On Wed, Mar 18 2015, Denys Vlasenko <vda.li...@googlemail.com> wrote:
> 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)
>
[On little-endian, I'm pretty sure the <<16 should be applied to the
second and fourth decpair value.]
Thanks for the suggestion. However, I don't see any change in the size
of the generated code (gcc 4.7), and, at least on my Xeon machine,
converting both ULONG_MAX and uniformly random u64s becomes slightly
slower (54 vs 56 cycles and 61 vs 65 cycles).
Rasmus
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