Re: [PATCH][Middle-end]change char type to unsigned char type when expanding strcmp/strncmp

[Qing Zhao <qing dot zhao at oracle dot com>]

> On Jul 19, 2018, at 2:24 PM, Jakub Jelinek <jakub@redhat.com> wrote:
> 
> On Thu, Jul 19, 2018 at 02:06:16PM -0500, Qing Zhao wrote:
>>> If you expand it as (int) ((unsigned char *)p)[n] - (int) ((unsigned char *)q)[n]
>>> then aren't you relying on int type to have wider precision than unsigned char
>>> (or unit_mode being narrower than mode)?
>> 
>> do you imply that we should only expand it as  (int) ((unsigned char *)p)[n] - (int) ((unsigned char *)q)[n] when we are sure
>> int type is wider than unsigned char? 
> 
> Yes.
> 
>>> I don't see anywhere where you'd
>>> give up on doing the inline expansion on targets where e.g. lowest
>>> addressable unit would be 16-bit and int would be 16-bit too.
>> 
>> even on this targets, is char type still 8-bit?
>> then int type is still wider than char?
> 
> C requires that int is at least 16-bit wide, so the sizeof (int) == sizeof
> (char) case is only possible say with 16-bit char and 16-bit int, or 32-bit
> char and 32-bit int etc.
> 
>>> On targets where int is as wide as char, one would need to expand it instead
>>> as something like:
>>> if (((unsigned char *)p)[n] == ((unsigned char *)q)[n]) loop;
>>> ret = ((unsigned char *)p)[n] < ((unsigned char *)q)[n] ? -1 : 1;
>>> or similar or just use the library routine.
>> 
>> 
>> even when int type is as wide as char,  expand it as (int) ((unsigned char *)p)[n] - (int) ((unsigned char *)q)[n]
>> should still be correct (even though not optimal), doesn’t it?
> 
> No.  Consider p[n] being e.g. 1 and q[n] being __SCHAR_MAX__ + 3U and 16-bit
> int and 16-bit char.  Then (unsigned char) 0x0001 < (unsigned char) 0x8002,
> so it should return a negative number.  But (int) (0x0001U - 0x8002U) is
> 0x7fff, which is a positive int.  Now, if int is 17-bit and char is 16-bit,
> this works fine, because is then -0x8001 and thus negative.

Okay, I see now.
really appreciate for your detailed explanation.
> 
> The above really works only if int is at least one bit wider than unsigned
> char.

Then, I will add a check to exclude the inlining when int is NOT wider than unsigned char on the target.

is the following the correct check:  (exp is the call to strcmp)

 if (CHAR_TYPE_SIZE >= TYPE_PRECISION (TREE_TYPE (exp)))

 
Thanks.

Qing