Edit report at https://bugs.php.net/bug.php?id=70173&edit=1
ID: 70173
Comment by: rainer dot jung at kippdata dot de
Reported by: rainer dot jung at kippdata dot de
Summary: ZVAL_COPY_VALUE_EX broken for 32bit Solaris Sparc
Status: Open
Type: Bug
Package: *General Issues
Operating System: Solaris 10 Sparc
PHP Version: 7.0.0beta3
Block user comment: N
Private report: N
New Comment:
I extracted a C standalone reproduction case from the PHP files.
I will attach it.
The zval.value layout on this playform is such, that lval and dval both start at the lower address
but dval extends above lval.
Now because of ZEND_ENDIAN_LOHI in the definition of ww, the address of w2 is the low one (same as
lval and dval), and the address of w1 is the high one.
Copying in ZVAL_COPY_VALUE copies the 32 bit contents of the low address, and then additionally
z->value.ww.w2 = _w2, which is again the low hald of dval.
So the high half stays uninitialized resp. 0.
So for this platform the use of ZEND_ENDIAN_LOHI doesn't work as expected.
I will attach a small C program to show the problem. It can be compiled standalone on Solaris Sparc
(I used gcc), no dependencies needed. I will attach it as a patch, because I don't see any
other way of attaching source files.
Previous Comments:
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[2015-08-08 20:57:19] rainer dot jung at kippdata dot de
Changed Summary, because I narrowed down the root cause.
Unfortunately there is no "Zend" in the issue tracker "Package" drop down.
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[2015-08-08 20:53:59] rainer dot jung at kippdata dot de
OK, I used the debugger. The problem is inside ZVAL_COPY_VALUE_EX. Only half of the double is being
copied, only 32 bits of the 64 bits. ZVAL_COPY_VALUE_EX is meant to fix this by using the "#if
SIZEOF_SIZE_T == 4" definition of it it additionally copies value.ww.w2. But on Solaris Sparc,
value.ww.w2 has already been copied and what is missing is value.ww.w1 ! For some reason the struct
layout is not the expected one, w1 and w2 have the wrong order. It seems the ZEND_ENDIAN_LOHI()
logic for w1, w2 resp. the refcount field which is used in the basic copying doesn't work on
sparc. It is a big endian platform and WORDS_BIGENDIAN is defined, but is nevertheless doesn't
work.
I hope that's enough debugging for you guys to be able to go the last mile. I can test on the
target platform whatever change you want me to.
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[2015-08-08 14:32:49] rainer dot jung at kippdata dot de
OK, another step forward, but I gues that's how far I get without any hints:
Simplest test script:
<?php
$var = 2900000000;
var_dump($var);
?>
Expected result:
----------------
float(2900000000)
Actual result:
--------------
float(2899998720)
It works for PHP 5.6, fails for 7.
- The result of zend_strtod() when printed with fprintf and %f format is
"2900000000.000000". So this is OK.
- The double assignment in Zend/zend_language_scanner.c in line 2757 assigns the correct value
(verified with fprintf and %f). If I call php_var_dump(zendlval, 0) there it writes the correct
string "float(2900000000)".
- In the var_dump() call of next script line, the float argument is already wrong. If checked via
fprointf and %f it is "2899998720.000000". The address of struc in php_var_dump() has also
changed from the previous address of zendlval in zend_language_scanner.c.
I don't know where the change happens, I could just narrow it down so far.
Just to make sure: I don't think this is an expected float precision problem. The test works
for 5.6 and the observed precision is 7 instead of 14. It makes a lot of standard test suite tests
fail.
------------------------------------------------------------------------
[2015-08-08 12:53:52] rainer dot jung at kippdata dot de
I added debug statements to zend_strtod(). The call to it gets the correct input string, and if I
print the result before returning with fprintf(stderr, "%f") it is also correct. Only the
var_dump() output float(...) is wrong. So it seems the problem is more about the formatting of float
in var_dump().
------------------------------------------------------------------------
[2015-08-08 12:41:47] rainer dot jung at kippdata dot de
Concerning zend_strtod():
- I first tried the original library available under www.netlib.org/fp. It didn't show the
problem.
- I then tried to shrink the delta between the PHP copy and the original and ended up with a test
case, that doesn't show the problem even with PHP zend_strtod(). So it seems it is something in
PHP before the calls to zend_strtod(), or the problem is in the output rendering of the float.
Here's my test:
1) Create a file dtoa-main.c containing:
#include <stdio.h>
#include <zend_strtod.h>
int main() {
fprintf(stdout, "%f\n", zend_strtod("29000000", NULL));
fprintf(stdout, "%f\n", zend_strtod("290000000", NULL));
fprintf(stdout, "%f\n", zend_strtod("2900000000", NULL));
fprintf(stdout, "%f\n", zend_strtod("29000000000", NULL));
fprintf(stdout, "%f\n", zend_strtod("290000000000", NULL));
fprintf(stdout, "%f\n", zend_strtod("2900000000000", NULL));
fprintf(stdout, "%f\n", zend_strtod("29000000000000", NULL));
fprintf(stdout, "%f\n", zend_strtod("290000000000000", NULL));
fprintf(stdout, "%f\n", zend_strtod("2900000000000000", NULL));
}
The numbers are the same as in the test.php I originally use to produce the problem.
Now compile this snippet against the original zend_strtod.o object file:
solaris10.sparc apache% gcc -I /path/to/php/bldir/Zend -I /path/to/php/bldir/TSRM -I
/path/to/php/bldir -Wl,-z -Wl,nodefs -o dtoa-test dtoa-main.c
/path/to/php/bldir/Zend/.libs/zend_strtod.o
The "-z nodefs" linker argument is only needed, because the object file references the
zend_error_noreturn symbol, which is not used in this test case. I didn't want to soak in many
more object files.
Now run the binary:
./dtoa-test
29000000.000000
290000000.000000
2900000000.000000
29000000000.000000
290000000000.000000
2900000000000.000000
29000000000000.000000
290000000000000.000000
2900000000000000.000000
So the float seems to be OK.
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