Re: Can LTO minor version be updated in backward compatible way ?

[Florian Weimer <fweimer@redhat.com>]

* Jeff Law:

> On 7/17/19 11:29 AM, Andi Kleen wrote:
>> Romain Geissler <romain.geissler@amadeus.com> writes:
>>>
>>> I have no idea of the LTO format and if indeed it can easily be updated
>>> in a backward compatible way. But I would say it would be nice if it
>>> could, and would allow adoption for projects spread on many teams
>>> depending on each others and unable to re-build everything at each
>>> toolchain update.
>> 
>> Right now any change to an compiler option breaks the LTO format
>> in subtle ways. In fact even the minor changes that are currently
>> done are not frequent enough to catch all such cases.
>> 
>> So it's unlikely to really work.

> Right and stable LTO bytecode really isn't on the radar at this time.

Maybe it's better to serialize the non-preprocessed source code instead.
It would need some (hash-based?) deduplication.  For #include
directives, the hash of the file would be captured for reproducibility.
Then if the initial #defines are known, the source code after processing
can be reproduced exactly.

Compressed source code is a surprisingly compact representation of a
program, usually smaller than any (compressed) IR dump.

Thanks,
Florian