From: "Paweł Sikora" <pluto@agmk.net>
To: gcc-help@gcc.gnu.org
Subject: question about equivalent x87/x64-64 fpu code...
Date: Fri, 13 May 2011 18:12:00 -0000 [thread overview]
Message-ID: <201105132011.34453.pluto@agmk.net> (raw)
Hi,
i'm using a 3rd-party engine http://glaros.dtc.umn.edu/gkhome/metis/metis/overview
for partitioning some complex data. it worked fine for years until today (may 13)...
observations:
- the 32-bit metis build produces nice and balanced partitons.
- the 64-bit metis build produces bad and unbalanced partitons.
the metis' engine uses arrays of integers on the public interface and internally
some float-based and unsafe in terms of precison (x<y and x==y) operations.
so, i've built/tested following metis variants:
1). -m32 -march=pentium4 -O1 - works fine.
2). -m32 -march=pentium4 -O1 -mfpmath=sse - works fine.
3). -m64 -march=x86-64 -O1 - bad/unbalanced partitions.
4). -m64 -march=x86-64 -O1 -mfpmath=387 - bad/unbalanced partitions.
at this point i've expected wrong results (< 80-bit precision) from variants 2/3
and good results from variants 1/4 but the real world differs.
next, i've isolated a one place in sources with float x<y stmt and changed it
to (x-y)<0.00001. with such change both native 1/3 variants give nice/equivalent results.
so, where is the problem? is the variants 1/4 really equivalent?
BR,
PaweÅ.
next reply other threads:[~2011-05-13 18:12 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-05-13 18:12 Paweł Sikora [this message]
2011-05-16 11:03 ` Andrew Haley
2011-05-16 22:58 ` Pawel Sikora
2011-05-16 23:57 ` Andrew Haley
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=201105132011.34453.pluto@agmk.net \
--to=pluto@agmk.net \
--cc=gcc-help@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).