PS And additional expressions could be crashed to rule out
flush-to-zero, treating all subnormals and zero, and other non-compliant
modes of operation.
-Joe
On 10/20/2022 1:05 PM, Joseph D. Darcy wrote:
On 10/20/2022 10:43 AM, Andrew Haley wrote:
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
So here's a thought:
Reading and writing floating-point control flags can be expensive
because they're serializing operations. However, we can discover
whether the processor has been put into an "odd" rounding mode with
just a few floating-point instructions, so we can do:
if (epsilon + epsilon == 0) {
rtn = fesetenv(&default_fenv)
}
Assuming the rounding mode could be one of the four classic rounding
modes (to nearest even, to +infinity, to -infinity, to zero), two
calculations are needed to determine if the mode is set to nearest
even, as required by the JVM.
Candidate calculations are
1.0 + ROUND_THRESH == 1.0
-1.0 - ROUND_THRESH == -1.0
with the decoding
false false => to nearest
false true => to positive infinity
true false => to negative infinity
true true => to zero
For double, the double rounding threshold is 2^–53 + 2^–105 ≈
1.1102230246251568e –16. An analogous constant can be derived for
float, if desired
HTH,
-Joe
