[arm-gnu] gcc-arm Compiler produce different object file for the same source file

[Abdurahman Alfeky]

there is no chnagein source code , and there no use of __DATE__ or time
macros , no regenerated headers and by looking into the output of objdump
on object file which is compiled without debugging information i can see
too many differences in assembly instructions. i heard about using
-frandom-seed=string to avoid this but it is not clear how the compiler
deal with it ?

copied from man page "
http://gcc.gnu.org/onlinedocs/gcc/Debugging-Options.html"

> -frandom-seed=stringThis option provides a seed that GCC uses in place of
> random numbers in generating certain symbol names that have to be different
> in every compiled file. It is also used to place unique stamps in coverage
> data files and the object files that produce them. You can use the
> -frandom-seed option to produce reproducibly identical object files.
>
> The string should be different for every file you compile
>



On Thu, Jun 13, 2013 at 9:30 AM, David Brown <david at westcontrol.com> wrote:

> On 12/06/13 17:59, Abdurahman Alfeky wrote:
> > Hi All ,
> >
> > Have anyone face the following problem before:
> >
> > Using the GCC compiler for ARM (windows) :
> >
> >     /arm-none-eabi-gcc.exe (Sourcery CodeBench Lite 2012.09-63) 4.7.2/
> >     version
> >
> > I have got different object file produced every ~5 times i compiled the
> > same source file.
> >
> > The optimization level 3 is used (aggressive) is used, compiler options
> > used:
> >
> >     -O3 -mcpu=cortex-a8 -mfpu=neon -mfloat-abi=softfp -fshort-wchar
> >     -fshort-enums  -funsafe-math-optimizations -mvectorize-with-neon-quad
> >
> > The dump of the different object files shows too many differences in
> > assembly instructions , registers and addresses used.
> >
> >   *
> >
> >     Is it normal that compiler optimize/compile exactly the same source
> >     file differently and produce different object files ?! is it a
> >     compiler bug ?
> >
> >   *
> >
> >     How to avoid this behavior without turning off aggressive
> optimization ?
> >
> > Thanks
> >
>
> There are usually differences in the object file, because it includes
> things like compilation date in the debugging information.  But I don't
> think you should get differences in the actual generated code.  Are you
> sure there is nothing changing, like headers, use of __DATE__ macros,
> etc.?  Minor changes in the source code can sometimes lead to apparently
> large changes in the object code such as different register choices or
> different ordering of code (the behaviour of such code is identical, but
> it can look very different).
>
> If you are looking at the final linked code, then remember that the
> order of the object files in the link can often affect the order of the
> generated code.
>
>
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>



-- 
Abdurahman Alfeky,
Software Engineer
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