RFR: JDK-8287288: Fix some typos in C1

[David Holmes]

On Thu, 26 May 2022 02:15:08 GMT, Zhuojun Miao <zmiao at openjdk.org> wrote:

>> src/hotspot/share/c1/c1_LIR.hpp line 203:
>> 
>>> 201:   //     data      opr-type  opr-kind
>>> 202:   // +-----------+----------+-------+
>>> 203:   // [max........|6  5  4  3|2  1  0]
>> 
>> I don't think your change is right. The code below indicates there are 3 bits for kind. So I think the original should actually look like this:
>> 
>> 
>> //     data       opr-type opr-kind
>> // +--------------+-------+--------+--+
>> // [max...........|7 6 5 4| 3 2 1  | 0]
>
> According to the definitions of `Opr_kind` and `kind_mask` below, it can be seen that `opr-kind` uses the lowest 3 bits, and if the lowest bit is 0, it means that this is a pointer.

I was reading:

enum OprBits {
      pointer_bits   = 1
    , kind_bits      = 3
    , type_bits      = 4
    , size_bits      = 2
    , destroys_bits  = 1

as a bitfield description so 1 pointer bit followed by 3 kind bits, followed by 4 type bits etc. But I agree the later mask value only does a shift of 3 not 4.

Also the code indicates there are a lot more non-data bits before we get to the data so the diagram is incomplete in other ways.

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PR: https://git.openjdk.java.net/jdk/pull/8880