RFR: 8342662: C2: Add new phase for backend-specific lowering

[Quan Anh Mai]

On Wed, 23 Oct 2024 17:28:25 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> src/hotspot/share/opto/compile.cpp line 2466:
>> 
>>> 2464:     print_method(PHASE_BEFORE_LOWERING, 3);
>>> 2465: 
>>> 2466:     PhaseLowering lower(&igvn);
>> 
>> Any specific reason to have lowering after loop optimizations ?
>> Lowered nodes may change the loop body size thereby impacting unrolling decisions.
>
> Because lowering is a transformation that increases the complexity of the graph.
> 
> - A `d = ExtractD(z,  4)` expanded into `x = VectorExtract(z, 2); d = ExtractD(x, 0)` increases the number of nodes by 1.
> - A logic cone transformed into a `MacroLogicV` introduces another kind of node that may not be recognized by other nodes.
> 
> As a result, we should do this as the last step when other transformation has finished their jobs. For the concerns regarding loop body size, we still have a function in `Matcher` for that purpose.

Another reason is that lowering being done late allows us to have more freedom to break some invariants of the nodes, such as looking through `VectorReinterpret`. An example is this (really crafted) case:

    Int256Vector v;
    int a = v.lane(5);
    float b = v.reinterpretAsFloats().lane(7);

This would be transformed into:

    vector<i,8> v;
    vector<i,4> v0 = VectorExtract(v, 1);
    int a = ExtractI(v0, 1);
    vector<f,8> v1 = VectorReinterpret(v, <f,8>);
    vector<f,4> v2 = VectorExtract(v1, 1);
    float b = ExtractF(v2, 3);

By allowing lowering to look through `VectorReinterpret` and break the invariant of `Extract` nodes that the element types of their inputs and outputs must be the same, we can `gvn` `v1` and `v`, `v2` and `v0`. Simplify the graph:

    vector<i,8> v;
    vector<i,4> v0 = VectorExtract(v, 1);
    int a = ExtractI(v0, 1);
    float b = ExtractF(v0, 3);

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PR Review Comment: https://git.openjdk.org/jdk/pull/21599#discussion_r1813252989