linux-kernel - Re: [PATCH] lib: test_scanf: Add explicit type cast to result initialization in test_number

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Date:   Fri, 4 Aug 2023 07:48:59 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     pmladek@...e.com, rostedt@...dmis.org, senozhatsky@...omium.org,
        linux@...musvillemoes.dk, ndesaulniers@...gle.com, trix@...hat.com,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        patches@...ts.linux.dev
Subject: Re: [PATCH] lib: test_scanf: Add explicit type cast to result
 initialization in test_number_prefix()

On Fri, Aug 04, 2023 at 07:25:00AM +0300, Andy Shevchenko wrote:
> On Thu, Aug 03, 2023 at 11:14:42AM -0700, Nathan Chancellor wrote:
> > A recent change in clang allows it to consider more expressions as
> > compile time constants, which causes it to point out an implicit
> > conversion in the scanf tests:
> > 
> >   lib/test_scanf.c:661:2: warning: implicit conversion from 'int' to 'unsigned char' changes value from -168 to 88 [-Wconstant-conversion]
> >     661 |         test_number_prefix(unsigned char,       "0xA7", "%2hhx%hhx", 0, 0xa7, 2, check_uchar);
> >         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >   lib/test_scanf.c:609:29: note: expanded from macro 'test_number_prefix'
> >     609 |         T result[2] = {~expect[0], ~expect[1]};                                 \
> >         |                       ~            ^~~~~~~~~~
> >   1 warning generated.
> > 
> > The result of the bitwise negation is the type of the operand after
> > going through the integer promotion rules, so this truncation is
> > expected but harmless, as the initial values in the result array get
> > overwritten by _test() anyways. Add an explicit cast to the expected
> > type in test_number_prefix() to silence the warning. There is no
> > functional change, as all the tests still pass with GCC 13.1.0 and clang
> > 18.0.0.
> 
> >  do {										\
> >  	const T expect[2] = { expect0, expect1 };				\
> > -	T result[2] = {~expect[0], ~expect[1]};					\
> > +	T result[2] = {(T)~expect[0], (T)~expect[1]};				\
> 
> Can we add spaces as above, while at it?
> 
> 	T result[2] = { (T)~expect[0], (T)~expect[1] };				\

Sure. I can send a v2 on Monday to give folks a chance to chime in with
other comments.

> >  	_test(fn, &expect, str, scan_fmt, n_args, &result[0], &result[1]);	\
> >  } while (0)
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>