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

[Harold Seigel]

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

So, for class file version 61, this function in method.hpp will return false because the flag is not set?

    bool is_strict() const { return access_flags().is_strict(); }

If so, then won't that prevent strictfp semantics in share/c1/c1_GraphBuilder.cpp (and other places) that call is_strict(), such as:

  if (method()->is_strict()) {
    res = round_fp(res);
  }

-------------

PR: https://git.openjdk.java.net/jdk/pull/3891