JIT Compilation

Also Known As

TL;DR

Explanation

JIT compilers profile running code to identify hot paths (frequently executed), then compile those paths to native machine code. Two strategies: method JIT (compile entire methods) and tracing JIT (compile frequently executed traces across method boundaries). PHP 8's JIT uses a tracing approach. JIT benefits CPU-bound code (math, algorithms, image processing); I/O-bound PHP web apps see minimal improvement because they spend most time waiting for database/network, not executing CPU instructions.

Diagram

flowchart TD
    SRC[PHP Source] --> LEX[Lexer]
    LEX --> AST2[AST]
    AST2 --> OPC[Opcodes]
    OPC --> JIT{JIT enabled?}
    JIT -->|no| INTERP[Interpreter<br/>execute opcode by opcode]
    JIT -->|yes| PROFILE[Profile hot paths]
    PROFILE --> COMPILE[Compile to native x86<br/>machine code]
    COMPILE --> NATIVE[Execute native code<br/>much faster for CPU-bound]
    NATIVE -.->|cache in opcache| COMPILE
    INFO[JIT helps: math, compression, image processing<br/>JIT does NOT help: DB or IO bound apps]
style NATIVE fill:#238636,color:#fff
style INTERP fill:#d29922,color:#fff
style INFO fill:#1f6feb,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Enabling JIT without benchmarking — JIT adds memory overhead; verify it actually improves your specific workload.
• Expecting JIT to fix slow database queries — JIT only affects CPU execution time, not network and disk I/O.
• Not setting opcache.jit_buffer_size — JIT silently disabled without a buffer allocation.
• Conflating OPcache (always beneficial) with JIT (workload dependent).

Code Examples

; php.ini — JIT enabled for typical web CRUD app:
opcache.jit=1255
opcache.jit_buffer_size=256M
; Web app spends 95% of time in MySQL queries
; JIT improves the 5% PHP execution — negligible real-world improvement
; 256MB memory wasted on unused JIT buffer

; php.ini — JIT for CPU-bound workload (image processing, ML inference):
opcache.jit=1255          ; Tracing JIT
opcache.jit_buffer_size=128M
; Benchmark first: wrk -t4 -c100 -d30s http://localhost/
; Compare: with/without JIT on your specific workload
; For typical CRUD: keep jit_buffer_size small or disable JIT entirely

References

• ↗ https://wiki.php.net/rfc/jit

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