Re: Using POPCNT and other advanced bit manipulation instructions

On 2019-Feb-15, Kyotaro HORIGUCHI wrote:

> I understand that the behavior of __builtin_c[tl]z(0) is
> undefined from the reason, they convert to bs[rf]. So if we use
> these builtins, additional check is required.
> 
> https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

Okay -- the functions check for a 0 argument:

+static inline int
+pg_rightmost_one32(uint32 word)
+{
+   int         result = 0;
+
+   Assert(word != 0);
+
+#ifdef HAVE__BUILTIN_CTZ
+   result = __builtin_ctz(word);
+#else
+   while ((word & 255) == 0)
+   {
+       word >>= 8;
+       result += 8;
+   }
+   result += rightmost_one_pos[word & 255];
+#endif                         /* HAVE__BUILTIN_CTZ */
+
+   return result;
+}

so we're fine.

> And even worse lzcntx is accidentially "fallback"s to bsrx on
> unsupported CPUs, which leads to bogus results.
> __builtin_clzll(8) = 3, which should be 60.

I'm not sure I understand this point.  Are you saying that if we use
-mlzcnt to compile, then the compiler builtin is broken in platforms
that don't support the lzcnt instruction?  That's horrible.  Let's stay
away from -mlzcnt then.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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