feat(prof): optimise allocation profiling for PHP >= 8.4

[realFlowControl]

Description

Since #3360 and with #3361 we've disabled the code path for PHP 8.4's zend_mm_set_custom_handlers_ex(). This PR re-enables that code path again bringing down overhead for allocation profiling.

This uses the same approach as PHP <= 8.3 allocation profiling with the difference that for alloc(), realloc() and free() calls we do not "prepare" the ZendMM heap by manipulating the heap->use_custom_heap flag anymore. This was necessary for when the allocation fails, ZendMM might call zend_mm_gc() to try and free memory for a new chunk which would just return 0; in case a custom allocator is installed.

With us being able to hook into zend_mm_gc() we can stop manipulating the heap->use_custom_heap flag in the allocator hot path and only do so in case a zend_mm_gc() was triggered.

Additionally this adds a prof correctness test with a custom allocator being used by setting USE_ZEND_ALLOC=0 but validating against the JSON that we use for allocation profiling correctness -> even with a custom allocator we should emit the same profile as without.

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.92%. Comparing base (7ae15a5) to head (3ca8f0e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3550      +/-   ##
==========================================
- Coverage   62.01%   61.92%   -0.10%     
==========================================
  Files         140      140              
  Lines       13309    13309              
  Branches     1762     1762              
==========================================
- Hits         8253     8241      -12     
- Misses       4268     4278      +10     
- Partials      788      790       +2     

see 2 files with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7ae15a5...3ca8f0e. Read the comment docs.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[pr-commenter]

Benchmarks [ profiler ]

Benchmark execution time: 2026-01-16 23:23:05

Comparing candidate commit 3ca8f0e in PR branch florian/alloc-php-84 with baseline commit 7ae15a5 in branch master.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 29 metrics, 6 unstable metrics.

scenario:php-profiler-timeline-memory-with-profiler-and-timeline

• 🟥 execution_time [+27.176ms; +50.093ms] or [+2.211%; +4.075%]

[datadog-official]

⚠️ Tests

✨ Fix all issues with Cursor

⚠️ Warnings

🧪 1022 Tests failed

    testSearchPhpBinaries from integration.DDTrace\Tests\Integration\PHPInstallerTest (Fix with Cursor)

    testSimplePushAndProcess from laravel-58-test.DDTrace\Tests\Integrations\Laravel\V5_8\QueueTest (Fix with Cursor)

testSimplePushAndProcess from laravel-8x-test.DDTrace\Tests\Integrations\Laravel\V8_x\QueueTest (Datadog) (Fix with Cursor)

DDTrace\Tests\Integrations\Laravel\V8_x\QueueTest::testSimplePushAndProcess
Test code or tested code printed unexpected output: spanLinksTraceId: 696ac637000000000fcd4089ba98eb42
tid: 696ac63700000000
hexProcessTraceId: 0fcd4089ba98eb42
hexProcessSpanId: 6c180d766aae3125
processTraceId: 1138637241079884610
processSpanId: 7788990357784375589

phpvfscomposer://tests/vendor/phpunit/phpunit/phpunit:106

View all

ℹ️ Info

❄️ No new flaky tests detected

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 3ca8f0e | Docs | Datadog PR Page | Was this helpful? Give us feedback!}

[morrisonlevi]

I merged in master to fix the merge conflict. Looks good. Hopefully no other weird edges pop up on 8.4+ and the API works as intended.