From: Marc Glisse <marc.glisse@inria.fr>
To: Richard Biener <richard.guenther@gmail.com>
Cc: "gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>,
"Hurugalawadi,
Naveen" <Naveen.Hurugalawadi@caviumnetworks.com>
Subject: Re: [PR25529] Convert (unsigned t * 2)/2 into unsigned (t & 0x7FFFFFFF)
Date: Tue, 07 Jul 2015 09:24:00 -0000 [thread overview]
Message-ID: <alpine.DEB.2.20.1507071121510.1875@laptop-mg.saclay.inria.fr> (raw)
In-Reply-To: <CAFiYyc1c_NVr1eKVg3v5xv4829aMDbp0QnrmeB-BEYCmg6uQ+A@mail.gmail.com>
On Tue, 7 Jul 2015, Richard Biener wrote:
> On Tue, Jul 7, 2015 at 8:06 AM, Marc Glisse <marc.glisse@inria.fr> wrote:
>> On Tue, 7 Jul 2015, Hurugalawadi, Naveen wrote:
>>
>>> Please find attached the patch "PR25529.patch" that converts the pattern:-
>>> (unsigned * 2)/2 is into unsigned &0x7FFFFFFF
>>
>>
>> +/* Simplify (unsigned t * 2)/2 -> unsigned t & 0x7FFFFFFF. */
>> +(for div (trunc_div ceil_div floor_div round_div exact_div)
>> + (simplify
>> + (div (mult @0 INTEGER_CST@1) INTEGER_CST@1)
>>
>> You don't need to repeat INTEGER_CST, the second time @1 is enough.
>>
>> + (with { tree n2 = build_int_cst (TREE_TYPE (@0),
>> + wi::exact_log2 (@1)); }
>> + (if (TYPE_UNSIGNED (TREE_TYPE (@0)))
>> + (bit_and @0 (rshift (lshift { build_minus_one_cst (TREE_TYPE (@0)); }
>> + { n2; }) { n2; }))))))
>>
>> What happens if you write t*3/3?
>
> Huh, and you posted this patch twice? See my reply to the other copy
> for the correctness issues and better handling of exact_div
They are not the same, one is for left shifts and the other one for right
shifts. And that makes a big difference: in t*c/c, the division is always
exact, so all divisions are equivalent. This is not the case for t/c*c.
--
Marc Glisse
next prev parent reply other threads:[~2015-07-07 9:24 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-07 4:52 Hurugalawadi, Naveen
2015-07-07 6:06 ` Marc Glisse
2015-07-07 9:12 ` Richard Biener
2015-07-07 9:24 ` Marc Glisse [this message]
2015-07-07 9:35 ` Richard Biener
2015-07-21 9:16 ` Hurugalawadi, Naveen
2015-07-22 12:10 ` Richard Biener
2015-07-23 3:59 ` Hurugalawadi, Naveen
2015-07-23 13:36 ` Richard Biener
2015-08-07 8:44 ` Hurugalawadi, Naveen
2015-08-11 12:50 ` Richard Biener
2015-08-11 12:51 ` Richard Biener
2015-08-20 20:10 ` H.J. Lu
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