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: c++ >> and signed numbers?

From: Jim Wilson <wilson at specifixinc dot com>
To: Jerry Quinn <jlquinn at optonline dot net>
Cc: libstdc++ at gcc dot gnu dot org, gcc at gcc dot gnu dot org
Date: Mon, 15 Sep 2003 20:10:13 -0700
Subject: Re: c++ >> and signed numbers?
References: <16229.9603.107584.379651@gargle.gargle.HOWL>

Jerry Quinn wrote:

According to the c++ spec, operator >> is implementation-defined as to
what it does with negative numbers.  So, does g++ sign-extend negative
numbers shifted to the right?  Or is it triplet-specific?

It is undocumented, and thus no result is guaranteed.

The intent is that the right shift sign-extends negative numbers (assuming we have a signed type). We convert it to a sign-extend shift in the intermediate language (IL), and we expect that the target will emit a sign-extending shift instruction. for this IL construct. However, no where do we guarantee this.

In practice, I don't know of any gcc target that doesn't follow this convention, and I would be very surprised to see one. -- Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com

Follow-Ups:
- Re: c++ >> and signed numbers?
  - From: Joe Buck

References:
- c++ >> and signed numbers?
  - From: Jerry Quinn

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