RFR: 8297539: Consolidate the uses of the int<->float union conversion trick [v2]

[Kim Barrett]

On Mon, 6 Feb 2023 10:31:21 GMT, Afshin Zafari <duke at openjdk.org> wrote:

>> ### Discussion
>> 
>> There are different ways of casting/converting integral types to floating point types and vice versa in the code. 
>> These conversions are changed to invocation of a centralised `cast<To>(From)` method in `primitiveConversions.hpp`. To make this changes build and run, some new `cast` methods are introduced and header dependencies changed. 
>> 
>> ### Patch
>> 
>> - All instances of using `union` technique that are used for int<->float and long<->double type conversions are replaced by `PrimitiveConversions::cast<To>(From)` method.
>>   - Instances of, for example, converting `int`<->`intptr_t` are not changed.
>> - After this change, `JavaValue` also uses the `PrimitiveConversions::cast<To>(From)` templates. Therefore, all the `union` conversions for `int`<-> `float` and `long` <-> `double` in hotspot are now centralised in one location.
>> - Templates in `PrimitiveConversions` class are numbered from 1-9 and all lines of code that use `PrimitivEConversions::cast<To>(From)` have comments saying which templated `cast<To>(From)` matches. Issue number 8297539 is also in the comments to make the search/reviews more easier.
>> - The new 'cast<To>(From)' templates are numbered 6-9 for some instances where existing templates (1-5) did not match.
>> - `globalDefinitions.hpp` now includes `primitiveConversions.hpp`.
>> - `primitiveConversions.hpp` does NOT include `globalDefinitions.hpp` any more. Some typedef's are added to it to compensate the missing types.
>> - The followings are not changed:
>>   - `jni.hpp`
>>   - `ciConstant.hpp` is not changed, since uinon is not used for converting `int` <->`float` or `long` <-> `double`. There are some asserts that allow setter/getter used only for the same types.
>>   - `bytecodeInterpreter_zero.hpp` is not changed. Writing to and reading from the `VMJavaVal64` are always with the same type and no conversion is made.
>>   - union with bit fields are not changed.
>>   - `json.hpp` and `json.cpp` in src/hotspot/share/utilities.
>> 
>> ### Test
>> mach5 tiers 1-5 passed, tiers 6-8 in progress
>
> Afshin Zafari has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8297539: Consolidate the uses of the int<->float union conversion trick

I think there are some significant problems with the [v2] version (webrev 1).  Afshin and I are going to meet, for better
bandwidth vs discussing in PR.  Will summarize here later, but other folks might want to hold off looking at that version.

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PR: https://git.openjdk.org/jdk/pull/12384