Date: Wed, 13 Feb 2008 11:46:38 +0100 From: =?utf-8?Q?Dag-Erling_Sm=C3=B8rgrav?= <des@des.no> To: deeptech71@gmail.com Cc: freebsd-chat@freebsd.org Subject: Re: bits wrap when leftshifting non-constant amounts Message-ID: <86abm56smp.fsf@ds4.des.no> In-Reply-To: <47B24480.9020001@gmail.com> (deeptech's message of "Wed\, 13 Feb 2008 02\:14\:40 %2B0100") References: <47B24480.9020001@gmail.com>
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deeptech71@gmail.com writes: > My gcc 3.4.6 behaves weirdly when left shifting, and I couldn't find > any info on this. [...] It is very clear in ISO/IEC 9899:1999 =C2=A76.5.7: 3 The integer promotions are performed on each of the operands. The type of the result is that of the promoted left operand. It the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behaviour is undefined. In other words, the result can be anything at all, and is not even required to be consistent from one evaluation to the next. In fact, gcc is free to wipe your hard drive, shoot your dog and burn down your house for all the standard cares; which I guess is a roundabout way of saying "don't do that". DES --=20 Dag-Erling Sm=C3=B8rgrav - des@des.no
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