role of executable_checksum & LTO?

[Basile Starynkevitch]

On Mon, 2010-06-28 at 16:58 -0700, Ian Lance Taylor wrote:
> Basile Starynkevitch <basile@starynkevitch.net> writes:
> 
> > What is the role of executable_checksum from c-common.h & generated by
> > genchecksum.
> 
> It is used for precompiled headers.  It verifies that the precompiled
> header was generated by precisely the same compiler as the one trying to
> use the precompiled header.  This approach is used because a precompiled
> header is essentially a memory dump of the compiler after parsing the
> header.  It would not work to use a memory of one compiler in another
> compiler.

Thanks for the nice explanation. 

However, I see a slightly more general use of executable_checksum (or
something similar) in plugins.  Imagine a plugin that store some
information somewhere (e.g. in a database) and which might reload that
information later.  It could be very useful (for that or such plugin[s])
to store a [nearly] unique identifier of the GCC compiler using it with
the data (to avoid reusing the same data with a slightly different GCC
compiler, eg 4.5.1 vs 4.5.0).  Then that plugin would be happy to use
the executable_checksum to avoid nightmares when incorrectly reusing
some data with a slightly different compiler. And version information is
not exactly adequate (the same gcc 4.5.0 could be built & configured
differently).

> > What is the reason of not having executable_checksum inside lto1 binary?
> 
> lto1 does not use precompiled headers, and as such does not need
> executable_checksum.  The executable_checksum variable only appears in
> those frontends which need it, namely the C family frontends.


But that lto1 could use a plugin such as I imagined above.  Perhaps the
better way might be to have a checksum identifying libbackend.a?

Would that be sensible to add, a libbackend_checksum?

Cheers.