Re: Different object code generated each time file is compiled

[Brian Dessent <brian at dessent dot net>]

Burt Bicksler wrote:

> We have tracked the difference down to the following line in the
> assembly code that is emited:
> 
> <_GLOBAL__I_software_foo_fah_snmp_AFSecZoneMib.cpp_8D2A3925_B0CAE5BA>:

You can use c++filt to help you here:

$ echo
'_GLOBAL__I_software_foo_fah_snmp_AFSecZoneMib.cpp_8D2A3925_B0CAE5BA' |
c++filt
global constructors keyed to
software_foo_fah_snmp_AFSecZoneMib.cpp_8D2A3925_B0CAE5BA

So this is the name of a function that runs static constructors for some
object in AFSecZoneMib.cpp.  It's been uniquified so that it can't clash
with any other object in the link from any other TU.

> The last four HEX bytes, B0CAE5BA, are what is different on each run.
> The rest of the line is the same.
> 
> Searching around Google hasn't turned up too much information yet.  One
> reference was to a global constructor tag, but there was no mention of
> how the suffix is derived.  Not sure if this has anything to do with the
> anonymous namespace suggestion made earlier.  I think that we'll try the
> -frandom-seed switch to see if that will do it, since the description
> there says "certain" symbol names that have to be different...

Look at gcc/tree.c:get_file_function_name().  The first group is a CRC32
of the object name and the second is a CRC32 of some randomness, so in
other words -frandom-seed is what you need.

Brian