Hello,

I noticed that multiple calls to substr with some long parameter (eg. 512kB) makes xdebug profiling very slow. Here's a simplified test case:

#php.ini:

zend_extension=xdebug.so

xdebug.mode=trace

xdebug.start_with_request=yes

xdebug.trace_format=1

#test.php:

$string = str_repeat('a', 512000);

for ($i = 0; $i <= 20000; $i++) {

substr($string, 1, 1);

}

$ time php test.php (without xdebug)

real    0m0.035s

user    0m0.026s

sys 0m0.009s

$ time php -c php.ini test.php (with xdebug)

real    0m9.847s

user    0m9.668s

sys 0m0.175s

After some digging I found 3 problems:

1. The reason for the slow execution is that trace_computerized.c calls xdebug_var_export_line() for each function call. Then xdebug_var_export_line calls php_addcslashes() which is very slow, because it copies the string byte by byte. After the string is escaped the function uses only max_data bytes (default is 512) of the string, so most of the slow work done by php_addcslashes is discarded.

2. I think there's problem with the truncation, because the string could be truncated ending on a special char, so the last char will be '\'. Then when the ending ' is appended, the final string will not be parsable.

xdebug_var_export_line:

#if PHP_VERSION_ID >= 70300

tmp_zstr = php_addcslashes(Z_STR_P(struc), (char) "'\\0..\37", 7);

#else

tmp_zstr = php_addcslashes(Z_STR_P(struc), 0, (char) "'\\0..\37", 7);

#endif

                    if (options->no_decoration) {
                            xdebug_str_add_zstr(str, tmp_zstr);
                    } else if ((size_t) Z_STRLEN_P(*struc) <= (size_t) options->max_data) {
                            xdebug_str_addc(str, '\'');
                            xdebug_str_add_zstr(str, tmp_zstr);
                            xdebug_str_addc(str, '\'');
                    } else {
                            xdebug_str_addc(str, '\'');
                            // the string could be truncated at a special char, so the last char will be \.
                            // This will break the parsing of the produced output file
                            xdebug_str_addl(str, ZSTR_VAL(tmp_zstr), options->max_data, 0);
                            xdebug_str_addc(str, '\'');
        }

3. The time spent on computations in xdebug_trace_computerized_function_entry is calculated for the called PHP function. This produces incorrect profiling results. In the above case the substr function is accounted for the whole amount of time, though it takes only fraction of second to complete.