RFR: 8359419: AArch64: Relax min vector length to 32-bit for short vectors [v3]

[Bhavana Kilambi]

On Tue, 8 Jul 2025 01:55:55 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>>> > https://github.com/openjdk/jdk/blob/f2d2eef988c57cc9f6194a8fd5b2b422035ee68f/test/hotspot/jtreg/compiler/vectorization/runner/ArrayTypeConvertTest.java#L388-L392
>>> 
>>> Actually I didn't change the min vector size for `char` vectors in this patch. Relaxing `short` vectors to 32-bit is to support the vector cast for Vector API, and there is no `char` species in it. Do you think it's better to do the same change for `char` as well? This will just benefit auto-vectorization.
>> 
>> Hi @XiaohongGong thanks for asking. In many auto-vectorization cases involving `char`, the vector elements are represented using `T_SHORT` as the `BasicType`, rather than `T_CHAR`.
>> 
>> This is because, in Java, operands of subword types are always promoted to `int` before any arithmetic operation. As a result, when handling a node like `ConvD2I`, we don’t initially know its actual subword type. Later, the SuperWord phase propagates a narrowed integer type backward to help determine the correct subword type. See:
>> https://github.com/openjdk/jdk/blob/f2d2eef988c57cc9f6194a8fd5b2b422035ee68f/src/hotspot/share/opto/superword.cpp#L2551-L2558
>> 
>> Since SuperWord assigns `T_SHORT` to `StoreC` early on https://github.com/openjdk/jdk/blob/f2d2eef988c57cc9f6194a8fd5b2b422035ee68f/src/hotspot/share/opto/superword.cpp#L2646-L2650
>> the entire propagation chain tends to use `T_SHORT` as well. 
>> 
>> This applies to most operations, with the exception of a few like `RShiftI`, `Abs`, and `ReverseBytesI`, which are handled separately.
>> 
>> So your change already benefits many char-related vectorization cases like `convertDoubleToChar` above. That’s why we can safely relax the IR condition mentioned earlier.
>
>> > > https://github.com/openjdk/jdk/blob/f2d2eef988c57cc9f6194a8fd5b2b422035ee68f/test/hotspot/jtreg/compiler/vectorization/runner/ArrayTypeConvertTest.java#L388-L392
>> > 
>> > 
>> > Actually I didn't change the min vector size for `char` vectors in this patch. Relaxing `short` vectors to 32-bit is to support the vector cast for Vector API, and there is no `char` species in it. Do you think it's better to do the same change for `char` as well? This will just benefit auto-vectorization.
>> 
>> Hi @XiaohongGong thanks for asking. In many auto-vectorization cases involving `char`, the vector elements are represented using `T_SHORT` as the `BasicType`, rather than `T_CHAR`.
>> 
>> This is because, in Java, operands of subword types are always promoted to `int` before any arithmetic operation. As a result, when handling a node like `ConvD2I`, we don’t initially know its actual subword type. Later, the SuperWord phase propagates a narrowed integer type backward to help determine the correct subword type. See:
>> 
>> https://github.com/openjdk/jdk/blob/f2d2eef988c57cc9f6194a8fd5b2b422035ee68f/src/hotspot/share/opto/superword.cpp#L2551-L2558
>> 
>> Since SuperWord assigns `T_SHORT` to `StoreC` early on
>> 
>> https://github.com/openjdk/jdk/blob/f2d2eef988c57cc9f6194a8fd5b2b422035ee68f/src/hotspot/share/opto/superword.cpp#L2646-L2650
>> 
>> 
>> the entire propagation chain tends to use `T_SHORT` as well.
>> This applies to most operations, with the exception of a few like `RShiftI`, `Abs`, and `ReverseBytesI`, which are handled separately.
>> 
>> So your change already benefits many char-related vectorization cases like `convertDoubleToChar` above. That’s why we can safely relax the IR condition mentioned earlier.
> 
> Thanks for your input! It's really helpful to me. Does this mean it always use `T_SHORT` for char vectors in SLP? If so, it's safe that we do not need to consider `T_CHAR` in vector IRs in backend?

> > Hi @XiaohongGong, is there any way we can implement 2HF -> 2S and 2S -> 2HF in these match rules ?
> > https://github.com/openjdk/jdk/blob/f2d2eef988c57cc9f6194a8fd5b2b422035ee68f/src/hotspot/cpu/aarch64/aarch64_vector.ad#L4697
> > 
> > https://github.com/openjdk/jdk/blob/f2d2eef988c57cc9f6194a8fd5b2b422035ee68f/src/hotspot/cpu/aarch64/aarch64_vector.ad#L4679
> > 
> > The `fcvtn` and `fcvtl` instructions do not support these arrangements. I was wondering if there is any other way we can implement these by any chance?
> 
> Do you mean `2HF -> 2F` and `2F -> 2HF` ?
> 
> Yes, it does not support the 32-bit arrangements. Vector conversion is a kind of lanewise vector operation. For such cases, we usually use the same arrangements with 64-bit vector size for 32-bit ones. That means we can reuse the `T4H` and `T4S` to implement it. Hence, current match rules can cover the conversions between `2HF` and `2F`.
> 
> Consider there is no such conversion cases in Vector API, I didn't change the comment in the match rules. I think this may benefit auto-vectorization. Currently, do we have cases that can match these rules with SLP?

Sorry yes I meant 2HF <-> 2F.
Yes, currently there are no such cases in VectorAPI as we do not support Float16 Vectors yet but this will benefit autovectorization cases. I think in this case this may also benefit 2D <-> 2HF as well (eventually we might add support for D <-> HF as well). 
Yes we have some JTREG tests that match these rules currently like - `test/hotspot/jtreg/compiler/vectorization/TestFloat16VectorConvChain.java`, `test/hotspot/jtreg/compiler/vectorization/TestFloatConversionsVector.java`.

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PR Comment: https://git.openjdk.org/jdk/pull/26057#issuecomment-3047838866