On 4/5/23 6:34 PM, Paul wrote:
On Tuesday, 4 April 2023 at 22:20:52 UTC, H. S. Teoh wrote:
Best practices for arrays in hot loops:
- Avoid appending if possible; instead, pre-allocate outside the loop.
- Where possible, reuse existing arrays instead of discarding old ones
and allocating new ones.
- Use slices where possible instead of making copies of subarrays (this
esp. applies to strings).
- Where possible, prefer sequential access over random access (take
advantage of the CPU cache hierarchy).
Thanks for sharing Teoh! Very helpful.
would this be random access? for(size_t i; i<arr.length; i++) using
indices?
...and this be sequential foreach(a;arr) ?
No, random access is access out of sequence. Those two lines are pretty
much equivalent, and even a naive compiler is going to produce exactly
the same generated code from both of them.
A classic example is processing a 2d array:
```d
for(int i = 0; i < arr[0].length; ++i)
for(int j = 0; j < arr.length; ++j)
arr[j][i]++;
// vs
for(int j = 0; j < arr.length; ++j)
for(int i = 0; i < arr[0].length; ++i)
arr[j][i]++;
```
The first accesses elements *by column*, which means that the array data
is accessed non-linearly in memory.
To be fair, both are "linear" in terms of algorithm, but one is going to
be faster because of cache coherency (you are accessing sequential
*hardware addresses*).
-Steve
