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

[Andrew Haley]

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

Thanks everyone for your contributions.

I think we have a good solution:
1. Add my "fast" alternative RestoreMXCSR code, and use =1 and =2 levels of checking, as suggested by John Rose. Don't enable RestoreMXCSR by default: it should be 0 (no check).
2. Warn and restore the MXCSR at safepoints. This could be a hard error rather than a warning; not sure. At least we wouldn't have silent corruption.
Everyone happy?

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