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 07:25:00 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Nathan Chancellor <nathan@...nel.org>
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 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] };				\

>  	_test(fn, &expect, str, scan_fmt, n_args, &result[0], &result[1]);	\
>  } while (0)

-- 
With Best Regards,
Andy Shevchenko