Il 22/12/2013 12:32, Aurelien Jarno ha scritto:
find_first_bit has started to be used heavily in TCG code. The current
implementation based on find_next_bit is not optimal and can't be
optimized be the compiler if the bit array has a fixed size, which is
the case most of the time.
If you mean by fully unrolling the loop, that's right.
However...
- return find_next_bit(addr, size, 0);
+ unsigned long result, tmp;
+
+ for (result = 0; result < size; result += BITS_PER_LONG) {
+ tmp = *addr++;
+ if (tmp) {
+ result += ctzl(tmp);
+ return result < size ? result : size;
+ }
+ }
+ /* Not found */
+ return size;
}
/**
... you probably want to limit this to bitmaps that are of constant
size, and small enough that the compiler will unroll them.
So it probably would be a good idea to add an
if (!__builtin_constant_p(size) || size > 8 * BITS_PER_LONG)
return find_next_bit(addr, size, 0);
Not urgent since TCG is the only user of find_first_bit right now, but
worth considering.
Paolo
