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From: "Jiangning Liu" <jiangning.liu@arm.com>
To: "'Richard Guenther'" <richard.guenther@gmail.com>
Cc: <gcc@gcc.gnu.org>
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Subject: RE: A case that PRE optimization hurts performance
Date: Mon, 26 Sep 2011 08:34:00 -0000
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> > * Without PRE,
> >
> > Path1:
> > =A0 =A0 =A0 =A0movl =A0 =A0$2, %eax
> > =A0 =A0 =A0 =A0cmpl =A0 =A0$1, %eax
> > =A0 =A0 =A0 =A0je =A0 =A0 =A0.L3
> >
> > Path2:
> > =A0 =A0 =A0 =A0movb =A0 =A0$3, %al
> > =A0 =A0 =A0 =A0cmpl =A0 =A0$1, %eax
> > =A0 =A0 =A0 =A0je =A0 =A0 =A0.L3
> >
> > Path3:
> > =A0 =A0 =A0 =A0cmpl =A0 =A0$1, %eax
> > =A0 =A0 =A0 =A0jne =A0 =A0 .L14
> >
> > * With PRE,
> >
> > Path1:
> > =A0 =A0 =A0 =A0movl =A0 =A0$1, %ebx
> > =A0 =A0 =A0 =A0movl =A0 =A0$2, %eax
> > =A0 =A0 =A0 =A0testb =A0 %bl, %bl
> > =A0 =A0 =A0 =A0je =A0 =A0 =A0.L3
> >
> > Path2:
> > =A0 =A0 =A0 =A0movl =A0 =A0$1, %ebx
> > =A0 =A0 =A0 =A0movb =A0 =A0$3, %al
> > =A0 =A0 =A0 =A0testb =A0 %bl, %bl
> > =A0 =A0 =A0 =A0je =A0 =A0 =A0.L3
> >
> > Path3:
> > =A0 =A0 =A0 =A0cmpl =A0 =A0$1, %eax
> > =A0 =A0 =A0 =A0setne =A0 %bl
> > =A0 =A0 =A0 =A0testb =A0 %bl, %bl
> > =A0 =A0 =A0 =A0jne =A0 =A0 .L14
> >
> > Do you have any more thoughts?
>=20
> It seems to me that with PRE all the testb %bl, %bl
> should be evaluated at compile-time considering the
> preceeding movl $1, %ebx.  Am I missing something?
>=20

Yes. Can this be done by PRE or any other optimizations in middle end?

Thanks,
-Jiangning

> Richard.
>=20