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Date: Wed, 14 Jan 2015 09:24:00 -0000
Message-ID: <CAFiYyc1kPsO+ZuyU47MKfUqeW3b-OUrKMGqBy_OjO-My3U5t9A@mail.gmail.com>
Subject: Re: shift/extract SHIFT_COUNT_TRUNCATED combine bug
From: Richard Biener <richard.guenther@gmail.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Jeff Law <law@redhat.com>, Mike Stump <mikestump@comcast.net>, 	GCC Patches <gcc-patches@gcc.gnu.org>, Richard Sandiford <rdsandiford@googlemail.com>, 	Eric Botcazou <ebotcazou@adacore.com>
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On Tue, Jan 13, 2015 at 6:38 PM, Segher Boessenkool
<segher@kernel.crashing.org> wrote:
> On Tue, Jan 13, 2015 at 10:51:27AM +0100, Richard Biener wrote:
>> 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.
>
> Note that that catches less though, e.g. in
>
> int f(int x, int n) { return x << ((2*n) & 31); }
>
> without SHIFT_COUNT_TRUNCATED it will try to match an AND with 30,
> not with 31.

But even with SHIFT_COUNT_TRUNCATED you cannot omit the
and as it clears the LSB.  Only at a higher level we might be tempted
to drop the & 31 while it still persists in its original form (not sure
if fold does that - I don't see SHIFT_COUNT_TRUNCATED mentioned there).

Richard.

>
> Segher