On Sat, Feb 21, 2015 at 12:51 AM, Rasmus Villemoes
<li...@rasmusvillemoes.dk> wrote:
> The most expensive part of decimal conversion is the divisions by 10
> (albeit done using reciprocal multiplication with appropriately chosen
> constants). I decided to see if one could eliminate around half of
> these multiplications by emitting two digits at a time, at the cost of
> a 200 byte lookup table, and it does indeed seem like there is
> something to be gained, especially on 64 bits.
...
...
> +char *put_dec_trunc8(char *buf, unsigned r)
> {
> + unsigned q;
> +
> + /* 1 <= r < 10^8 */
> + if (r < 100)
> + goto out_r;
> +
> + /* 100 <= r < 10^8 */
> + q = (r * (u64)0x28f5c29) >> 32;
> + *((u16 *)buf) = decpair[r - 100*q];
> + buf += 2;
> +
> + /* 1 <= q < 10^6 */
> + if (q < 100)
> + goto out_q;
> +
> + /* 100 <= q < 10^6 */
> + r = (q * (u64)0x28f5c29) >> 32;
> + *((u16 *)buf) = decpair[q - 100*r];
> + buf += 2;
> +
> + /* 1 <= r < 10^4 */
> + if (r < 100)
> + goto out_r;
> +
> + /* 100 <= r < 10^4 */
> + q = (r * 0x147b) >> 19;
> + *((u16 *)buf) = decpair[r - 100*q];
> + buf += 2;
> +out_q:
> + /* 1 <= q < 100 */
> + r = q;
> +out_r:
> + /* 1 <= r < 100 */
> + *((u16 *)buf) = decpair[r];
> + buf += 2;
> + if (buf[-1] == '0')
> + buf--;
> return buf;
> }
Your code does four 16-bit stores.
The version below does two 32-bit ones instead,
and it is also marginally smaller.
char *put_dec_full8(char *buf, unsigned r)
{
unsigned q;
u32 v;
/* 0 <= r < 10^8 */
q = (r * (u64)0x28f5c29) >> 32;
v = (u32)decpair[r - 100*q] << 16;
/* 0 <= q < 10^6 */
r = (q * (u64)0x28f5c29) >> 32;
v = v | decpair[q - 100*r];
((u32*)buf)[0] = v;
/* 0 <= r < 10^4 */
q = (r * 0x147b) >> 19;
v = (u32)decpair[r - 100*q] << 16;
/* 0 <= q < 100 */
v = v | decpair[q];
((u32*)buf)[1] = v;
return buf + 8;
}
It may be faster not only because of having fewer stores,
but because on x86, this code (moving 16-bit halves):
movw decpair(%ebx,%ebx), %dx
movw %dx, 4(%eax)
movw decpair(%ecx,%ecx), %dx
movw %dx, 6(%eax)
suffers from register merge stall when 16-bit value
is read into lower part of %edx. 32-bit code
has no such stalls:
movzwl decpair(%ebx,%ebx), %edx
sall $16, %edx
movzwl decpair(%ecx,%ecx), %ecx
orl %ecx, %edx
movl %edx, 4(%eax)
Before you ask:
I didn't manage to extend this trick to a code
with one 64-bit store which is not larger.
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