Re: [PATCH 0/2] Require that constraints are used to reference global regs

[Michael Matz <matz at suse dot de>]

Hi,

On Tue, 24 Apr 2018, Alexander Monakov wrote:

> > Sure but even for that we need to decide if we want to go that or the 
> > opposite way, and that's not easy when a deadline is lurking behind 
> > you.
> 
> I am surprised there is any question. Even gcc-3.4 optimizes reg vars 
> over asms, on a testcase not unlike the very one you've used I see:

Thanks for checking.  Seems all the code that was supposed to care for 
enabling this was always incomplete since a long time, which certainly 
speaks for going the explicit dependency way.

> Besides, I don't really understand the objection about changing the docs 
> for gcc-8.1. Even if there's a doubt about future development, improving 
> the documentation to explain the behavior of a released compiler should 
> be good. Right now the docs are simply wrong.

Well, documentation is both: a description and specification.  If we 
change the documentation now we might not be in the position anymore to 
change behaviour and docu back because people might in the meanwhile rely 
on the specification.  Which is why documentation changes that promise a 
certain behaviour are exactly the ones that shouldn't be rushed (even less 
than actual implementation changes).

But in the meantime you seem to have established that everyone thinking 
that asms ever had implicit dependencies on global reg vars was merely 
wishful.  I retract my objection to the documentation change (i.e. the 
specification of non implicit dependencies).  Thanks for persisting :)


Ciao,
Michael.