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| author | Andreas Rumpf <rumpf_a@web.de> | 2020-07-15 23:00:06 +0200 |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2020-07-15 23:00:06 +0200 |
| commit | c5358b0d4b1d27db04b974a0183c9b7312bc7bdc (patch) | |
| tree | ca3a71953358c8f9475188045ba61c5dfb5caf87 /doc/destructors.rst | |
| parent | 813dd1b670b953b0ac8348b04079faadace46c29 (diff) | |
| download | Nim-c5358b0d4b1d27db04b974a0183c9b7312bc7bdc.tar.gz | |
An optimizer for ARC (#14962)
* WIP: an optimizer for ARC * do not optimize away destructors in 'finally' if unstructured control flow is involved * optimized the optimizer * minor code cleanup * first steps to .cursor inference * cursor inference: big steps to a working solution * baby steps * better .cursor inference * new feature: expandArc for easy inspection of the AST after ARC transformations * added topt_cursor test * adapt tests * cleanups, make tests green * optimize common traversal patterns * moved test case * fixes .cursor inference so that npeg compiles once again * cursor inference: more bugfixes Co-authored-by: Clyybber <darkmine956@gmail.com>
Diffstat (limited to 'doc/destructors.rst')
| -rw-r--r-- | doc/destructors.rst | 28 |
1 files changed, 27 insertions, 1 deletions
diff --git a/doc/destructors.rst b/doc/destructors.rst index fefc646f5..6cab6e1d2 100644 --- a/doc/destructors.rst +++ b/doc/destructors.rst @@ -354,7 +354,8 @@ Destructor removal ``wasMoved(x);`` followed by a `=destroy(x)` operation cancel each other out. An implementation is encouraged to exploit this in order to improve -efficiency and code sizes. +efficiency and code sizes. The current implementation does perform this +optimization. Self assignments @@ -509,6 +510,31 @@ to be safe, but for ``ptr`` the compiler has to remain silent about possible problems. +Cursor inference / copy elision +=============================== + +The current implementation also performs `.cursor` inference. Cursor inference is +a form of copy elision. + +To see how and when we can do that, think about this question: In `dest = src` when +do we really have to *materialize* the full copy? - Only if `dest` or `src` are mutated +afterwards. If `dest` is a local variable that is simple to analyse. And if `src` is a +location derived from a formal parameter, we also know it is not mutated! In other +words, we do a compile-time copy-on-write analysis. + +This means that "borrowed" views can be written naturally and without explicit pointer +indirections: + +.. code-block:: nim + + proc main(tab: Table[string, string]) = + let v = tab["key"] # inferred as .cursor because 'tab' is not mutated. + # no copy into 'v', no destruction of 'v'. + use(v) + useItAgain(v) + + + Owned refs ========== |