Re: [PATCH ARM]Extend thumb1_reorg to save more comparison instructions

[Richard Earnshaw <rearnsha at arm dot com>]

On 18/04/13 09:20, Steven Bosscher wrote:
> On Thu, Apr 18, 2013 at 7:34 AM, Bin Cheng wrote:
> > Hi,
> > Before thumb1_reorg, ARM backend uses peephole to save comparison
> > instructions when a flag setting move is found before branch instruction.
> > Since we are using thumb1_reog now, it can be extended to catch more
> > opportunities by searching flag setting move instruction before branch,
> > rather than only the exact one before branch.
> > For example:
> >
> > mov r0, r1
> > //other insns does not kill r0
> > branch if (r0 == 0)
> > //other insns
> >
> > Tested on thumb1, is it OK?
>
> Isn't this something you could use compare-elim.c for instead of a
> target specific pass?
>
> Ciao!
> Steven

I haven't looked at compare-elim, but probably not.  Thumb1 doesn't use 
cc1 and doesn't expose flag setting instructions in a MI manner (since 
add instructions clobber the flags).  As such it needs private back-end 
data to work out what's safe.