Re: [PATCH v5 5/9] powerpc/mm/book3s64: Update tlb flush routines to take a page walk cache flush argument

[Guenter Roeck <linux@roeck-us.net>]

On Wed, May 19, 2021 at 09:20:38AM -0500, Segher Boessenkool wrote:
> On Wed, May 19, 2021 at 06:37:44AM -0700, Guenter Roeck wrote:
> > On 5/19/21 5:03 AM, Segher Boessenkool wrote:
> > >On Tue, May 18, 2021 at 07:45:14PM -0500, Segher Boessenkool wrote:
> > >>And it actually explicitly is undefined behaviour in C90 already
> > >>(3.6.6.4 in C90, 6.8.6.4 in C99 and later).
> > >
> > >... but there is a GCC extension that allows this by default:
> > ><https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wreturn-type>
> > >   For C only, warn about a 'return' statement with an expression in a
> > >   function whose return type is 'void', unless the expression type is
> > >   also 'void'.  As a GNU extension, the latter case is accepted
> > >   without a warning unless '-Wpedantic' is used.
> > 
> > In C99:
> > 
> > "6.8.6.4 The return statement
> > Constraints
> > 
> > A return statement with an expression shall not appear in a function whose 
> > return type
> > is void. A return statement without an expression shall only appear in a 
> > function
> > whose return type is void."
> > 
> > Sounds like invalid to me, not just undefined behavior.
> 
> I don't know what "invalid" would mean here other than UB, it isn't a
> specific defined term, unlike the latter, which is precisely defined in
> 3.4.3/1:
>   undefined behavior
>   behavior, upon use of a nonportable or erroneous program construct or
>   of erroneous data, for which this International Standard imposes no
>   requirements
> 
> This is the strongest thing the standard can say, it is not Law, it does
> not prohibit anyone from doing anything :-)
> 
> "Shall" and "shall not" X means it is undefined behaviour if X (or its
> inverse)  is violated.  See 4.2:
>   If a ''shall'' or ''shall not'' requirement that appears outside of a
>   constraint or runtime-constraint is violated, the behavior is
>   undefined.  Undefined behavior is otherwise indicated in this
>   International Standard by the words ''undefined behavior'' or by the
>   omission of any explicit definition of behavior.  There is no
>   difference in emphasis among these three; they all describe ''behavior
>   that is undefined''.
> which also explains that what you call "invalid" has undefined behaviour
> just as well, most likely.
> 

I'd have assumed that "shall not" is syntactically wrong, but I stand
corrected.

Guenter