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RE: Strange shifting behaviour
- From: "John Yates" <jyates at netezza dot com>
- To: "Eljay Love-Jensen" <eljay at adobe dot com>,"Ulf Magnusson" <ulfalizer at gmail dot com>,<gcc-help at gcc dot gnu dot org>
- Date: Wed, 13 Jul 2005 14:10:35 -0400
- Subject: RE: Strange shifting behaviour
Eljay,
Though I agree with your point about the standards and
undefined behavior, I do believe that Ulf has identified
a quality of implementation issue.
Would you not agree that compile-time expression evaluation
should mimic run-time as much as possible? Or to put it
another way, the more often compile-time and run-time
evaluated results diverge, the lower the subject quality
of the compiler.
If the shift operator at run-time examines only the lower
order 5 bits of the shift count (as Ulf's x86 does) then
a "high-quality" compile-time expression evaluator ought
to do the same.
/john
-----Original Message-----
From: Eljay Love-Jensen [mailto:eljay@adobe.com]
Sent: Tuesday, July 12, 2005 6:46 PM
To: Ulf Magnusson; gcc-help@gcc.gnu.org
Subject: Re: Strange shifting behaviour
Hi Ulf,
>When shifting an int by its size in bits...
That is undefined behavior (implementation dependent), as per C and C++ standards. Ever since C was first taking it's first baby steps.
By "undefined behavior", that means any given particular implementation can:
+ not do anything
+ do what you expect
+ SEGV
+ format your hard drive
--Eljay