Dead Code

Frequently I've been asked why Xdebug sees "dead code" in places where people don't expect it. Most often this is related to PHPUnit's Code Coverage in the following situations:

1:  <?php
2:  function foo()
3:  {
4:      if ( false )
5:      {
6:          throw new Exception();
7:      } /* line with dead code */
9:      return 42;
10: } /* line with dead code */
11: ?>

The explanation for this is rather simple. Xdebug checks code coverage by adding hooks into certain opcodes. Opcodes are the building blocks of oparrays. PHP converts each element in your script—main body, method, function—to oparrays when it parses them. The PHP engine then executes those oparrays by running some code for each opcode. Opcodes are generated, but they are not optimised. Which means that it does not remove opcodes that can not be executed.

With vld we can see which opcodes are generated. For the above script, there are two elements. The main body of the script, and the foo function. I used vld to show their opcodes, and after some trimming the main script body looks like:

line     #* I O op               ext  return  operands
   2     0  >   EXT_STMT
         1      NOP
  12     2      EXT_STMT
         3    > RETURN                        1

We'll ignore this one mostly, as there is nothing much in it, but do notice the RETURN opcode, which represents a return statement in a PHP script. We did not add a return statement, but PHP's parser always puts a RETURN opcode at the end of each oparray.

The foo function's oparray looks like:

line     #* I O op               ext  return  operands
   2     0  >   EXT_NOP
   5     1      EXT_STMT
         2    > JMPZ                          false, ->11
   6     3  >   EXT_STMT
         4      FETCH_CLASS        4  :0      'Exception'
         5      EXT_FCALL_BEGIN
         6      NEW                   $1      :0
         7      DO_FCALL_BY_NAME   0
         8      EXT_FCALL_END
         9    > THROW              0          $1
   7    10*     JMP                           ->11
   9    11  >   EXT_STMT
        12    > RETURN                        42
  10    13*     EXT_STMT
        14*   > RETURN                        null

Xdebug's code coverage marks line 7 and 10 as "dead code". When we look at the vld output above, we see that line 10 has an EXT_STMT and a RETURN statement. But they can never be reached as there is no path through the code that does not hit the RETURN on line 9 first. vld marks dead code with a *. The > in the I and O columns indicate points in the oparray that that are the end point of a jump instruction (ie., the start of a branch) and a location from where a jump is initiated respectively (ie., the exit point out of a branch).

vld actually tells you which branches and paths are found:

branch: #  0; line: 2- 5; sop:  0; eop:  2; out1:   3; out2:  11
branch: #  3; line: 6- 6; sop:  3; eop:  9
branch: # 11; line: 9-10; sop: 11; eop: 14
path #1: 0, 3,
path #2: 0, 11,

Each branch is "named" by its starting opcode entry. For each of the branches, Xdebug, and vld, check whether there is a premature unconditional exit. Conditional exits and jumps are already checked when the oparray is split into branches.

From the three branch definitions you can already see that opcode 10 is not part of any branch as it sits between an exit point and an entry point. Hence it's marked as dead code on line 7. This line contains the closing brace (}) of the if statement.

In the branch covering opcodes 3 to 9 the THROW in opcode 9 is the exit point. For if statements, PHP's code generator always generates an extra JMP at the end. This opcode would simply jump to the next opcode (the jump target is shown as ->11). However, if the branch is exitted prematurely (due to the THROW) in this case, it's not hit. Because it's the only opcode on line 7, the whole line gets marked as "dead code".

In the branch covering opcodes 11 to 14, the RETURN statement in opcode 12 on line 9 is the exit point of the branch, and hence opcodes 13-14 are marked as dead code.

Hopefully this explains that sometimes lines which seem to have code, are marked as dead code. And this is in the cases where PHP gets the line numbers for opcodes right... which isn't always the case either.

For Xdebug, I am improving code coverage to also include path and branch coverage, which should come in Xdebug 2.3.


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