RFR: 8338449: ubsan: division by zero in sharedRuntimeTrans.cpp

[Lutz Schmidt]

On Wed, 16 Oct 2024 07:22:38 GMT, Matthias Baesken <mbaesken at openjdk.org> wrote:

>>> I'm not sure what the implications of this actually are. Do we really need it? Maybe @jddarcy can comment?
>> 
>> It needs to return -Inf. ubsan seems wrong here: this IEEE-754 arithmetic is well defined, and has been for a very long while.
>
> Ubsan is just following the C++ standard 
> https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf
> 5.6 Multiplicative operators  
>  ....
> 'The binary / operator yields the quotient, and the binary % operator yields the remainder from the division
> of the first expression by the second. If the second operand of / or % is zero the behavior is undefined.'
> 
> See also https://stackoverflow.com/questions/42926763/the-behaviour-of-floating-point-division-by-zero .
> 
> However on our set of platforms in OpenJDK we probably get away with the expected results when dividing by 0  (because they all seem to follow IEEE-754).
> That's why I just added the static_assert.

To me, this feels like the C++ standard is not compatible with IEEE-754 arithmetic. Undefined behavior would give the optimizer freedom to do whatever it likes to do. That is in contrast to the well-defined IEEE-754 requirement.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21500#discussion_r1808300823