donell [this message]
2015-03-08 21:30         ` John David Anglin
2015-03-09 12:36           ` Carlos O'Donell
2015-03-07 17:13     ` John David Anglin
2015-03-08 19:53       ` Carlos O'Donell
2015-03-11  6:29       ` Carlos O'Donell
2015-02-27  6:46 ` Mike Frysinger

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=54FCA873.3030008@redhat.com \
    --to=carlos@redhat.com \
    --cc=carlos@systemhalted.org \
    --cc=dave.anglin@bell.net \
    --cc=libc-alpha@sourceware.org \
    --cc=vapier@gentoo.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).