On Tue, 8 Jul 2025 01:55:55 GMT, Xiaohong Gong <xg...@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`. ------------- PR Comment: https://git.openjdk.org/jdk/pull/26057#issuecomment-3047838866
