From: Jeff Law <law@redhat.com>
To: Richard Biener <richard.guenther@gmail.com>
Cc: Mike Stump <mikestump@comcast.net>,
GCC Patches <gcc-patches@gcc.gnu.org>,
Richard Sandiford <rdsandiford@googlemail.com>,
Eric Botcazou <ebotcazou@adacore.com>
Subject: Re: shift/extract SHIFT_COUNT_TRUNCATED combine bug
Date: Tue, 13 Jan 2015 17:00:00 -0000 [thread overview]
Message-ID: <54B54A14.2090606@redhat.com> (raw)
In-Reply-To: <CAFiYyc2t83m8mccgSdfa0k6wDXub-9uTwehXJxCmq9WzcNPVwg@mail.gmail.com>
On 01/13/15 02:51, Richard Biener wrote:
>> On a SHIFT_COUNT_TRUNCATED target, I don't think it's ever OK to widen a
>> shift, variable or constant.
>>
>> In the case of a variable shift, we could easily have eliminated the masking
>> code before or during combine. For a constant shift amount we could have
>> adjusted the constant (see SHIFT_COUNT_TRUNCATED in cse.c)
>>
>> I think it's just an oversight and it has simply never bit us before.
>
> IMHO SHIFT_COUNT_TRUNCATED should be removed and instead
> backends should provide shift patterns with a (and:QI ...) for the
> shift amount which simply will omit that operation if suitable.
Perhaps. I'm certainly not wed to concept of SHIFT_COUNT_TRUNCATED. I
don't see that getting addressed in the gcc-5 timeframe.
aarch64, alpha, epiphany, iq2000, lm32, m32r, mep, microblaze, mips,
mn103, nds32, pa, sparc, stormy16, tilepro, v850 and xtensa are the
current SHIFT_COUNT_TRUNCATED targets.
Jeff
next prev parent reply other threads:[~2015-01-13 16:38 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-04-08 20:09 Mike Stump
2015-01-12 22:25 ` Jeff Law
2015-01-13 10:11 ` Richard Biener
2015-01-13 17:00 ` Jeff Law [this message]
2015-01-13 18:25 ` Segher Boessenkool
2015-01-14 9:24 ` Richard Biener
2015-01-14 14:35 ` Segher Boessenkool
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