Floating point bug in range operation on Apple Silicon hardware

[judahnator]

Description

The following code:

<?php
print_r(range(-0.03, 0.03, 0.01));

Resulted in this output:

Array
(
    [0] => -0.03
    [1] => -0.02
    [2] => -0.01
    [3] => 1.7347234759768E-18
    [4] => 0.01
    [5] => 0.02
)

But I expected this output instead:

Array
(
    [0] => -0.03
    [1] => -0.02
    [2] => -0.01
    [3] => 0
    [4] => 0.01
    [5] => 0.02
    [6] => 0.03
)

I tested this on a few devices with my peers. Every x86 processor we tested worked as expected, but reliably got the erroneous result using different Apple M2 devices.

PHP Version

PHP 8.3.6

Operating System

Sonoma 14.4.1

[SakiTakamachi]

I'll investigate this issue.

[SakiTakamachi]

The investigation is complete. This is probably an Apple bug.

The following code:

// double.c
int main(void)
{
    int i = 3;
    double v = 0.03;
    double v2 = 0.01;

    double ret = v - (i * v2);

    return 0;
}

compile:

clang -o double -g double.c

exec:

./double

If this continues, nothing will be output, so debug with gdb:

# gdb ./double

....

(gdb) b main
Breakpoint 1 at 0x724: file double.c, line 4.
(gdb) run

....

Breakpoint 1, main () at double.c:4
4           int i = 3;
(gdb) n
5           double v = 0.03;
(gdb) 
6           double v2 = 0.01;
(gdb) 
8           double ret = v - (i * v2);
(gdb) 
10          return 0;
(gdb) p ret
$1 = -1.7347234759768071e-18
(gdb) p v
$2 = 0.029999999999999999
(gdb) p v2
$3 = 0.01
(gdb) p i
$4 = 3
(gdb) p i * v2
$5 = 0.029999999999999999
(gdb) p v - (i * v2)
$6 = 0

We can see that the calculation result ret is incorrect.

[SakiTakamachi]

However, I'm just trying to figure out if there's a workaround for this

[devnexen]

If that helps, I have the same issue with my raspberry 4.

[erjotek]

same problem occurred between 5.0 and 5.3.6 checking on 3v4l

[SakiTakamachi]

Thanks. hmm, then, is this an arm-specific phenomenon? It might be a good idea to push the test code to Circle CI arm.

[SakiTakamachi]

On a separate matter, I was investigating the behavior when using gcc and arm together.
So I discovered by chance that this only happens when compiled with clang. Compiling with gcc gives normal results.

[youkidearitai]

In M1 MacBook Air is reproduced.

MacBook-Air-2:php-src tekimen$ sapi/cli/php -r 'print_r(range(-0.03, 0.03, 0.01));'
Array
(
    [0] => -0.03
    [1] => -0.02
    [2] => -0.01
    [3] => 1.7347234759768E-18
    [4] => 0.01
    [5] => 0.02
)

In Raspberry Pi 4 (build in GCC), result is bellow:

tekimen@raspberrypi:~/src/php-src $ sapi/cli/php -r 'print_r(range(-0.03, 0.03, 0.01));'
Array
(
    [0] => -0.03
    [1] => -0.02
    [2] => -0.01
    [3] => 0
    [4] => 0.01
    [5] => 0.02
    [6] => 0.03
)

However, In Raspberry Pi 4 (build in clang), result is reproduced:

tekimen@raspberrypi:~/src/php-src $ cat config.nice
#! /bin/sh
#
# Created by configure

CFLAGS='-g' \
LDFLAGS='-v' \
CC='clang' \
'./configure' \
'--enable-debug' \
'--enable-mbstring' \
'--prefix=/home/tekimen/php83' \
"$@"
tekimen@raspberrypi:~/src/php-src $ sapi/cli/php -r 'print_r(range(-0.03, 0.03, 0.01));'
Array
(
    [0] => -0.03
    [1] => -0.02
    [2] => -0.01
    [3] => 1.7347234759768E-18
    [4] => 0.01
    [5] => 0.02
)

[SakiTakamachi]

The combination of intel and clang works fine.

[SakiTakamachi]

I reported it
llvm/llvm-project#91224

[SakiTakamachi]

I was able to get an answer right away. This is a problem with clang's compilation flags, and if -ffp-contract=off is not specified, calculation results may be slightly different from those of gcc etc.

Therefore, if you are building it yourself, please pass CFLAGS='-ffp-contract=off' in ./configure.

[youkidearitai]

I confirmed correct behavior in M1 MacBook Air.

MacBook-Air-2:php-src tekimen$ cat config.nice
#! /bin/sh
#
# Created by configure

CFLAGS='-ffp-contract=off' \
'./configure' \
'--enable-mbstring' \
'--enable-debug' \
'--prefix=/Users/tekimen/php_master' \
'--with-iconv=/opt/homebrew/opt/libiconv' \
'PKG_CONFIG_PATH=/opt/homebrew/opt/icu4c/lib/pkgconfig' \
'--enable-bcmath' \
'--with-zlib' \
'--enable-intl' \
"$@"
MacBook-Air-2:php-src tekimen$ sapi/cli/php -r 'print_r(range(-0.03, 0.03, 0.01));'
Array
(
    [0] => -0.03
    [1] => -0.02
    [2] => -0.01
    [3] => 0
    [4] => 0.01
    [5] => 0.02
    [6] => 0.03
)

[github-actions]

No feedback was provided. The issue is being suspended because we assume that you are no longer experiencing the problem. If this is not the case and you are able to provide the information that was requested earlier, please do so. Thank you.

[erjotek]

@shivammathur Hi, do you plan to change this flag in yours brew packages?

[SakiTakamachi]

This issue was resolved by the following PR:

#14162

[shivammathur]

@erjotek

@shivammathur Hi, do you plan to change this flag in yours brew packages?

Yes, with the next releases.

[judahnator]

Still having the problem, thanks GitHub actions.