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Bug #79353 bad optimizing "Null Coalescing Assignment"
Submitted: 2020-03-08 02:46 UTC Modified: 2020-03-10 10:44 UTC
From: bugreports at gmail dot com Assigned: nikic (profile)
Status: Assigned Package: opcache
PHP Version: 7.4Git-2020-03-08 (Git) OS:
Private report: No CVE-ID: None
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 [2020-03-08 02:46 UTC] bugreports at gmail dot com
Description:
------------
with opcache other than "Null coalescing operator" the "Null Coalescing Assignment" is not completly optimized out

function a()
{
 $test = $test ?? TRUE;
 return $test;
}

function b()
{
 $test ??= TRUE;
 return $test;
}

runtime difference with 50000000 calls:
a(): 3.1884
b(): 3.3666

function name:  a
number of ops:  2
----------------------------------------------------------------------------------
   5     0*       COALESCE                                         ~1      !0
   6     1*       RETURN                                                   <true>

function name:  b
number of ops:  3
----------------------------------------------------------------------------------
  11     0*       COALESCE                                         ~1      !0
         1*       ASSIGN                                           $2      !0, <true>
  12     2*       RETURN                                                   !0


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 [2020-03-09 14:34 UTC] nikic@php.net
-Assigned To: +Assigned To: nikic
 [2020-03-09 14:34 UTC] nikic@php.net
I think we mainly need to add pi node placement for coalesce.
 [2020-03-09 15:21 UTC] nikic@php.net
https://github.com/php/php-src/commit/d9c45d86f9cd3d20f66ebf38384a9f53113415e5

Pi node placement turned out to be more complicated than anticipated, because it did not pass our existing profitability heuristic.
 [2020-03-10 10:29 UTC] bugreports at gmail dot com
as i understand that it will be part of PHP8 and not optimized for 7.4?
 [2020-03-10 10:44 UTC] nikic@php.net
Yes, this is only for master, as the change is non-trivial.

I'm also keeping this issue open, because we should still try to remove the COALESCE itself.
 
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