Re: inline asm clobbers

[David Wohlferd <dw at LimeGreenSocks dot com>]

Resending due to bounced email.

On 3/11/2015 6:19 PM, Ian Lance Taylor wrote:
> On Wed, Mar 11, 2015 at 5:51 PM, David Wohlferd <dw@limegreensocks.com> wrote:
> > The reason I believe the order can change is this comment from i386.h:
> >
> > /* Order in which to allocate registers.  Each register must be
> >     listed once, even those in FIXED_REGISTERS.  List frame pointer
> >     late and fixed registers last.  Note that, in general, we prefer
> >     registers listed in CALL_USED_REGISTERS, keeping the others
> >     available for storage of persistent values.
> >
> >     The ADJUST_REG_ALLOC_ORDER actually overwrite the order,
> >     so this is just empty initializer for array.  */
> That is REG_ALLOC_ORDER.  The index that appears in an asm statement
> is the hard register number.  REG_ALLOC_ORDER is an array holding hard
> register numbers.  The hard register numbers can change in principle,
> by changing the source code, but I actually can't recall that ever
> happening.
To wrap this up:

Like Ian said, the order of registers here apparently never changes.  I 
read more into that comment than I should have.  For good luck, I 
experimented with -fomit-frame-pointer, -ffixed-, etc, and nothing has 
any impact here.  The list is the list.

In fact, it turns out you can use this same format with register variables:

    register int x asm("3"); // i386: ebx

So while I find it ugly, unnecessarily complex, and lacking in 
self-documenting-ness, it is not inherently buggy the way I feared it 
was, so I can't think of any good arguments for pulling it out.

Thanks to Ian and Paul for straightening me out.

dw