RFR: 8256655: rework long counted loop handling [v4]

[Tobias Hartmann]

On Fri, 27 Nov 2020 08:15:16 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> Currently the transformation of a long counted loop into a loop nest
>> with an inner int counted loop is performed in 2 steps:
>> 
>> 1- recognize the counted loop shape and build the loop nest
>> 2- transform the inner loop into a counted loop
>> 
>> I propose changing this to a 3 steps process:
>> 
>> 1- recognize the counted loop shape (transform the loop into a LongCountedLoop)
>> 2- build the loop nest
>> 3- transform the inner loop into a counted loop
>> 
>> The benefits are:
>> 
>> - the logic is cleaner because step 1 and 2 are now separated
>> - Simple optimizations (loop iv type, empty loop elimination, parallel
>>   iv) can be implemented for LongCountedLoop by refactoring existing
>>   code
>> 
>> 1- above is achieved by refactoring the
>> PhaseIdealLoop::is_counted_loop() so it takes an extra parameter (the
>> kind of counted loop, int or long).
>> 
>> 2- is the existing loop nest construction logic. But now that it takes
>> a LongCountedLoop as input, the shape of the loop is known to be that
>> of a canonicalized counted loop. As a result, the loop nest
>> construction is simpler.
>> 
>> This change also refactors PhiNode::Value() so that it works for both
>> CountedLoop and LongCountedLoop.
>> 
>> I also changed ConvL2INode to be a TypeNode (ConvI2LNode is a type
>> node) and:
>> 
>>     jlong init_p = (jlong)init_t->_lo + stride_con;
>>     if (init_p > (jlong)max_jint || init_p > (jlong)limit_t->_hi)
>>       return false; // cyclic loop or this loop trips only once
>> 
>> to:
>> 
>>     if (init_t->lo_as_long() > max_signed_integer(iv_bt) - stride_con) {
>> 
>> because if the loop has a single iteration transforming it to a
>> CountedLoop should allow the backedge to be optimized out.
>
> Roland Westrelin 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 eight additional commits since the last revision:
> 
>  - test failure
>  - Merge branch 'master' into JDK-8256655
>  - assert fix
>  - Merge branch 'master' into JDK-8256655
>  - build fixes
>  - fix trailing whitespace
>  - fix comment
>  - long counted loop refactoring

Looks good to me. I'll re-run testing and report back once it finished.

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

Marked as reviewed by thartmann (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1316