Optimizing PHP with JIT Compilation

PHP 8 introduced a significant leap in performance with the inclusion of a Just-In-Time (JIT) compiler. This addition marks a pivotal moment for the language, traditionally known for its interpreted nature. While previous PHP versions relied heavily on Opcode caching to mitigate interpretation overhead, JIT takes optimization a step further by compiling frequently executed code segments into native machine code. This post will delve into the intricacies of PHP 8's JIT, explore its performance implications through benchmarking, and provide insights into its underlying compiler internals.

Understanding PHP 8 JIT

Prior to PHP 8, PHP code executed through a multi-step process: source code was parsed into an Abstract Syntax Tree (AST), then compiled into Opcode (Zend Opcodes), and finally executed by the Zend Engine. Opcaching, introduced in earlier versions, significantly improved performance by caching these Opcodes, avoiding recompilation on every request.

PHP 8's JIT compiler integrates directly with OPcache. Instead of merely interpreting Opcodes, JIT identifies "hot" code paths – those executed frequently – and translates them into highly optimized machine code at runtime. This machine code can then be executed directly by the CPU, bypassing the Zend Engine's interpretation overhead for those specific sections.

There are two main JIT compilation modes in PHP 8:

• Tracing JIT (default): This mode focuses on optimizing "hot" code traces (sequences of operations) as they are executed. It identifies frequently run paths and compiles them.
• Function JIT: This mode compiles entire functions, regardless of whether they are frequently executed. While potentially offering broader optimization, it can also lead to more overhead for less-used code.

JIT is not a magic bullet for all PHP applications. Its benefits are most pronounced in CPU-intensive workloads, such as:

• Complex mathematical computations
• Image processing
• Data analysis
• Long-running scripts

Traditional web applications, which often spend significant time on I/O operations (database queries, network requests), might see less dramatic improvements, as the bottleneck often lies outside of pure CPU execution.

Performance Benchmarking

Benchmarking PHP 8 with JIT enabled reveals varying degrees of performance improvements depending on the workload. Synthetic benchmarks, often involving pure mathematical calculations or tight loops, show the most impressive gains.

For example, initial tests by PHP core developers demonstrated that the PHP-Parser became approximately 1.3 times faster with JIT. Some synthetic benchmarks have even reported 3-5x speedups for specific CPU-bound tasks.

However, in real-world web applications, the performance uplift is generally more modest. Studies and community benchmarks indicate that for typical web applications, which are often I/O bound, the gains range from minimal to around 10-20%. Some reports suggest up to a 40-50% improvement in specific scenarios. It's crucial to understand that JIT's impact is workload-dependent.

Here's an example of how you might enable JIT in your php.ini:

opcache.enable=1
opcache.jit_buffer_size=100M
opcache.jit=1255

Let's break down the opcache.jit value (e.g., 1255):

Each digit represents a flag for JIT compilation strategy:

• Digit 1 (Optimization Level):
  • 0: No JIT
  • 1: Minimal JIT (basic block compilation)
  • 2: Function JIT
  • 3: Tracing JIT (default and generally recommended)
  • 4: Aggressive Tracing JIT
• Digit 2 (CPU-specific optimizations):
  • 0: No CPU-specific optimizations
  • 1: Enable CPU-specific optimizations (e.g., SSE, AVX)
• Digit 3 (Register allocation):
  • 0: No register allocation
  • 1: Local register allocation
  • 2: Global register allocation
• Digit 4 (Loop unrolling):
  • 0: No loop unrolling
  • 1: Enable loop unrolling

A common recommended setting for opcache.jit for many real-world applications is 1255, which enables tracing JIT with aggressive optimizations. Experimentation with different opcache.jit values is recommended to find the optimal setting for your specific application.

For comprehensive benchmarking, consider using tools like:

• PHPBench: A benchmarking framework for PHP.
• Blackfire.io: A powerful profiler that can help identify performance bottlenecks.

Remember to run benchmarks multiple times with consistent environments to ensure accurate and reliable results.

Compiler Internals

The PHP JIT compiler is implemented as an integral part of the OPcache extension. It leverages a dynamic assembler library called DynASM (Dynamic Assembler), which is also used by LuaJIT. DynASM allows the JIT to generate native machine code directly from PHP's intermediate representation (Opcodes).

Here's a simplified overview of how the JIT process works:

1. PHP Code -> Opcodes: Your PHP code is first parsed and compiled into Zend Opcodes, which are platform-independent bytecode instructions.
2. OPcache: These Opcodes are stored in shared memory by OPcache to avoid repeated parsing and compilation.
3. JIT Monitoring: When JIT is enabled, it monitors the execution of these Opcodes. It identifies "hot" regions of code—sections that are executed frequently or within tight loops.
4. Tracing/Function Compilation:
   • Tracing JIT: When a "hot" trace is identified, JIT records the sequence of Opcodes executed within that trace.
   • Function JIT: The entire function's Opcodes are considered for compilation.
5. Machine Code Generation: The recorded Opcodes (or function Opcodes) are then passed to the DynASM engine. DynASM translates these Opcodes into highly optimized, native machine code specific to the underlying CPU architecture.
6. Execution: The generated machine code is stored in the OPcache shared memory. Subsequent executions of the same "hot" code path directly invoke this compiled machine code, bypassing the Zend Engine's interpreter.

It's important to note that the JIT compiler in PHP 8 is a specialized implementation. Unlike highly optimizing compilers found in languages like C++ or Java (JVM), PHP's JIT focuses on specific types of optimizations. It doesn't introduce an additional Intermediate Representation (IR) form but instead works directly with Opcodes and DynASM to achieve its goals. The design prioritizes pragmatic performance gains for PHP workloads without introducing excessive compilation overhead.

Conclusion

PHP 8's JIT compiler is a significant advancement that brings substantial performance improvements to CPU-intensive PHP applications. While it may not revolutionize the speed of typical I/O-bound web applications, it provides a powerful tool for optimizing specific bottlenecks and pushing the boundaries of what PHP can achieve. Understanding the different JIT modes, carefully benchmarking your applications, and exploring the compiler's internals will empower you to leverage this feature effectively.

As PHP continues to evolve, the JIT compiler represents a commitment to performance and a testament to the language's adaptability. We encourage you to experiment with JIT in your projects and contribute to the ongoing efforts to make PHP even faster.

Resources

• PHP.Watch - PHP JIT in Depth
• Stitcher.io - PHP 8: JIT performance in real-life web applications
• PHP RFC: JIT
• Kinsta - What's New in PHP 8 (Features, Improvements, and the JIT Compiler)
• DynASM (Dynamic Assembler)

Next Steps

• Experiment with different opcache.jit settings in your development environment.
• Profile your PHP applications to identify CPU-bound sections that could benefit most from JIT.
• Explore the PHP internals documentation for a deeper dive into the Zend Engine and OPcache.