c/7710: GCC warns "<var> might be used uninitialized" when optimisation turned on

[royhills at hotmail dot com]

>Number:         7710
>Category:       c
>Synopsis:       GCC warns "<var> might be used uninitialized" when optimisation turned on
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Aug 24 05:26:00 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     royhills@hotmail.com
>Release:        GCC 2.95.2
>Organization:
>Environment:
Debian Linux "Potato" (2.2r7) running on i386 with custom 2.2.19 Kernel.
>Description:
GCC 2.95.2 on Debian Linux 2.2r7 gives the warning:
"`fp' might be used uninitialized in this function"
when optimisation is turned on with -O1 or -O2, but no
warning is issued if optimisation is turned off.

I've simplified the code to a 13 line program which I attach to this bug report.  Lookig at the code, I can see no way that the variable "fp" could be used uninitialised.

This issue doesn't seem to affect the real program which works fine with -O2 (so far anyway...).

Example:

# Program compiles fine without optimisation
rsh@roy:/usr/home/rsh/src$ gcc -Wall gcc-fp-unused.c

# But it gives the warning with optimisation on
rsh@roy:/usr/home/rsh/src$ gcc -O2 -Wall gcc-fp-unused.c
gcc-fp-unused.c: In function `main':
gcc-fp-unused.c:6: warning: `fp' might be used uninitialized in this function
>How-To-Repeat:
Bug is fully repeatable on my system.  Warning occurs every time the file shown in the attachment is compiled with optimisation turned on.
>Fix:
Compile without optimisation :-)
>Release-Note:
>Audit-Trail:
>Unformatted:
----gnatsweb-attachment----
Content-Type: application/octet-stream; name="gcc-fp-unused.ts"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="gcc-fp-unused.ts"

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