Re: [PATCH] Add cast to unsigned char to strverscmp

Hans-Bernhard Bröker wrote:
> Am 08.11.2024 um 18:36 schrieb Christian Franke:
>
>> This variant does not depend on signedness or integer overflow:
>>
>>    if ('1' <= l[dp] && l[dp] <= '9' && '1' <= r[dp] && r[dp] <= '9') {
>
> Or one could actually use isdigit() for what it's intended to do? Yeah 
> right, that would be just too ridiculous to even consider...
>
>

No, because the '0' is intentionally excluded here.

I never use isdigit() in my own code because it forbids to directly pass 
negative signed char values and typically results in unnecessary (locale 
specific) code.

"Proof" for newlib:

int isdigit_slow(int c)
{
   return isdigit(c);
}

int isdigit_fast(int c)
{
   return ('0' <= c && c <= '9');
}

Code generated with 'gcc -O2 ...':

isdigit_slow:
     pushq    %rbx
     subq    $32, %rsp
     movslq    %ecx, %rbx
     call    __locale_ctype_ptr
     movzbl    1(%rbx,%rax), %eax
     andl    $4, %eax
     addq    $32, %rsp
     popq    %rbx
     ret

isdigit_fast:
     xorl    %eax, %eax
     subl    $48, %ecx
     cmpl    $9, %ecx
     setbe    %al
     ret

Speedup factor is somewhere between 5x and 10x.

If compiled as C++ with 'g++ -O2 ...', identical (fast) code is 
generated for both functions. This is because the is*() macros in 
ctype.h are disabled for C++. Then GCC uses its builtin isdigit(). Not 
really consistent, IMO :-)

-- 
Regards,
Christian