On Tue, 8 Jul 2025 09:00:53 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. >>> >>> 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? >> >> No, we don't always use `T_SHORT` for char vectors. As mentioned earlier, >> for operations like `RShiftI`, `Abs`, and `ReverseBytesI`, the compiler >> needs to preserve the higher-order bits of the first operand. Therefore, >> SuperWord still needs to assign them precise subword types. See: >> https://github.com/openjdk/jdk/blob/f2d2eef988c57cc9f6194a8fd5b2b422035ee68f/src/hotspot/share/opto/superword.cpp#L2583-L2589 > >> > > > > 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? >> >> No, we don't always use `T_SHORT` for char vectors. As mentioned earlier, >> for operations like `RShiftI`, `Abs`, and `ReverseBytesI`, the compiler >> needs to preserve the higher-order bits of the first operand. Therefore, >> SuperWord still needs to assign them precise subword types. See: >> >> https://github.com/openjdk/jdk/blob/f2d2eef988c57cc9f6194a8fd5b2b422035ee68f/src/hotspot/share/opto/superword.cpp#L2583-L2589 > > Yes, I see. Thanks! What I mean is for cases that SLP will use the subword > types, it will be actu... > > > 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`. Thanks! So per my understanding, things that I just need is updating comment (e.g. `// 4HF to 4F`) of rules like `vcvtHFtoF`, right? For conversions between double and HF, we do not need any new rules as it will be actually `double -> float -> HF`, right? ------------- PR Comment: https://git.openjdk.org/jdk/pull/26057#issuecomment-3048019109
