Bug 21188 - gcc over-optimizes volatile double
gcc over-optimizes volatile double
Status: CLOSED RAWHIDE
Product: Red Hat Linux
Classification: Retired
Component: gcc (Show other bugs)
7.0
i386 Linux
medium Severity medium
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Assigned To: Jakub Jelinek
David Lawrence
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Depends On:
Blocks: 21191
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Reported: 2000-11-21 11:58 EST by Plummer, Martyn
Modified: 2007-04-18 12:29 EDT (History)
0 users

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Fixed In Version:
Doc Type: Bug Fix
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Last Closed: 2000-11-27 11:28:09 EST
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Attachments (Terms of Use)
Example C program to highlight bug (8.84 KB, text/plain)
2000-11-21 12:00 EST, Plummer, Martyn
no flags Details

  None (edit)
Description Plummer, Martyn 2000-11-21 11:58:16 EST
gcc 2.96 ignores the volatile keyword when optimizing C code.

This bug came to light in the R language (see http://www.r-project.org and
the R-base RPM on powertools), which uses a routine called "machar" to
determine the machine characteristics at runtime.  As a result of the bug,
machar reports double precision to be 2^-63 instead of the correct value
2^-52 on an i686.

A search of the gcc mailing list archives shows that this bug was found
independently by Tom Truscott and reported by him on 2 August, but
without any follow-up. See
http://gcc.gnu.org/ml/gcc-bugs/2000-08/msg00037.html
He includes a short program which aborts when the bug is triggered.
See also
http://gcc.gnu.org/ml/gcc-bugs/2000-08/msg00038.html
for an obviously related bug.

The machar routine from the R language is too long to include here, but you
can grab a copy from
http://calvin.iarc.fr/gcc/machar.c
This includes a main() function that prints the value of the double
precision. Compare with and without optimization.
Comment 1 Plummer, Martyn 2000-11-21 12:00:06 EST
Created attachment 5622 [details]
Example C program to highlight bug
Comment 2 Jakub Jelinek 2000-11-22 07:27:45 EST
I've posted a fix for this at
http://gcc.gnu.org/ml/gcc-patches/2000-11/msg01227.html
and am waiting now for feedback on it. If it is accepted, it will make it
into next gcc rpms.
Comment 3 Jakub Jelinek 2000-11-27 11:28:06 EST
It was not accepted, but it will take quite some time before Richard Kenner
writes a fix he wants to see, so I'll be putting this patch into gcc-2.96-65
and above.

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