Re: Does PHP ever stack allocate?

2021-01-05 10:51 GMT, Nikita Popov <nikita.ppv@gmail.com>: > On Mon, Jan 4, 2021 at 11:53 AM Olle Härstedt <olleharstedt@gmail.com> > wrote: > >> 2021-01-03 17:43 GMT, tyson andre <tysonandre775@hotmail.com>: >> > Hi Olle, >> > >> >> Thanks Sara! I realize I should have been more precise: Can PHP >> >> allocate non-reference counted memory that automatically is freed when >> >> leaving scope, similar to what Go does with escape analysis? >> >> >> >> Article describing the Go mechanism: >> >> >> >> https://segment.com/blog/allocation-efficiency-in-high-performance-go-services/ >> > >> > Could you give some concrete examples of what type of code you're >> > talking >> > about? >> > As Sara Golemon said, scalars (null, bool, int, float) are allocated on >> > a >> > php call frame, >> > and the call frames go on a stack. That stack is separate from the C >> stack, >> > but still a stack >> > >> > The call frame is "freed" when leaving scope - i.e. that part of the >> stack >> > will be reused on subsequent calls. >> >> Sure. Consider the following trivial snippet: >> >> ```php >> class Point { >> public $x; >> public $y; >> } >> function foo() { >> $p = new Point(); >> return $p->x + $p->y; >> } >> ``` >> >> Since we know the lifetime of $p, we don't have to ref count it. >> Escape analysis helps with checking lifetimes and to remove needles >> ref counting (and heap allocations in compiled languages). There is a >> file in php-src for this: >> >> >> https://github.com/php/php-src/blob/master/ext/opcache/Optimizer/escape_analysis.c >> >> My question was about how this functionality is used. >> > > I believe the escape analysis is currently only used as part of SCCP to > perform constant folding on objects. For example, we would be able to fold > the following function down to "return 3;": > > function foo() { > $p = new Point(); > $p->x = 1; > $p->y = 2; > return $p->x + $p->y; > } > > However, I think the optimization you have in mind is more along the lines > of dropping the object and storing its properties as local variables > instead. Hm, this is called scalar replacement, I think. Olle