RFR: 8266530: HotSpot changes for JEP 306 [v3]

[Joe Darcy]

On 5/10/2021 5:20 PM, David Holmes wrote:
> On 11/05/2021 5:14 am, Harold Seigel wrote:
>> On Fri, 7 May 2021 04:58:22 GMT, Joe Darcy <darcy at openjdk.org> wrote:
>>
>>>> Please review these VM changes in support of JEP 306: Restore 
>>>> Always-Strict Floating-Point Semantics.
>>>>
>>>> JEP 306 is considering the following changes to the JVMS:
>>>>
>>>> 1) Require "strict" floating-point evaluation in all circumstances.
>>>> 2) For class file version 61 and higher, undefine ACC_STRICT, 
>>>> moving that bit position back to undefined for methods. For class 
>>>> file versions, 46 through, ACC_STRICT would remain a defined modifier.
>>>>
>>>> Since all floating-point evaluation is done strictly, a modifier 
>>>> bit is not needed to indicate this anymore. There are a few 
>>>> mandated modifier checks when ACC_STRICT is defined, such as not 
>>>> allowing an "abstract strictfp" method. These checks in 
>>>> classFileParser are now conditional on the class file version.
>>>>
>>>> The tests verify  "abstract strictfp" triggers a ClassFormatError  
>>>> for version 60 and does not trigger an error or exception for 
>>>> version 61. (As an undefined bit in version 61, the VM is supposed 
>>>> to ignore it.)
>>>>
>>>> FYI, the javac and core libs portions of JEP 306 are out for review 
>>>> under https://github.com/openjdk/jdk/pull/3831.
>>>>
>>>> The HotSpot and javac/core libs parts of JEP 306 are independent in 
>>>> that neither relies on the other:
>>>>   * javac will not output class files which expose the difference 
>>>> in class parsing behavior; the ACC_STRICT modifier will not be set 
>>>> in any version 61 class file emitted by javac.
>>>>   *  since HotSpot already does all floating-point evaluation 
>>>> strictly, no changes are needed to support strict evaluation.
>>>
>>> Joe Darcy has updated the pull request with a new target base due to 
>>> a merge or a rebase. The incremental webrev excludes the unrelated 
>>> changes brought in by the merge/rebase. The pull request contains 
>>> three additional commits since the last revision:
>>>
>>>   - Merge branch 'master' into 8266530
>>>   - Respond to review feedback.
>>>   - 8266530: HotSpot changes for JEP 306
>>
>> Hi Joe,
>> If the code that calls is_strict() (and other code that looks at 
>> ACC_STRICT) is no longer needed then it should be removed. Otherwise, 
>> that code should be changed to also check if ik->major_version() >= 61.
>
> Right - the normal process would be to update the code based on 
> is_strict() always returning true thus eliding it, but retaining the 
> code it causes to be called.
>
I've pushed another changeset which defines is_strict in method.hpp as 
true under IA32. If IA32 covers all platforms that could use the x87 FPU 
stack, this should be sufficient for correctness.

Thanks,

-Joe