Optimizing assembly for spl_object_id(), fdiv(), etc in the JIT?

On Sat, Apr 25, 2020 at 6:44 PM tyson andre tysonandre775@hotmail.com
wrote:

Hi internals,

I was wondering if the maintainers of the Opcache Jit would be open to
work on optimizing functions such as intdiv(), fdiv(), and spl_object_id()
in the JIT to assembly code when it was safe to do so.
These functions have simple C implementations, so the performance overhead
of calling PHP functions in the generated assembly
would be noticeable compared to emitting optimized assembly.

I'd expect that it'd be safe to do so under the following circumstances:

• All arguments are Compiled Variables(CV) in the opcodes (i.e. $var)
  (
  http://nikic.github.io/2017/04/14/PHP-7-Virtual-machine.html#variable-types
  )
• The types and count of the function arguments are known to be strictly
  correct, e.g.
  • Opcache infers they will not throw a TypeError or emit undefined
    variable warnings
  • maybe strictly accept floats but not integers for fdiv
• The return value of the expression won't throw
  (e.g. don't perform this optimization for $y = spl_object_id($x) when
  a CV is the return value)
• The function in question exists and is not disabled.

For example, these operands would be seen by the JIT for this sequence of
opcodes

/*
0000 CV0($x) = RECV 1
0001 INIT_FCALL 1 96 string("spl_object_id")
0002 SEND_VAR CV0($x) 1
0003 V2 = DO_ICALL
0004 T1 = INIT_ARRAY 1 (packed) CV0($x) V2
0005 RETURN T1
*/
function create_set(stdClass $x) : array {
    return [spl_object_id($x) => $x];
}

I expect it to be technically feasible to check for the sequence of
opcodes INIT_FCALL, SEND_VAR (repeated), and DO_ICALL, in
ext/opcache/jit/zend_jit.c. The resulting assembly would be smaller and
much faster.
(this would be done before emitting any assembly - skip over the opcodes
for INIT_ARRAY and SEND_VAR and DO_ICALL)

• What do the maintainers of the JIT module think of this idea?
  (e.g. are there concerns about this making it harder to understand the
  JIT codebase, adding potential bugs, or slowing down the generation of
  assembly)
• Are there any general guidelines (or talks/articles) you'd recommend for
  new contributors to the Opcache JIT? I couldn't find a README and
  https://wiki.php.net/rfc/jit doesn't seem to make recommendations that
  apply to my question.

// core fdiv C implementation (does not throw for a divisor of 0)
        RETURN_DOUBLE(dividend / divisor);
// core of spl_object_id C implementation
        RETURN_LONG((zend_long)Z_OBJ_HANDLE_P(obj));

I started looking into this because I work on an application that heavily
uses spl_object_id()
and would see a small performance benefit from this,
and was reminded of this when looking at the get_resource_id() proposal.
(
https://github.com/phan/phan/search?q=spl_object_id&unscoped_q=spl_object_id
)

Thanks,

• Tyson

Hi Tyson,

Our general approach to this is to first add a VM opcode for the operation,
which will also provide a benefit if the JIT is not used. There's already
plenty of those, see ZEND_STRLEN for example. Adding JIT support for the
opcode would then be the natural second step.

There aren't any hard rules for when this should be done, but I believe
having some evidence that the operation is both common and performance
critical would be good.

Nikita