From: Segher Boessenkool <segher@kernel.crashing.org>
To: vogt@linux.vnet.ibm.com, gcc-patches@gcc.gnu.org,
Andreas Krebbel <krebbel@linux.vnet.ibm.com>
Subject: Re: [PATCH] combine: Omit redundant AND in change_zero_ext.
Date: Wed, 14 Dec 2016 19:55:00 -0000 [thread overview]
Message-ID: <20161214193248.GY30845@gate.crashing.org> (raw)
In-Reply-To: <20161214100147.GA3427@linux.vnet.ibm.com>
On Wed, Dec 14, 2016 at 11:01:47AM +0100, Dominik Vogt wrote:
> This is another micro-optimisation in change_zero_ext. If an
>
> (and (lshiftrt ... (N)) (M))
>
> generated by change_zero_ext is equivalent to just
>
> (lshiftrt ... (N))
>
> (because the AND constant selects the N rightmost bits of the
> result), strip off the AND.
>
> _But_ I'm still not completely convinced whether this is a good
> idea. It may become necessary to add md patterns to deal with
> just the LSHIFTRT. On the other hand it saves the need for
> another special case in change_zero_ext, and a less obvious, very
> specific risbg pattern on s390
For PowerPC we should already have all such patterns with a "bare" shift
(they can be created in other ways, too).
> Bootstrapped and regression tested on s390x and s390. (Targets
> with risbg-like instructions (Power, others?) may need some
> tuning.)
But, it is also possible I missed some. So please wait until I have
tested it.
> diff --git a/gcc/combine.c b/gcc/combine.c
> index 19851a2..5ebf31c 100644
> --- a/gcc/combine.c
> +++ b/gcc/combine.c
> @@ -11280,8 +11280,13 @@ change_zero_ext (rtx pat)
> else
> continue;
>
> - wide_int mask = wi::mask (size, false, GET_MODE_PRECISION (mode));
> - x = gen_rtx_AND (mode, x, immed_wide_int_const (mask, mode));
> + if (!(GET_CODE (x) == LSHIFTRT
> + && CONST_INT_P (XEXP (x, 1))
> + && size + INTVAL (XEXP (x, 1)) == GET_MODE_PRECISION (mode)))
> + {
> + wide_int mask = wi::mask (size, false, GET_MODE_PRECISION (mode));
> + x = gen_rtx_AND (mode, x, immed_wide_int_const (mask, mode));
> + }
One could argue that this should have been an lshiftrt in the first place
then, not a zero_ext*. Hrm.
Segher
next prev parent reply other threads:[~2016-12-14 19:33 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-12-14 10:15 Dominik Vogt
2016-12-14 19:55 ` Segher Boessenkool [this message]
2016-12-15 9:16 ` Dominik Vogt
2016-12-15 9:19 ` Segher Boessenkool
2016-12-19 9:57 ` Andreas Krebbel
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