Re: patch to add #pushdef, #popdef to cccp,cpplib

[Jeffrey A Law <law at cygnus dot com>]

  In message <35AD5BA1.5B32@tssc.co.nz>you write:
  > Jeffrey A Law wrote:
  > > Often they do not have a choice -- consider what's happened with asms
  > > that have drifted into system include files.  And in the process of
  > > getting wider exposure, they have uncovered fundamental flaws in the
  > > design of the asm extensions.
  > 
  > This is pure curiosity: is an example of these flaws the problem with
  > (eg) `asm ("foo %1,%0"::"r","r");' in C++ (parse error from the `::')?
I hadn't thought of that one :-)

There's no clean way to say things like:

  * I need to clobber a register of a certain class (we can only clobber
    explicit registers).

  * I need a particular value to appear in an input register at the
    start of the asm, then the register is available as a scratch
    register for the asm.  You can fake this with a dummy output
    operand, but there should be a cleaner way to describe this since
    it's a very common operation.
   
  * Fundamental incompatabilities between SMALL_REGISTER_CLASS machines
    and clobbering specific registers.

  * No way to expose what register classes correspond to constraint
    letters for input/output operands.

  * How do you describe to an asm-user how many registers they are
    allowed to use in an asm.    Sometimes it is not obvious that
    a particular operand may need multiple registers (to satisfy
    reloading).  Thus it is hard for a user to write a correct asms.

There's more, but thats what's come up within the last year.

  > Now, I'm not meaning to argue with you, but I thought the extended asm
  > statements were pretty good.  Mind you, getting the constraints and the
  > operand modifiers (eg %k0) right is a PITA, especially when they're
  > pooly documented.  So really, just what are the fundamental flaws in the
  > dedisng of the asm extensions?  Maybe they can be fixed (yeah, right. I
  > can imagine the caos).
asms are probably the best designed & implemented extension in the
compiler, yet we still have major problems with them.

Most of those problems can be fixed.  We'll probably end up doing it
at some point.

jeff