This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.

Index Nav:	[Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav:	[Date Prev] [Date Next]	[Thread Prev] [Thread Next]
Other format:	[Raw text]

Re: Strange optimization in GCC 4.7.2

From: David Given <dg at cowlark dot com>
To: gcc at gcc dot gnu dot org
Date: Tue, 23 Jul 2013 13:57:42 +0100
Subject: Re: Strange optimization in GCC 4.7.2
References: <CADn89gTyW9ds-ZdQ+Rpwvzb+tPFTV_0j60GJe5_GqHivVarxqg at mail dot gmail dot com>

Konstantin Vladimirov wrote:
[...]
> x = (y & ~(1 << 7)) | (((value >> 9) & 1) << 7);
[...]
> x = y & 4294967167 | (value >> 9) << 7 & 255; <--------- WAT?


   ((value >> 9) & 1) << 7
== ((value >> 9) << 7) & (1 << 7)
== ((value >> 9) << 7) & 0x80
== ((value >> 9) << 7) & 0xff

...I think.

That last step is probably being done because anding with 0xff is really
cheap on x86 (you just pick the appropriate subreg --- al instead of
eax, for example).

-- 
ââââ ïïïïïïïïïïïïïï âââââ http://www.cowlark.com âââââ
â "USER'S MANUAL VERSION 1.0:  The information presented in this
â publication has been carefully for reliability." --- anonymous
â computer hardware manual

Attachment: signature.asc
Description: OpenPGP digital signature

Follow-Ups:
- Re: Strange optimization in GCC 4.7.2
  - From: Konstantin Vladimirov

References:
- Strange optimization in GCC 4.7.2
  - From: Konstantin Vladimirov

Index Nav:	[Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav:	[Date Prev] [Date Next]	[Thread Prev] [Thread Next]