Re: [Emacs-diffs] master d826037 3/3: Remove the need for temacs.in

[Paul Eggert <eggert@cs.ucla.edu>]

>> checksumming the .o files ought to be enough to
>> generate a fingerprint good enough for the intended purpose of the
>> checksum.
> 
> No: that's simply wrong. pdumper *does* care about the low-level layout of
> objects within Emacs. We have dump-to-emacs relocations based on offsets
> from a known symbol within Emacs.

Ah, sorry, I didn't know that.

> The linker deciding to lay out objects
> within sections in a different order will break the dump.

But why would the linker do that?  It sounds like you're worrying that, even if 
we give the linker the same object files in the same order and ask it to link 
Emacs again, then the linker might generate a different executable, one that is 
incompatible with the previous one.  But if that's the case, the temacs.in 
solution has the same problem so we're no worse off than before. And if it's not 
the case, then what exactly is the failure scenario here? I'm not seeing any 
failure scenarios for the current approach that can't also be failure scenarios 
for the previous approach, even now that I know that pdumper cares about object 
order within a section.

>> The mechanism could also "break" with whole-program optimization that
>> inlines the fingerprint array, or with other plausible future changes to
>> linkers as they get smarter.
> 
> The optimization you've described doesn't matter: as long as changing the
> *value* of the fingerprint array (not its length!) preserves Emacs *object
> layout* change in Emacs, we're good.

The optimization I describe could migrate some or all of the fingerprint array's 
contents into the code, with the amount of migration depending on the contents 
of the fingerprint array, and with the migrated contents omitted from the 
fingerprint array. I don't see how object layout (in the sense that you 
describe) would be preserved in that scenario.