Re: [PATCH] diagnose unsupported uses of hardware register variables (PR 88000)

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Nov 14, 2018 at 05:35:06AM -0600, Segher Boessenkool wrote:
> On Wed, Nov 14, 2018 at 10:47:57AM +0100, Jakub Jelinek wrote:
> > This makes no sense to me.  There is nothing unsupported in passing
> > a local hard register variable to a function, that is well defined,
> > and as your testcase changes show, you broke quite some completely valid
> > testcases with that.
> 
> What could perhaps be useful is a warning for a local register var that
> is not used in any asm?
> 
> > What doesn't work as the reporter expect is assumption that local hard
> > register variables that live across function calls must have their values
> > preserved; they can be modified by the callees.
> 
> It would be really nice if we could fix that :-)

You mean for all local hard register variables living across function calls
save them into temporary and restore them around the calls?
One issue is that those variables aren't in SSA form, so liveness analysis
is harder.  And, would we want to have an exception for the stack pointer?
I mean there is no need for register void *sp __asm ("rsp"); to be
saved/restored that way, it shouldn't change value across function calls.
Plus, as has been mentioned, function calls aren't the only issue here,
e.g. division/modulo etc. that require specific hard registers might clobber
them too.

	Jakub