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The Significance of Compiler-Optimized Functions in PHP


When delving into open-source codebases, you may have encountered internal functions that are either implicitly imported using “use function array_map;” as seen in Doctrine or prefixed with the global namespace separator like “\is_string($foo)” in Symfony. This practice might raise questions about its necessity. After all, don’t function calls automatically fall back into the global namespace? In this article, we’ll explore the reasons behind this coding convention and shed light on the technical aspects of compiler-optimized functions in PHP.

Compiler-Optimized Functions in PHP

Compiler-optimized functions are a subtle but important feature in PHP. Although you technically don’t need to use “use function” or the global namespace separator when calling functions, there’s a valid rationale for doing so. Some PHP internal functions have compiler-optimized versions that significantly reduce the internal overhead of function calls within the PHP engine. These optimized functions can be found in the zend_compile.c file of PHP.

Why Opt for Micro-Optimizations?

You might wonder if this level of optimization is genuinely worthwhile. In open-source code, it’s considered good practice to minimize overhead wherever possible. While this optimization might seem micro in nature, it can make a difference in certain scenarios. For instance, when your application calls these compiler-optimized functions hundreds of thousands of times and aims for low response times in the 20-200ms range. While such cases are rare, understanding this optimization can prove valuable.

Automating the Import of Compiler Optimized Functions

Benefiting from compiler-optimized functions becomes even more accessible when using modern code formatting automation tools like php-cs-fixer and phpcs. Importing these optimized functions can be fully automated with minimal effort. For php-cs-fixer, the “native_function_invocation” rule with the “@compiler_optimized” configuration setting can be used:

For phpcs, the “SlevomatCodingStandard.Namespaces.ReferenceUsedNamesOnly” sniff from slevomat/coding-standard can be applied. However, note that it goes beyond importing compiler-optimized functions:

Profiler Insights into Compiler-Optimized Functions

Understanding how profilers interact with compiler-optimized functions provides valuable insights into PHP’s inner workings. Compiler-optimized functions can bypass the hooks that profilers use to track execution. Let’s consider an example where a userland function “my_own_str_repeat” mimics the behavior of “str_repeat” and employs “stolen” within it:

When profiling this code with tools like Tideways Callgraph Profiler or Xdebug, you’ll notice that “stolen” gets executed 100,000 times. This happens because the global namespace import is missing, preventing the use of the compiler-optimized version. Tideways even suggest a potential optimization opportunity, accounting for 24% of the total execution time.

As a side note, the sharp drop from 7.24ms to 1.81ms can be attributed to profiling overhead, as discussed elsewhere, caused by an internal function being called excessively.

However, once you add a leading namespace slash to import the function, the profiler no longer observes the execution. It appears as if “my_own_str_repeat” has no child calls, and the 100,000 optimized “strlen” functions remain hidden from the profiler. This illustrates how the PHP engine’s shortcut can render profilers oblivious to certain function calls.

Conclusion: The Relevance of Importing Compiler-Optimized Functions

The decision to import functions that the compiler can optimize ultimately depends on the context. When overhead significantly impacts performance, importing these functions is a wise choice. While this optimization may seem minor, understanding its implications can empower developers to make informed decisions about when and how to apply it in their code. By embracing best practices and leveraging automation tools, developers can harness the benefits of compiler-optimized functions while maintaining code efficiency.

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