RFR: 8305324: C2: Wrong execution of vectorizing Interger.reverseBytes

[Pengfei Li]

On Mon, 10 Apr 2023 13:21:29 GMT, Daohan Qu <duke at openjdk.org> wrote:

> This patch should fix [JDK-8305324](https://bugs.openjdk.org/browse/JDK-8305324).
> 
> `SuperWord::compute_vector_element_type()` implemented in `jdk/src/hotspot/share/opto/superword.cpp` propagates backward a narrower integer type when the upper bits of the value are not needed. However, `Integer.reverseBytes()` depends on higher-order bits of an integer and should be prevented from being narrowed and vectorized. Instead, it needs to be treated like `Math.abs()` (which is represented by `Op_AbsI` in the following code).
> 
> https://github.com/openjdk/jdk/blob/0243da2e4adc1b7ab6fcd5b10778532101158dce/src/hotspot/share/opto/superword.cpp#L3935-L3945
> 
> I have tested this patch for tier 1-3 on x86-64.

src/hotspot/share/opto/superword.cpp line 3941:

> 3939:             // RShiftI or AbsI operations, the compiler has to know the precise
> 3940:             // signedness info of the 1st operand. These operations shouldn't be
> 3941:             // vectorized if the signedness info is imprecise.

Could you update the comments I wrote before?

src/hotspot/share/opto/superword.cpp line 3944:

> 3942:             const Type* vt = vtn;
> 3943:             int op = in->Opcode();
> 3944:             if (VectorNode::is_shift_opcode(op) || op == Op_AbsI || op == Op_ReverseBytesI) {

(another suggestion) This list may be longer and longer as we vectorize more operations. Shall we move this check into a static function of `VectorNode`, like `VectorNode::requires_higher_order_bits(op)`, and put the comments inside?

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PR Review Comment: https://git.openjdk.org/jdk/pull/13406#discussion_r1162232224
PR Review Comment: https://git.openjdk.org/jdk/pull/13406#discussion_r1162236801