[vm] Floating point discrepancy (-0.0 vs +0.0) between -emu_flavor emu and -emu_flavor jit

[RobinMorisset]

Describe the bug
On the following code:

-module(jit13564).
-compile([export_all]).

f(0) ->
    -f(ok);
f(_) ->
    0 / 18446744073709551615.

wrapper0() ->
    io:write(f(0)).

Running it with cerl -emu_flavor emu -noshell -pa . -s jit13564 wrapper0 -s init stop results in -0.0 (which seems correct to me).
But running it with cerl -emu_flavor jit -noshell -pa . -s jit13564 wrapper0 -s init stop results in 0.0.

Expected behavior
I'd expect to get the same result from arithmetic operations regardless of whether the JIT is active or not.

Affected versions

• master
• master + compiler: Various type-related fixes and improvements #6651

Additional context
Replacing f by

f(0) ->
    f(ok);
f(_) ->
    - (0 / 18446744073709551615).

makes the program return -0.0 in both emu and jit flavors.

[jhogberg]

Thanks for your report!

It's the JIT that does the wrong thing (see 200842d). I've fixed it in #6726

[jhogberg]

Fixed in #6726, to be released in OTP 25.3. Thanks again for your report! :-)

[Olivier-Boudeville]

Hi, this is probably not a problem, but as, depending on these changes, it may be surprising, this might be worth reporting: certainly that floats shall not be compared based on strict equality, but this allowed me to notice that apparently (perhaps in link with this issue) Erlang 25.3 evaluates some floating-point operations a bit differently from the past versions.
For example an expression that has been evaluating consistently to 2.442000e+01 (probably for years) is now 2.44200000000000017053e+01. Both values are OK, but as a result change may be the sign of a problem, I preferred to report it.

More context: this was detected in a simple test in CI, https://github.com/Olivier-Boudeville/Ceylan-Myriad/blob/352890f711d9f5c128360522c52a98d597f54d25/test/maths/vector4_test.erl#L102 which with 25.3 is now failing with

{"init terminating in do_boot",{{badmatch,[2.200000e+01,5.400000e+01,-6.000000e+00,2.442000e+01]},[{vector4_test,run,0,[{file,"vector4_test.erl"},{line,102}]},{erl_eval,do_apply,7,[{file,"erl_eval.erl"},{line,744}]},{init,start_it,1,[]},{init,start_em,1,[]},{init,do_boot,3,[]}]}}
init terminating in do_boot ({{badmatch,[2.20000000000000000000e+01,5.40000000000000000000e+01,-6.00000000000000000000e+00,2.44200000000000017053e+01]},[{vector4_test,run,0,[{_},{_}]},{erl_eval,do_apply,7,[{_},{_}]},{init,start_it,1,[]},{init,start_em,1,[]},{init,do_boot,3,[]}]})

(again: not a bug, but a surprise)