RFR: 8295159: DSO created with -ffast-math breaks Java floating-point arithmetic [v7]

[Vladimir Ivanov]

On Wed, 12 Oct 2022 17:00:15 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> A bug in GCC causes shared libraries linked with -ffast-math to disable denormal arithmetic. This breaks Java's floating-point semantics.
>> 
>> The bug is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55522
>> 
>> One solution is to save and restore the floating-point control word around System.loadLibrary(). This isn't perfect, because some shared library might load another shared library at runtime, but it's a lot better than what we do now. 
>> 
>> However, this fix is not complete. `dlopen()` is called from many places in the JDK. I guess the best thing to do is find and wrap them all. I'd like to hear people's opinions.
>
> Andrew Haley has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8295159: DSO created with -ffast-math breaks Java floating-point arithmetic

Some additional info to consider: the library modifies only MXCSR register and `-XX:+RestoreMXCSROnJNICalls` makes the test to pass. 


$ objdump -D libfast-math.so
...
0000000000000560 <set_fast_math>:
 560:   f3 0f 1e fa             endbr64
 564:   0f ae 5c 24 fc          stmxcsr -0x4(%rsp)
 569:   81 4c 24 fc 40 80 00    orl    $0x8040,-0x4(%rsp)
 570:   00
 571:   0f ae 54 24 fc          ldmxcsr -0x4(%rsp)
 576:   c3                      ret

-------------

PR: https://git.openjdk.org/jdk/pull/10661