On Sunday, 21 July 2024 at 10:33:38 UTC, Nick Treleaven wrote:
Just to mention that if you assign to the static array it
works: `a = [1,3,6,9];`.
Bonkers. `array[]` is meant to be 'all of `array` as a slice', so
you'd think that's how you copy a slice to a static array, but no!
My understanding is that they do not allocate if used to
initialize or assign a static array. That includes passing an
array literal as an argument to a static array function
parameter.
D is pretty eager to make array literals into slices; thus have I
developed a general distrust for using array literals in the
vicinity of static arrays.
Case and point:
A scope slice can also be initialized from an array literal in
@nogc code:
https://dlang.org/changelog/2.102.0.html#dmd.scope-array-on-stack
But assigning a literal to a scope slice is not allowed in
@nogc code.
If there is enough spare capacity in a's allocation, no
allocation will occur.
Obviously for a long array literal, the benefit of knowing its
length upfront (and the readability) would probably outweigh
the allocation; but for small array literals, is splitting
them into separate concatenations going to yield faster code,
or will I waste my time and screen space?
Note that concatenation always allocates:
Concatenation always creates a copy of its operands, even if
one of the operands is a 0 length array
https://dlang.org/spec/arrays.html#array-concatenation
Thank you for all this info!
P.S. I am mostly addressing LDC2 & GDC's output, since I am
aware that DMD's optimisations are usually minimal.
While people may say that on the forum, dmd's optimizer does
actually do data flow analysis:
https://forum.dlang.org/post/uqhgoi$31a7$1...@digitalmars.com
People frequently come here to complain that 'D is slow' when
they're using DMD, and often not even using `-O`. The responses
will then usually contain some version of 'don't use DMD to
generate optimised code'.
