On Wed, 12 Oct 2022 17:00:15 GMT, Andrew Haley <a...@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 On 10/31/22 15:49, Joe Darcy wrote: > In terms of the overhead of using floating-point expression evaluation > as a guard, are there still platforms where operating on subnormal > values is pathologically slower? Some generations of SPARC chips had > that behavior where a subnormal multiply would take, say 10,000 cycles, > rather than 3 or 4 since the subnormal operations were implemented via > trap handling. That's a very interesting point. I know it used to be the case that denormals were handled by trapping to microcode, but there are good hardware algorithms since Schwarz et al, 2003 [1]. This paper showed how with a little hardware, such numbers can be handled close to the speed of normalized numbers. I deliberately ran my tests on a ten-year-old CPU, but I guess I'd have to go further back to find a bad case. Anyway, I plan to a. Restore the FPU CR after calls to dlopen(3). b. Detect FPU CR corruption at safepoints, and print a warning. At least the user might find out that something is wrong. I think this will avoid most cases of badness. I guess I'll need a CSR for this? [1] Hardware implementations of denormalized numbers, DOI:10.1109/ARITH.2003.1207662 Conference: Computer Arithmetic, 2003. Proceedings. 16th IEEE Symposium on ------------- PR: https://git.openjdk.org/jdk/pull/10661
