3 ms·

You are still introducing a conditional by detecting the need to subtract, and iterating backward through memory is horrific for cache performance. If you need

by kitsuac 10y ago

You are still introducing a conditional by detecting the need to subtract, and iterating backward through memory is horrific for cache performance. If you need a specific, non power-of-2 sized buffer, then of course you make that design decision and pay the performance penalty. But I restate it's odd that you weren't even aware of the cost in 1992 as a system level programmer.

jgrahamc 10y ago
The 286/386 didn't have a cache so that wasn't a worry at that time.
coldtea 10y ago
>But I restate it's odd that you weren't even aware of the cost in 1992 as a system level programmer. Costs were very different in the pipelines (or lack thereof) of eighties/early nineties hardware.
to3m 10y ago
Have you tested this recently? I haven't for some years now but performance was identical regardless of direction. Maybe I need to try it again. I'd expect going backwards to be no worse than "not as good" - like say perhaps the prefetching mechanism doesn't cater for this case - but maybe my standards aren't high enough and this is enough to tip things over into the horrific.
tbirdz 10y ago
>iterating backward through memory is horrific for cache performance This isn't true for Intel chips since Netburst Pentium 4. The hardware prefetchers can handle predicting iterating through an array forwards, backwards, and even strided accesses [0]. The arrays takes up the same number of cache lines in both cases, so going forwards or backwards are still going to have the same number of cache misses. 0: https://software.intel.com/en-us/articles/optimizing-application-performance-on-intel-coret-microarchitecture-using-hardware-implemented-prefetchers/ https://software.intel.com/en-us/articles/optimizing-applica...
SamReidHughes 10y ago
You don't need a conditional. You can set up a mask using sbb. ; precondition: 0 <= x <= N ; (N is constant) mov y, 0 ; set up mask cmp x, N-1 ; set carry flag if x >= N sbb y, 0 ; subtract 1 from y if carry flag set and x, y ; set x to zero if x == N
- jnordwick 10y ago
  The cmov looks better than sbb, but both have data dependencies than a predicted branch wouldn't.
  - SamReidHughes 10y ago
    Ahh! Serves me right for reading books from before the 486 :P

by kitsuac 10y ago

jgrahamc 10y ago

The 286/386 didn't have a cache so that wasn't a worry at that time.

coldtea 10y ago

>But I restate it's odd that you weren't even aware of the cost in 1992 as a system level programmer. Costs were very different in the pipelines (or lack thereof) of eighties/early nineties hardware.

to3m 10y ago

Have you tested this recently? I haven't for some years now but performance was identical regardless of direction. Maybe I need to try it again. I'd expect going backwards to be no worse than "not as good" - like say perhaps the prefetching mechanism doesn't cater for this case - but maybe my standards aren't high enough and this is enough to tip things over into the horrific.

tbirdz 10y ago

>iterating backward through memory is horrific for cache performance This isn't true for Intel chips since Netburst Pentium 4. The hardware prefetchers can handle predicting iterating through an array forwards, backwards, and even strided accesses [0]. The arrays takes up the same number of cache lines in both cases, so going forwards or backwards are still going to have the same number of cache misses. 0: https://software.intel.com/en-us/articles/optimizing-application-performance-on-intel-coret-microarchitecture-using-hardware-implemented-prefetchers/ https://software.intel.com/en-us/articles/optimizing-applica...

SamReidHughes 10y ago

You don't need a conditional. You can set up a mask using sbb. ; precondition: 0 <= x <= N ; (N is constant) mov y, 0 ; set up mask cmp x, N-1 ; set carry flag if x >= N sbb y, 0 ; subtract 1 from y if carry flag set and x, y ; set x to zero if x == N

jnordwick 10y ago
The cmov looks better than sbb, but both have data dependencies than a predicted branch wouldn't.
- SamReidHughes 10y ago
  Ahh! Serves me right for reading books from before the 486 :P