Improve performance of WeakReference/WeakMap. Avoid hash collisions on pointers.

[TysonAndre]

Shift pointers by ZEND_MM_ALIGNMENT_LOG2
to avoid the noticeable performance degradation caused by hash table collisions.
in EG(weakrefs) and WeakMap entries

On 64-bit platforms, pointers are usually aligned to at least 8 bytes,
so only one in 8 hash buckets were actually getting used.
(With the metadata needed to track allocations,
alignment might be at least 16 bytes in practice)

Before this change with taskset 1 sapi/cli/php -d memory_limit=2G -d zend_extension=$PWD/modules/opcache.so -d opcache.enable_cli=1 -d opcache.enable=1 bench.php in a non-debug build

Benchmarking note: SplObjectStorage is slow because there's no native write handler (SplObjectStorage implementation was removed because it needed to call
offsetSet when subclasses overrid it and there were historically bugs), where it's the other way around for WeakMap which has a native write handler
(I forgot about that)

    /**
     * @param object $object
     * @tentative-return-type
     * @implementation-alias SplObjectStorage::attach
     * @no-verify Cannot specify arg type because ArrayAccess does not
     */
    public function offsetSet(mixed $object, mixed $info = null): void {}

# Before optimization
           bench_weak_map:  149048272 bytes create=0.356473 total=0.536920
 bench_spl_object_storage:  122724856 bytes create=0.336846 total=0.424876

           bench_weak_map:   98724768 bytes create=0.258280 total=0.437024
 bench_spl_object_storage:  122724856 bytes create=0.254590 total=0.345208

           bench_weak_map:   98724768 bytes create=0.258501 total=0.437509
 bench_spl_object_storage:  122724856 bytes create=0.223680 total=0.317106

           bench_weak_map:   98724768 bytes create=0.258847 total=0.437875
 bench_spl_object_storage:  122724856 bytes create=0.235694 total=0.322943

           bench_weak_map:   98724768 bytes create=0.260643 total=0.440493
 bench_spl_object_storage:  122724856 bytes create=0.244720 total=0.336108

           bench_weak_map:   98724768 bytes create=0.255080 total=0.434092
 bench_spl_object_storage:  122724856 bytes create=0.198827 total=0.290071

           bench_weak_map:   98724768 bytes create=0.255159 total=0.434814
 bench_spl_object_storage:  122724856 bytes create=0.200624 total=0.293820

After this change in php 8.2, performance is comparable to SplObjectStorage(first run grows EG(weakrefs) and allocates memory for the first time and is slower:

Note that EG(weakrefs) doesn't shrink capacity, so the first entry's memory is a more accurate representation of weakref memory usage for a long-running program.

# After optimization (v1: Only avoid hash collisions for EG(weakrefs))
           bench_weak_map:  149048272 bytes create=0.265922 total=0.374167
 bench_spl_object_storage:  122724856 bytes create=0.330690 total=0.416207

           bench_weak_map:   98724768 bytes create=0.214908 total=0.324684
 bench_spl_object_storage:  122724856 bytes create=0.251910 total=0.343701

           bench_weak_map:   98724768 bytes create=0.206807 total=0.315133
 bench_spl_object_storage:  122724856 bytes create=0.222431 total=0.319022

           bench_weak_map:   98724768 bytes create=0.217553 total=0.333927
 bench_spl_object_storage:  122724856 bytes create=0.225307 total=0.310985

After optimizations V2: Avoid collisions both for global EG(weakrefs) and individual WeakMap HashTables (zend_weakmap->ht)
           bench_weak_map:  149048272 bytes create=0.177938 total=0.225081
 bench_spl_object_storage:  122724856 bytes create=0.348268 total=0.441844

           bench_weak_map:   98724768 bytes create=0.130229 total=0.178878
 bench_spl_object_storage:  122724856 bytes create=0.263178 total=0.361414

           bench_weak_map:   98724768 bytes create=0.128731 total=0.177533
 bench_spl_object_storage:  122724856 bytes create=0.230146 total=0.326304

           bench_weak_map:   98724768 bytes create=0.135435 total=0.183836
 bench_spl_object_storage:  122724856 bytes create=0.241961 total=0.332324

           bench_weak_map:   98724768 bytes create=0.134626 total=0.185558
 bench_spl_object_storage:  122724856 bytes create=0.253069 total=0.345240

           bench_weak_map:   98724768 bytes create=0.125791 total=0.175136
 bench_spl_object_storage:  122724856 bytes create=0.208869 total=0.303268

