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Re: [PATCH] Fix PR63259: bswap not recognized when finishing with rotation
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Jakub Jelinek <jakub at redhat dot com>
- Cc: "Thomas Preud'homme" <thomas dot preudhomme at arm dot com>, GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Wed, 8 Oct 2014 09:27:24 +0200
- Subject: Re: [PATCH] Fix PR63259: bswap not recognized when finishing with rotation
- Authentication-results: sourceware.org; auth=none
- References: <000101cfe29b$22541de0$66fc59a0$ at arm dot com> <20141008063843 dot GQ1986 at tucnak dot redhat dot com> <000201cfe2c3$1e3d7d00$5ab87700$ at arm dot com> <000301cfe2c5$08c47080$1a4d5180$ at arm dot com> <20141008070435 dot GR1986 at tucnak dot redhat dot com>
On Wed, Oct 8, 2014 at 9:04 AM, Jakub Jelinek <email@example.com> wrote:
> On Wed, Oct 08, 2014 at 02:56:46PM +0800, Thomas Preud'homme wrote:
>> > From: firstname.lastname@example.org [mailto:gcc-patches-
>> > email@example.com] On Behalf Of Thomas Preud'homme
>> > Sent: Wednesday, October 08, 2014 2:43 PM
>> > > Also, perhaps you could short-circuit this if the rotation isn't by constant
>> Note that do_shift_rotate already check for this. Is it enough?
> The question is only about the compile time cost needed through all the
> routines find_bswap_or_nop calls before it finds out it can't do anything
> with the rotate, the short-circuiting could avoid that. E.g.
> find_bswap_or_nop_1 first recurses on the argument etc.
I wouldn't worry about that too much. Indeed the question would be what
should be canonical on GIMPLE (expanders should choose the optimal
vairant from both). I think a tree code should be always prefered to a
builtin function call - which means a rotate is more canonical than a
>From the performance side the pass could be re-structured to populate
a lattice, thus work from def to use instead of the other way around. Which
means we visit each stmt exactly once, compute its value symbolically
and check it against a rotate.