[8u] RFR(S): 8244548: JDK 8u: sun.misc.Version.jdkUpdateVersion() returns wrong result

[Andrew Hughes]

On 11/05/2020 19:22, Simon Tooke wrote:
> That's horrible.  I will try again.  My wife claims I tell anecdotes
> like that too.
> 
> Original:
> 
> 1644     unsigned int jdk_version;    /* Consists of major, minor, micro
> (n.n.n) */
> 1645                                                /* and build number
> (xx) */
> 1646     unsigned int update_version : 8;         /* Update release
> version (uu) */
> 1647     unsigned int special_update_version : 8; /* Special update
> release version (c)*/
> 1648     unsigned int reserved1 : 16;
> 1649     unsigned int reserved2;
> 
> The proposed patch:
> 
> 1644     unsigned int jdk_version;   /* Consists of major, minor, micro
> (n.n.n) */
> 1645                                                /* and build number
> (xx) */
> 1646     unsigned int update_version : 16;        /* Update release
> version (uu) */
> 1647     unsigned int special_update_version : 8; /* Special update
> release version (c)*/
> 1648     unsigned int reserved1 : 16;
> 1649     unsigned int reserved2;
> 
> Note that the original sum of bitfields is 32 bits, so it will fit into
> a 4 byte int (which it is surrounded by)
> The proposed sum of bitfields is 40 bits, which will probably cause a
> new 4 byte bitfield to be generated under the hood.
> 
> -Simon

Thanks. That's a lot more readable :-)

Do we have any idea how much the three later fields are used in the
wild? In particular, could we not just steal those eight bits from
'special_update_version' (whatever that is) and keep the sum the same?

This would only apply to versions where update_version is broken
already, so reinterpretation of a structure of the existing size seems
safer than increasing the size by eight bits.

In code,

unsigned int jdk_version;
unsigned int update_version : 16;
unsigned int reserved1 : 16;
unsigned int reserved2;

or, if we're wary about losing the name special_update_version, we could
shrink reserved1:

unsigned int jdk_version;
unsigned int update_version : 16;
unsigned int special_update_version : 8;
unsigned int reserved1 : 8;
unsigned int reserved2;

Thanks,
-- 
Andrew :)

Senior Free Java Software Engineer
Red Hat, Inc. (http://www.redhat.com)

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