Zend Engine

Zend memory manager

General

The goal of the new memory manager (available since PHP 5.2) is to reduce memory allocation overhead and speedup memory management.

Debugging

Normal:

sapi/cli/php -r 'leak();'

Zend MM disabled:

USE_ZEND_ALLOC=0 valgrind --leak-check=full sapi/cli/php -r 'leak();'

Shared extensions

Since PHP 5.3.11 it is possible to prevent shared extensions from unloading so that valgrind can correctly track the memory leaks in shared extensions. For this there is the ZEND_DONT_UNLOAD_MODULES environment variable. If set, then DL_UNLOAD() is skipped during the shutdown of shared extensions.

ZEND_VM

ZEND_VM architecture allows specializing opcode handlers according to op_type fields and using different execution methods (call threading, switch threading and direct threading). As a result ZE2 got more than 20% speedup on raw PHP code execution (with specialized executor and direct threading execution method). As in most PHP applications raw execution speed isn't the limiting factor but system calls and database calls are, your mileage with this patch will vary.

Most parts of the old zend_execute.c go into zend_vm_def.h. Here you can find opcode handlers and helpers. The typical opcode handler template looks like this:

ZEND_VM_HANDLER(<OPCODE-NUMBER>, <OPCODE>, <OP1_TYPES>, <OP2_TYPES>)
{
    <HANDLER'S CODE>
}

<OPCODE-NUMBER> is a opcode number (0, 1, ...) <OPCODE> is an opcode name (ZEN_NOP, ZEND_ADD, :) <OP1_TYPES> and <OP2_TYPES> are masks for allowed operand op_types. Specializer will generate code only for defined combination of types. You can use any combination of the following op_types UNUSED, CONST, VAR, TMP and CV also you can use ANY mask to disable specialization according operand's op_type. <HANDLER'S CODE> is a handler's code itself. For most handlers it stills the same as in old zend_execute.c, but now it uses macros to access opcode operands and some internal executor data.

You can see the conformity of new macros to old code in the following list:

EXECUTE_DATA
    execute_data
ZEND_VM_DISPATCH_TO_HANDLER(<OP>)
    return <OP>_helper(ZEND_OPCODE_HANDLER_ARGS_PASSTHRU)
ZEND_VM_DISPATCH_TO_HELPER(<NAME>)
    return <NAME>(ZEND_OPCODE_HANDLER_ARGS_PASSTHRU)
ZEND_VM_DISPATCH_TO_HELPER_EX(<NAME>,<PARAM>,<VAL>)
    return <NAME>(<VAL>, ZEND_OPCODE_HANDLER_ARGS_PASSTHRU)
ZEND_VM_CONTINUE()
    return 0
ZEND_VM_NEXT_OPCODE()
    NEXT_OPCODE()
ZEND_VM_SET_OPCODE(<TARGET>
    SET_OPCODE(<TARGET>
ZEND_VM_INC_OPCODE()
    INC_OPCOD()
ZEND_VM_RETURN_FROM_EXECUTE_LOOP()
    RETURN_FROM_EXECUTE_LOOP()
ZEND_VM_C_LABEL(<LABEL>):
    <LABEL>:
ZEND_VM_C_GOTO(<LABEL>)
    goto <LABEL>
OP<X>_TYPE
    opline->op<X>.op_type
GET_OP<X>_ZVAL_PTR(<TYPE>)
    get_zval_ptr(&opline->op<X>, EX(Ts), &free_op<X>, <TYPE>)
GET_OP<X>_ZVAL_PTR_PTR(<TYPE>)
    get_zval_ptr_ptr(&opline->op<X>, EX(Ts), &free_op<X>, <TYPE>)
GET_OP<X>_OBJ_ZVAL_PTR(<TYPE>)
    get_obj_zval_ptr(&opline->op<X>, EX(Ts), &free_op<X>, <TYPE>)
GET_OP<X>_OBJ_ZVAL_PTR_PTR(<TYPE>)
    get_obj_zval_ptr_ptr(&opline->op<X>, EX(Ts), &free_op<X>, <TYPE>)
IS_OP<X>_TMP_FREE()
    IS_TMP_FREE(free_op<X>)
FREE_OP<X>()
    FREE_OP(free_op<X>)
FREE_OP<X>_IF_VAR()
    FREE_VAR(free_op<X>)
FREE_OP<X>_VAR_PTR()
    FREE_VAR_PTR(free_op<X>)

Executor's helpers can be defined without parameters or with one parameter. This is done with the following constructs:

ZEND_VM_HELPER(<HELPER-NAME>, <OP1_TYPES>, <OP2_TYPES>)
{
    <HELPER'S CODE>
}

ZEND_VM_HELPER_EX(<HELPER-NAME>, <OP1_TYPES>, <OP2_TYPES>, <PARAM_SPEC>)
{
    <HELPER'S CODE>
}

The executors code is generated by the PHP script zend_vm_gen.php. It uses zend_vm_def.h and zend_vm_execute.skl as input and produces zend_vm_opcodes.h and zend_vm_execute.h. The first file is a list of opcode definitions. It is included from zend_compile.h. The second one is an executor code itself. It is included from zend_execute.c.

zend_vm_gen.php can produce different kind of executors. You can select a different opcode threading model using --with-vm-kind=CALL|SWITCH|GOTO|HYBRID. You can disable opcode specialization using --without-specializer. At last you can debug the executor using the original zend_vm_def.h or the generated zend_vm_execute.h file. Debugging with the original file requires the --with-lines option. By default, Zend Engine uses the following command to generate the executor:

# Default VM kind is HYBRID
php zend_vm_gen.php --with-vm-kind=HYBRID