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

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

On 4/10/19 12:42 PM, Daniel Colascione wrote:
> PIE and shared libraries are irrelevant. The whole point of pdumper is to
> be invariant across different PIE and DSO configurations.

Yes, and that's part of the point. Because the pdumper doesn't care how
'write' is implemented so long as it's done correctly, the fingerprint
doesn't need to include a checksum of the implementation of 'write'.
There's a good chunk of the Emacs executable that is in the same
category as 'write' - that is, the chunk doesn't matter for the purposes
of the fingerprint. Whether we checksum the irrelevant chunk is a
pragmatic/efficiency issue; it's not needed for correctness.

With this in mind, checksumming the .o files ought to be enough to
generate a fingerprint good enough for the intended purpose of the
checksum. The checksum won't capture how 'write' is implemented, nor
will it capture detailed decisions about how the linker lays out Emacs's
low-level objects, but that's OK: the pdumper doesn't care about those
things so long as they're done correctly.

> The temacs.in mechanism
> covers all current and *and unknown future* binary layout modifications.
> It breaks only if linker symbol arrangement depends on randomness or on
> the precise content of the fingerprint array, and both of these
> possibilities are unlikely.

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. But none of this should matter, as any such
"breakage" should be irrelevant to the pdumper.