role of executable_checksum & LTO?

[David Brown]

On 29/06/2010 07:36, Basile Starynkevitch wrote:
> 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).
>

Are you thinking of something like ccache functionality being 
implemented as a gcc plugin rather than an external program?  You could 
do many nice things with that that are (I think) impossible with 
external ccache, such as caching parts of the compilation rather than 
just the whole compilation.  Pre-compiled headers could be effectively 
automated by such a system, and if the cache keys were based on internal 
structures rather than the cpp output (as ccache uses), then the 
pre-compiled sections could be re-used despite small changes in the code.