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Re: GCC turns &~ into | due to undefined bit-shift without warning
On Freitag, 22. März 2019 14:38:10 CET Andrew Haley wrote:
> 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.
True, but in the context of "things UBSAN warns about", that includes
architecture specific details.
And isn't unaligned access real undefined behavior that just happens to work
on x86 (and newer ARM)?
There are also stuff like type-punning unions which is not architecture
specific, technically undefined, but which GCC explicitly tolerates (and needs
to since some NEON intrinsics use it).
'Allan