Use-and-clobber insns and some ancient combine.c code

[Ulrich Weigand]

Richard Earnshaw wrote:

> CLOBBER is used to indicate that a value is destroyed by the insn, but
> it doesn't say precisely when this happens.  In the case of hard
> registers we have to be conservative, because there is no way to convey
> the equivalent of an early-clobber operand (a value that is destroyed
> before all the other inputs have been processed).

I had not been aware of this interpretation of CLOBBER.  This is
unfortunate, as it appears to make the use of use-and-clobber
type patterns unfeasible ...

Has it always been that way?  The documentation appears a bit ambiguous
on that point, e.g.:

 When a @code{clobber} expression for a register appears inside a
 @code{parallel} with other side effects, the register allocator
 guarantees that the register is unoccupied both before and after that
 insn.  However, the reload phase may allocate a register used for one of
 the inputs unless the @samp{&} constraint is specified for the selected
 alternative (@pxref{Modifiers}).

These two sentences appear to directly contradict each other.

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  weigand@informatik.uni-erlangen.de