Re: Warning from count-leading-zeros.h

[Paul Eggert]

On 12/14/18 4:09 AM, Reuben Thomas wrote:
> As far as I can tell, the following warning is not dependent on my 
> code, as it's inside a function in count-leading-zeros.h. Warning 
> obtained with GCC 7.3.0:
>
> bits_util.c: In function ‘smite_find_msbit’:
> ../lib/count-leading-zeros.h:39:26: warning: assuming signed overflow 
> does not occur when simplifying conditional to constant 
> [-Wstrict-overflow]
>    return x ? BUILTIN (x) : CHAR_BIT * sizeof x;
>           ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
> ../lib/count-leading-zeros.h:100:3: note: in expansion of macro 
> ‘COUNT_LEADING_ZEROS’
>    COUNT_LEADING_ZEROS (__builtin_clzl, _BitScanReverse, unsigned long 
> int);
>    ^~~~~~~~~~~~~~~~~~~

The warning could stem from your code, if your code calls 
count_leading_zeros (E) where E's value depends on a computation that 
could involve signed integer overflow (which means behavior is undefined 
so GCC can optimize the if-then-else away and pretend it yields 0 or 
whatever else it likes). The warning almost surely does not stem from 
the underlined expression (CHAR_BIT * sizeof x), as that expression 
should evaluate to 32 or 64 on current platforms and this doesn't 
involve integer overflow.