Bug 251096 - Difference in behaviour with compiler-optimised code (-O2)
Difference in behaviour with compiler-optimised code (-O2)
Product: Fedora
Classification: Fedora
Component: gcc (Show other bugs)
All Linux
low Severity low
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Assigned To: Jakub Jelinek
Depends On:
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Reported: 2007-08-06 23:56 EDT by David Campbell
Modified: 2007-11-30 17:12 EST (History)
0 users

See Also:
Fixed In Version:
Doc Type: Bug Fix
Doc Text:
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Clone Of:
Last Closed: 2007-08-07 02:56:59 EDT
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Attachments (Terms of Use)
bug.c to reproduce the problem (497 bytes, text/x-csrc)
2007-08-06 23:56 EDT, David Campbell
no flags Details

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Description David Campbell 2007-08-06 23:56:14 EDT
Description of problem:

This issue arose from tracing a problem with an infinite loop in the startup of
f7 build of xsane, the scanner software, but I've boiled the issue down to a
tiny C program that can be compiled with or without optimisation and in each of
those cases you can see that the generated executable gives a different result.

Version-Release number of selected component (if applicable):

gcc version 4.1.2 20070502 (Red Hat 4.1.2-12)

How reproducible:


Steps to Reproduce:
1. Save the attached bug.c C program
2. Compile:  gcc -o bug bug.c
3. Run: ./bug
4. Observe result 39019943
5. Compile:  gcc -O2 -o buggo bug.c
6. Run:  ./buggo
7. Observe result:  39019942

Also note that if you uncomment the commented printf statements in the attached
C source, you get a different result!!!!  Something is rather weird.

Actual results:

The two results were different

Expected results:

The two results should be the same
Comment 1 David Campbell 2007-08-06 23:56:15 EDT
Created attachment 160796 [details]
bug.c to reproduce the problem
Comment 2 David Campbell 2007-08-07 00:03:47 EDT
This difference in behaviour is also visible in gcc 2.96 on an old linux.
Comment 3 Jakub Jelinek 2007-08-07 02:56:59 EDT
That's caused by excess precision, not a bug, but a thing you need to take
into account when programming i?87.
See e.g. info gcc --index-search=ffloat-store
or e.g.
If this is undesirable in your program, you can either use
-ffloat-store option which will slow down your program, but ensure every
assignment to a double (or float) variable will do the rounding to that
precision, or if you have sufficiently new CPU, compile with
-msse2 -mfpmath=sse
or if you have 64-bit x86_64 CPU, compile 64-bit.

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