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

[David Brown]

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.