Re: libgcc: strange optimization

[Richard Guenther <richard.guenther@gmail.com>]

On Wed, Aug 3, 2011 at 3:27 PM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> On Wed, 3 Aug 2011, Richard Guenther wrote:
>
>> > Yes, that's reasonable.  As I understand the docs, in code like
>> >
>> > void foo ()
>> > {
>> >   register int var asm ("r1") = 10;
>> >   asm (";; use r1");
>> > }
>> >
>> > there is nothing that connects var to the asm and assuming that
>> > r1 holds 10 in the asm is a user error.
>> >
>> > The only place where the asm attached to a variable needs to have
>> > effect are the inline asm sequences that explicitly refer to
>> > respective variables.  If there is no inline asm referencing a
>> > local register variable, there is on difference to a non-register
>> > auto variable; there could even be a warning that in such a case
>> > that
>> >
>> >   register int var asm ("r1") = 10;
>> >
>> > is equivalent to
>> >
>> >   int var = 10;
>> >
>> > This would render local register variables even more functional
>> > because no one needed to care if there were implicit library calls or
>> > things like that.
>>
>> Yes, I like that idea.
>
> I do too.  Except it doesn't work :)
>
> There's a common idiom of accessing registers read-only by declaring local
> register vars.  E.g. to (*grasp*) the stack pointer.  There won't be a DEF
> for that register var, and hence at use-points we couldn't reload any
> sensible values into those registers (and we really shouldn't clobber the
> stack pointer in this way).
>
> We could introduce that special semantic only for non-reserved registers,
> and require no writes to register vars for reserved registers.
>
> Or we could simply do:
>
>  if (any_local_reg_vars)
>    optimize = 0;
>
> But I already see people wanting to _do_ optimization also with local reg
> vars, "just not the wrong optimizations" ;-/

I'd say we should start rejecting all these bogus constructs by default
(maybe accepting them with -fpermissive and then, well, maybe generate
some dwim code).  That is, local register var decls are only valid
with an initializer, they are implicitly constant (you can't re-assign to them).
Reserved registers are a no-go (like %esp), either global or local.

Richard.

>
> Ciao,
> Michael.