Re: GCC turns &~ into | due to undefined bit-shift without warning

[Andrew Haley <aph at redhat dot com>]

On 3/22/19 10:20 AM, Allan Sandfeld Jensen wrote:
> On Freitag, 22. März 2019 11:02:39 CET Andrew Haley wrote:
>> On 3/21/19 10:19 PM, Allan Sandfeld Jensen wrote:
>>> From having fixed UBSAN warnings, I have seen many cases where undefined
>>> behavior was performed, but where the code was aware of it and the final
>>> result of the expression was well defined nonetheless.
>>
>> Is this belief about undefined behaviour commonplace among C programmers?
>> There's nothing in the standard to justify it: any expression which contains
>> UB is undefined.
> 
> Yes, even GCC uses undefined behavior when it is considered defined for 
> specific architecture,

If it's defined for a specific architecture it's not undefined. Any compiler
is entitled to do anything with UB, and "anything" includes extending the
language to make it well defined.

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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