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Re: Backward insn combination?
- To: richard dot earnshaw at arm dot com
- Subject: Re: Backward insn combination?
- From: Jeffrey A Law <law at cygnus dot com>
- Date: Wed, 30 Jun 1999 06:44:26 -0600
- cc: Michael Hayes <m dot hayes at elec dot canterbury dot ac dot nz>, egcs at egcs dot cygnus dot com
- Reply-To: law at cygnus dot com
In message <199906301104.MAA22454@sun52.NIS.cambridge>you write:
>
> law@cygnus.com said:
> > What you may need is multiple iterations of the combiner at higher
> > optimization levels. Though I have no idea how safe that would be and
> > how many new combinations later passes would find. Probably worth some
> > experiments.
>
>
> Rather than doing multiple passes, each time you do make a successful
> combination you could restart at the insn after the one deleted (or when
> three are combined after the earliest insn deleted).
That does not work in the case Michael is talking about. For his case to
work you have to back up to an unknown point *before* the insn you delete.
A
B
C
D
Assume we can't combine A & C because of B. But we can combine B & D and
delete B. To then be able to combine A & C you have to restart at A, not
after B.
jeff