RFR(S): 8213419: C2 may hang in MulLNode::Ideal()/MulINode::Ideal() with gcc 8.2.1

[Andrew Haley]

On 11/07/2018 09:06 PM, dean.long at oracle.com wrote:

> OK, I just wanted to make sure the code was doing what you intended.
> Isn't this still undefined behavior:
> 
> jint abs_con = uabs(con);
> 
> if the unsigned value is too big to fit in a signed int?

No, because the conversion of unsigned to signed is not undefined but
implementation-defined.

For gcc the behavior is:

    The result of, or the signal raised by, converting an integer to a
    signed integer type when the value cannot be represented in an
    object of that type (C90 6.2.1.2, C99 6.3.1.3).

    For conversion to a type of width N, the value is reduced modulo
    2^N to be within range of the type; no signal is raised.

Nevertheless, I agree with the decision to make everything unsigned.
It's really to only correct way to do arithmetic in C++ on jints.  I
wonder if we could redefine a jint type correctly, which is to say it
had the same behaviour as Java. It'd need overloads for all the
arithmetic operations.

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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