Ping: PR25514: A problem with note distribution in combine

[Alan Modra]

On Wed, May 10, 2006 at 01:47:31PM +0100, Richard Sandiford wrote:
> > Alan added the comment:
> >
> >               /* You might think you could search back from FROM_INSN
> >                  rather than from I3, but combine tries to split invalid
> >                  combined instructions.  This can result in the old I2
> >                  or I1 moving later in the insn sequence.  */

If I remember correctly, I was seeing this sequence

	i2
	other insns
	i3

being combined to

	deleted
	other insns
	i2+i3

but i2+i3 was invalid so it was split to

	deleted
	other insns
	i2
	i3

A rather useless exercise on the part of combine but illustrates why a
from_insn pointing to the old i2 can't be used.  (Perhaps there was an
i1 involved too, I can't remember.)

-- 
Alan Modra
IBM OzLabs - Linux Technology Centre