RFR: 8332856: C2: Add new transform for bool eq/ne (cmp (and (urshift X const1) const2) 0) [v2]

[Jasmine Karthikeyan]

On Tue, 28 May 2024 20:11:35 GMT, Tobias Hotz <duke at openjdk.org> wrote:

>> This PR adds a new ideal optimization for the following pattern:
>> 
>> public boolean testFunc(int a) {
>>     int mask = 0b101;
>>     int shift = 12;
>>     return ((a >> shift) & mask) == 0;
>> }
>> 
>> Where the mask and shift are constant values and a is a variable. For this optimization to work, the right shift has to be idealized to a unsinged right shift earlier in the pipeline, which here: https://github.com/openjdk/jdk/blob/b92bd671835c37cff58e2cdcecd0fe4277557d7f/src/hotspot/share/opto/mulnode.cpp#L731
>> If the shift is already an unsiged bit shift, it works as well.
>> On AMD64 CPUs, this means that this whole line computation can be reduced to a simple `test` instruction.
>
> Tobias Hotz has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - LF endings...
>  - Add a benchmark to measure effect of new ideal transformation

I have mostly stylistic comments here, I think the patch itself looks good. Also, the copyright on `subnode.cpp` should be updated to 2024.

src/hotspot/share/opto/subnode.cpp line 1627:

> 1625: 
> 1626:   // Change "bool eq/ne (cmp (and (urshift X 4) 1) 0)" into "bool ne/eq (cmp (and X 8) 0)".
> 1627:   // Note: rshift gets converted to urshift in and Ideal

Suggestion:

  // Note: rshift gets converted to urshift in AndNode Ideal

src/hotspot/share/opto/subnode.cpp line 1633:

> 1631:     const TypeInt* shift_val_type = phase->type(cmp1->in(1)->in(2))->isa_int();
> 1632:     const TypeInt* mask_type = phase->type(cmp1->in(2))->isa_int();
> 1633:     if (shift_val_type && shift_val_type->is_con() && mask_type && mask_type->is_con()) {

Suggestion:

    if (shift_val_type != nullptr && shift_val_type->is_con() && mask_type != nullptr && mask_type->is_con()) {

test/micro/org/openjdk/bench/vm/compiler/ShiftAndCmpZeroIdeal.java line 2:

> 1: /*
> 2:  * Copyright (c) 2020, Oracle and/or its affiliates. All rights reserved.

Suggestion:

 * Copyright (c) 2024, Oracle and/or its affiliates. All rights reserved.

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PR Review: https://git.openjdk.org/jdk/pull/19310#pullrequestreview-2149762686
PR Review Comment: https://git.openjdk.org/jdk/pull/19310#discussion_r1659925210
PR Review Comment: https://git.openjdk.org/jdk/pull/19310#discussion_r1659925311
PR Review Comment: https://git.openjdk.org/jdk/pull/19310#discussion_r1659925357