[trunk] patch to clean up simulation of insns in df.

[Richard Sandiford]

Kenneth Zadeck <zadeck@naturalbridge.com> writes:
> +   It would be trivial to define a similar set of functions that work
> +   in the forwards direction.  The only changes would be to process
> +   the uses before the defs and properly rename the functions.  This
> +   has so far not been necessary.

But this isn't true, is it?  The comment seems to describe what
the removed function does, but I think the removed function is wrong.

With forward scanning, a value is born at the point of definition and
dies after the last use (if any).  The point of death is indicated
by REG_DEAD and REG_UNUSED notes.

So the forward version of df_simulate_on_insn would do something like:

  - kill all values in REG_DEAD notes
  - make all defined values live
  - kill all values in REG_UNUSED notes

Despite what the comment says, I don't think it would look at the
DF_REF_USES at all.

I'm not saying you should say all that in the comment ;)  But I think
it's one of those comments that's more misleading than helpful.

Richard