Re: [PATCH 3/4] cache.h: hex2chr() - avoid -Wsign-compare warnings

["René Scharfe" <l.s.r@web.de>]

Am 22.09.2017 um 07:47 schrieb Jeff King:
> On Thu, Sep 21, 2017 at 05:48:38PM +0100, Ramsay Jones wrote:
> 
>> diff --git a/cache.h b/cache.h
>> index a916bc79e..a0e3e362c 100644
>> --- a/cache.h
>> +++ b/cache.h
>> @@ -1243,8 +1243,8 @@ static inline unsigned int hexval(unsigned char c)
>>    */
>>   static inline int hex2chr(const char *s)
>>   {
>> -	int val = hexval(s[0]);
>> -	return (val < 0) ? val : (val << 4) | hexval(s[1]);
>> +	unsigned int val = hexval(s[0]);
>> +	return (val & ~0xf) ? val : (val << 4) | hexval(s[1]);
>>   }
> 
> Ironically, the unsigned return from hexval() comes from internally
> converting the signed char in hexval_table. And then we again return it
> as a signed int from hex2chr().
> 
> Would it make sense to return a signed int from hexval()? That would
> make hex2chr just work as it tries to above. I admit that shifting
> signed values is a little funny, but it should be fine here since we
> know they're no larger than 8 bits in the first place.

I'd say yes.  We can replace that shift with a multiplication --
compilers will nevertheless use a shift even with -O0 (if
beneficial).

> As an aside, I also see some uses of hexval() that don't appear to be
> quite as rigorous in checking for invalid characters. A few
> unconditionally shift the first nibble and assume that there will still
> be high bits set. I think that's generally true for twos-complement
> negative numbers, but isn't shifting off the left side of a signed
> integer undefined behavior?
> 
> And mailinfo's decode_q_segment() does not seem to check for errors at
> all.

It also allocates and returns a strbuf, which seems odd as well.

René