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

[Andrew Haley]

On 10/13/22 19:13, Vladimir Ivanov wrote:
> On Thu, 13 Oct 2022 07:47:20 GMT, Andrew Haley <aph at openjdk.org> wrote:
> 
>> The problem is that this bug is very insidious: the user probably doesn't know that there's anything wrong, and almost certainly has no idea that it's anything to do with JNI.
> 
> I'm still trying to grasp why the current problem is something different from what we experienced before.

I don't know anything about what we experienced before. In any case,
I'm not sure that precedent applies here: the problem is that there
are shared libraries in use which break IEEE semantics. This is
an insidious bug, and my proposed fix costs almost nothing.

> Some platforms (x86_32 and AArch32) provide a way to restore FP environment, but it is turned off by default. Why x86_64 case is different, so it requires the logic to be turned on by default? Does the same reasoning apply to those platforms as well and we want to have `AlwaysRestoreFPU` turned on by default?

My patch is not in any way x86_64-specific. At present it is Linux-specific,
but I have my doubts that it should be.
> $ grep -r AlwaysRestoreFPU src/hotspot/
> src/hotspot//cpu/x86/templateInterpreterGenerator_x86.cpp:  if (AlwaysRestoreFPU) {
> src/hotspot//cpu/x86/sharedRuntime_x86_32.cpp:  if (AlwaysRestoreFPU) {
> src/hotspot//cpu/arm/sharedRuntime_arm.cpp:  if (AlwaysRestoreFPU) {
> src/hotspot//cpu/arm/templateInterpreterGenerator_arm.cpp:  if (AlwaysRestoreFPU) {
> src/hotspot//share/runtime/globals.hpp:  product(bool, AlwaysRestoreFPU, false,                                    \
> 
> -------------
> 
> PR: https://git.openjdk.org/jdk/pull/10661

x86-32 has a very specific problem: its default floating-point precision is
AFAIK extended float, which does not meet Java specs. Also, restoring the
FPU after every JNI call is expensive.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671