<?php

class Example {
}

function bench_weak_map(int $n) {
    $wm = new WeakMap();
    $values = [];
    for ($i = 0; $i < $n; $i++) {
        $x = new Example();
        $wm[$x] = true;
        $values[] = $x;
    }
    return [$values, $wm];
}

function bench_spl_object_storage(int $n) {
    $wm = new SplObjectStorage();
    $values = [];
    for ($i = 0; $i < $n; $i++) {
        $x = new Example();
        $wm[$x] = true;
        $values[] = $x;
    }
    return [$values, $wm];
}

function main(int $n) {
    error_reporting(E_ALL & ~E_DEPRECATED);
    foreach (['bench_weak_map', 'bench_spl_object_storage'] as $f) {
        gc_collect_cycles();
        $t1 = hrtime(true);
        $m1 = memory_get_usage();
        $v = $f($n);
        $t2 = hrtime(true);
        $m2 = memory_get_usage();
        unset($v);
        $t3 = hrtime(true);
        printf("%25s: %10d bytes create=%.6f total=%.6f\n", $f, $m2 - $m1, ($t2 - $t1)/1e9, ($t3 - $t1)/1e9);
    }
    echo "\n";
}

while(true) {
    main(1000000);
}

[TysonAndre]

As seen in #7672 - php's WeakReference/WeakMap handling is spending a lot of time in hash table lookups/modifications. This improves performance by:

• Multiplying the usable hash entries by ZEND_MM_ALIGMENT (usually 8) and significantly reducing hash collisions
• Improving cache locality for sequentially allocated objects when accessing the hash tables used to track WeakReference/WeakMap instances (especially in the benchmark)

=====================================================================
FAILED TEST SUMMARY
---------------------------------------------------------------------
FPM: Process manager config pm.process_idle_timeout [sapi/fpm/tests/proc-idle-timeout.phpt]
=====================================================================

The Travis build failure is unrelated. The azure failures are caused by running out of CI minutes.

[nikic]

cc @bwoebi as the new commit changes keys for the zend_weakrefs_hash_add/del API -- they're no longer simply zend_object* cast to integer. Is that okay? It shouldn't matter if your just use zend_weakrefs_hash_add/del, but may matter is you use ZEND_HASH_FOREACH and interpret the keys.

[TysonAndre]

I assume implementations that needed to inspect internals could add macros like these - 8.2 alphas aren't even out yet. If debuggers or instrumentation tools needed this I guess this could be added to the public API in 8.2.

static zend_always_inline zend_ulong copyof_zend_object_ptr_to_weakref_key(const zend_object *object)
{
#if PHP_VERSION_ID >= 80200
	ZEND_ASSERT(((uintptr_t)object) % ZEND_MM_ALIGNMENT == 0);
	return ((uintptr_t) object) >> ZEND_MM_ALIGNMENT_LOG2;
#else
	return (zend_long) object;
#endif
}

static zend_always_inline zend_object *copyof_zend_weakref_key_to_zend_object_ptr(zend_ulong key)
{
#if PHP_VERSION_ID >= 80200
	return (zend_object *) (((uintptr_t) key) << ZEND_MM_ALIGNMENT_LOG2);
#else
	return (zend_long) object;
#endif
}

[bwoebi]

I suppose that's fine, we just should have some macro in weakrefs.h to convert the bare key then.

[TysonAndre]

I suppose that's fine, we just should have some macro in weakrefs.h to convert the bare key then.

That's done

[nikic]

ext/zend_test/tests/zend_weakmap.phpt is failing.

[TysonAndre]

ext/zend_test/tests/zend_weakmap.phpt is failing.

I'd missed that since I thought php was still out of Azure Pipelines minutes (thanks for fixing that). That's fixed now, it needed to use the new function in zend_weakref.h to convert the hash key to a pointer.