phparchitect-2019-08

36 \ August 2019 \ http://www.phparch.com

Generated Singletons

Internal Apparatus

We learned last month the code generator always produces
RETURN null for the closing brace of a function or method. We
see the same thing here at line 15 (generated line L2).

We know that a class constructor returns an object of that
class. That’s what a constructor does, but we don’t see that
here. We’ll see that in the next section of generated code.

Get Instance

The third section, Output 1 lines 17–41, is the generated
code for our static getInstance() method. Line 17 identi-
fies the method name getInstance and line 19 verifies we’re
dealing with lines 13-18 of our source code. Let’s walk
through the generated code, lines L0–L18.

L0 (13): EXT_NOP

We saw last month that any function/method declaration
becomes a no-op (NOP or, as here, EXT_NOP). The generator does
not produce any executable code for the function declaration.

L1 (14): EXT_STMT

L2 (14): V0 = FETCH_STATIC_PROP_R string("instance") (self)

(exception)

L3 (14): T1 = BOOL_NOT V0

L4 (14): JMPZ T1 L12

We also learned last month that every PHP statement

begins with a generated EXT_STMT line, as we see for line L1

here. This provides a safe, quiescent, place for the PHP virtual

machine to invoke debugger hooks such as with Xdebug.

We now see, with line L2, the PHP virtual machine has a

special instruction for reading a static property, FETCH_STATIC_

PROP_R. As we’ll see below, PHP has a series of instructions

for situations involving self, static, sand/or parent. This

line fetches the value of self::$instance and places it in the

temporary variable V0.

Line L3 implements the !self::$instance calculation. We

already have self::$instance in variable V0. We now need

to compute the Boolean “not” of that value. If V0 is “truthy,”

the result ill be 0 (false); if V0 is “falsy,” the result is 1 (true).

The BOOL_NOT instruction performs this calculation, placing

the result in the temporary variable T1.

Note that BOOL_NOT produces a numeric

zero or one rather than true or false.

Line L4 implements the if() block.

We have a double-negative here, so let

me explain. When the PHP if(...)

condition is not true, we do not execute

the inside of the if(...) {...} block.

Program execution jumps around

it, moving to the next section of code

(elseif, else, or the end of the if block).

The JMPZ instruction means “jump on

zero.” Line L4 reads “When temporary

variable T1 contains a zero (based on

the BOOL_NOT calculation), skip down to

line L12 and continue program execu-

tion at line L12.”

Lines L5–L11 are quite a bit of work

for our one line of code static::$in-

stance = new static;.

L5 (15): EXT_STMT

L6 (15): V3 = NEW 0 (self) (exception)

L7 (15): EXT_FCALL_BEGIN

L8 (15): DO_FCALL

L9 (15): EXT_FCALL_END

L10 (15): V2 = FETCH_STATIC_PROP_W

string("instance") (self) (exception)

L11 (15): ASSIGN V2 V3

Recall that our constructor method’s

generated code did not appear to be

returning a new object. That’s handled

here with the NEW instruction on line

L6. NEW tells the PHP virtual machine to

create the new object. The 0 says we are

Output 1

1. $_main: ; (lines=5, args=0, vars=0, tmps=1)


2. ; (before optimizer)


3. ; /PragmaticPHP/pragma/src/GeneratedSingleton/CamdenStatePark.php:1-20


4. L0 (2): NOP


5. L1 (4): NOP


6. L2 (6): EXT_STMT


7. L3 (6): NOP


8. L4 (20): RETURN int(1)


9. 
   


10. App\GeneratedSingleton\CamdenStatePark::__construct: ; (lines=3, args=0, vars=0, tmps=0)


11. ; (before optimizer)


12. ; /PragmaticPHP/pragma/src/GeneratedSingleton/CamdenStatePark.php:10-11


13. L0 (10): EXT_NOP


14. L1 (11): EXT_STMT


15. L2 (11): RETURN null


16. 
    


17. App\GeneratedSingleton\CamdenStatePark::getInstance: ; (lines=19, args=0, vars=0, tmps=7)


18. ; (before optimizer)


19. ; /PragmaticPHP/pragma/src/GeneratedSingleton/CamdenStatePark.php:13-18


20. L0 (13): EXT_NOP


21. L1 (14): EXT_STMT


22. L2 (14): V0 = FETCH_STATIC_PROP_R string("instance") (self) (exception)


23. L3 (14): T1 = BOOL_NOT V0


24. L4 (14): JMPZ T1 L12


25. L5 (15): EXT_STMT


26. L6 (15): V3 = NEW 0 (self) (exception)


27. L7 (15): EXT_FCALL_BEGIN


28. L8 (15): DO_FCALL


29. L9 (15): EXT_FCALL_END


30. L10 (15): V2 = FETCH_STATIC_PROP_W string("instance") (self) (exception)


31. L11 (15): ASSIGN V2 V3


32. L12 (17): EXT_STMT


33. L13 (17): V6 = FETCH_STATIC_PROP_R string("instance") (self) (exception)


34. L14 (17): VERIFY_RETURN_TYPE V6


35. L15 (17): RETURN V6


36. L16 (18): EXT_STMT


37. L17 (18): VERIFY_RETURN_TYPE


38. L18 (18): RETURN null


39. LIVE RANGES:


40. 3: L10 - L11 (tmp/var)


41. 6: L14 - L15 (tmp/var)