2.3 Operand Types

Operand type can be of following:

#define IS_UNUSED   0
#define IS_CONST    (1<<0)
#define IS_TMP_VAR  (1<<1)
#define IS_VAR      (1<<2)
#define IS_CV       (1<<3)  // Compiled variable

• If the operand is not used by the instruction, or the instruction doesn’t generate a result, the type of corresponding znode_op is IS_UNUSED.
• If the operand is a literal, its type is IS_CONST.
• If the operand is the temporary value returned by an expression, its type is IS_TMP_VAR.
• If the operand is a variable known at compile time, its type is IS_CV.
• If the operand is a variable returned by an expression, its type is IS_VAR.

For example, the following PHP code:

$a = 1;
$a + 1;
$b = $a + 1;
$a += 1;
$c = $b = $a += 1;

Compiles to:

#                        (op1     op2     result) type
ASSIGN     $a, 1       #  CV      CONST   UNUSED
ADD        $a, 1,  ~1  #  CV      CONST   TMP_VAR
FREE       ~1          #  TMP_VAR UNUSED  UNUSED
ADD        $a, 1,  ~2  #  CV      CONST   TMP_VAR
ASSIGN     $b, ~2      #  CV      TMP_VAR UNUSED
ASSIGN_ADD $a, 1       #  CV      CONST   UNUSED
ASSIGN_ADD $a, 1,  @5  #  CV      CONST   VAR
ASSIGN     $b, @5, @6  #  CV      VAR     VAR
ASSIGN     $c, @6      #  CV      VAR     UNUSED

We can see that, for an assignment instruction, whether it has a result depends on whether the result is used or not. But for non-assignment instructions, the result is always stored in a temporary variable, even when the result is unused, in case it needs to be freed.