Re: [PATCH] Cygwin: Make gcc-specific code in <sys/cpuset.h> compiler-agnostic

[Mark Geisert <mark@maxrnd.com>]

Hi Corinna,

Corinna Vinschen wrote:
> On Jul  7 00:41, Mark Geisert wrote:
>> The current version of <sys/cpuset.h> cannot be compiled by Clang due to
>> the use of __builtin* functions.  Their presence here was a dubious
>> optimization anyway, so their usage has been converted to standard
>> library functions.  A popcnt (population count of 1 bits in a word)
>> function is provided here because there isn't one in the standard library
>> or elsewhere in the Cygwin DLL.
> 
> And clang really doesn't provide it?  That's unfortunate.
> 
> Do you really think it's not worth to use it if it's available?

I don't know for sure.  I'd guess the popcnt op should be optimized if available; 
the others probably don't need it.

> You could workaround it like this:
> 
>> +/* Modern CPUs have popcnt* instructions but the need here is not worth
>> + * worrying about builtins or inline assembler for different compilers. */
>> +static inline int
>> +__maskpopcnt (__cpu_mask mask)
>> +{
> #if (__GNUC__ >= 4)
>       return __builtin_popcountl (mask);
> #else
>> +  int res = 0;
>> +  unsigned long ulmask = (unsigned long) mask;
>> +
>> +  while (ulmask != 0)
>> +    {
>> +      if (ulmask & 1)
>> +        ++res;
>> +      ulmask >>= 1;
>> +    }
>> +  return res;
> #endif
>> +}
>> +

The first version of the patch (unsubmitted) worked something like that, though it 
was a chore figuring out how to tell the difference between gcc and clang.  clang 
#defines __GNUC__ (?!) for example.  I ended up using __GNUC_PREREQ__ with the 
hope clang version numbers stay lower than gcc version numbers.  Has to be a 
better way than that.

On the other hand, one compilation with clang or clang++, I forget which, and with 
some optimization flag, recognized the 'while' loop in that function and turned it 
into the Hackers Delight algorithm for popcnt in ~20 instructions and no loop.

TL;DR let me ponder this over the weekend.
Thanks for listening,

..mark