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Date: Thu, 07 Jun 2012 22:18:22 +0200
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Subject: Re: [PHP-DEV] concatenation operator
From: johannes@schlueters.de (Johannes =?ISO-8859-1?Q?Schl=FCter?=)

On Thu, 2012-06-07 at 12:53 -0700, Adi Mutu wrote:
> Ok Johannes, thanks for the answer. I'll try to look deeper. 
> I basically just wanted to know what happens when you concatenate two
> strings? what emalloc/efree happens.

This depends. As always. As said what has to be done is one allocation
for the result value ... and then the zval magic, which depends on
refcount, references, ...

> Also can you tell me if possible how to put a breakpoint to
> emalloc/efree which are executed only after all core functions are
> registered? because it takes like a million years like this and a
> million F8 presses...

Depends on your debugger. Most allow conditional breakpoints or have a
breakpoint and while holding at some place add a few more ...

For such a question my preference is using DTrace (on Solaris, Mac or
BSD), something like this session:

        $ cat test.d
        #!/sbin/dtrace
        
        pid$target::concat_function:entry {
        	self->in_concat = 1;
        }
        
        pid$target::execute:return {
        	self->in_concat = 0;
        }
        
        pid$target::_emalloc:entry
        / self->in_concat /
        {
        	trace(arg0);
        	ustack();
        }
        
        pid$target::_erealloc:entry
        / self->in_concat /
        {
        	trace(arg0);
        	trace(arg1);
        	ustack();
        }
        
        $ cat test1.php
        <?php
        $a = "foo"; $b = "bar"; $a.$b;
        
        $ dtrace -s test.d -c 'php test1.php'
        dtrace: script 'test.d' matched 4 probes
        dtrace: pid 16406 has exited
        CPU     ID                    FUNCTION:NAME
          3 100372                   _emalloc:entry                 7
                      php`_emalloc
                      php`concat_function+0x270
                      php`ZEND_CONCAT_SPEC_CV_CV_HANDLER+0xcd
                      php`execute+0x3d9
                      php`dtrace_execute+0xe7
                      php`zend_execute_scripts+0xf5
                      php`php_execute_script+0x2e8
                      php`do_cli+0x864
                      php`main+0x6e2
                      php`_start+0x83
        
        $ cat test2.php
        <?php
        $a = 23; $b = "bar"; $a.$b;
        
        $ dtrace -s test.d -c 'php test2.php'
        dtrace: script 'test.d' matched 4 probes
        dtrace: pid 16425 has exited
        CPU     ID                    FUNCTION:NAME
          1 100373                  _erealloc:entry                 0               79
                      php`_erealloc
                      php`xbuf_format_converter+0x11ee
                      php`vspprintf+0x34
                      php`zend_spprintf+0x2f
                      php`_convert_to_string+0x174
                      php`zend_make_printable_zval+0x5ec
                      php`concat_function+0x3c
                      php`ZEND_CONCAT_SPEC_CV_CV_HANDLER+0xcd
                      php`execute+0x3d9
                      php`dtrace_execute+0xe7
                      php`zend_execute_scripts+0xf5
                      php`php_execute_script+0x2e8
                      php`do_cli+0x864
                      php`main+0x6e2
                      php`_start+0x83
        
          1 100372                   _emalloc:entry                 6
                      php`_emalloc
                      php`concat_function+0x270
                      php`ZEND_CONCAT_SPEC_CV_CV_HANDLER+0xcd
                      php`execute+0x3d9
                      php`dtrace_execute+0xe7
                      php`zend_execute_scripts+0xf5
                      php`php_execute_script+0x2e8
                      php`do_cli+0x864
                      php`main+0x6e2
                      php`_start+0x83
        
So, when having two constant strings there's a single malloc, in this
case allocating 7 bytes (strlen("foo")+strlen("bar")+1), if you have a
different type it has to be converted first ...


johannes