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  1. JDK
  2. JDK-8314618

RISC-V: -XX:MaxVectorSize does not work as expected

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    • b12
    • riscv
    • linux

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        Hi all, we found that when the specified -XX:MaxVectorSize=16 no bigger than the detected _initial_vector_length=32, it causes the MaxVectorSize to be set incorrectly.

        MaxVectorSize is updated in src/hotspot/cpu/riscv/vm_version_riscv.cpp#VM_Version::c2_initialize().

        ```
          if (UseRVV) {
            if (FLAG_IS_DEFAULT(MaxVectorSize)) {
              MaxVectorSize = _initial_vector_length;
            } else if (MaxVectorSize < 16) {
              warning("RVV does not support vector length less than 16 bytes. Disabling RVV.");
              UseRVV = false;
            } else if (is_power_of_2(MaxVectorSize)) {
              if (MaxVectorSize > _initial_vector_length) {
                warning("Current system only supports max RVV vector length %d. Set MaxVectorSize to %d",
                        _initial_vector_length, _initial_vector_length);
              }
              MaxVectorSize = _initial_vector_length;
            } else {
              vm_exit_during_initialization(err_msg("Unsupported MaxVectorSize: %d", (int)MaxVectorSize));
            }
          }
        ```

        It's that RISC-V only supports max-width vectorization at first, so it's unconditionally set to hardware max-width here. However, after https://github.com/openjdk/jdk/commit/43c71ddf923d442499449948f4bf8a7c79249af0, vectors with small widths are supported, so here it needs to be adjusted accordingly. The correct should be If MaxVectorSize is less than _initial_vector_length, then MaxVectorSize should be used as the final value.

        This issue affects C2 autovectorization and some specific Vector API interfaces such as VectorSupport.getMaxLaneCount on RISC-V.

        We can verify the problem using the following test case:

        ```
        import jdk.internal.vm.vector.VectorSupport;

        public class GetMaxVectorSizeTest {
            public static void main(String[] args) {
                final int maxLaneCount = VectorSupport.getMaxLaneCount(byte.class);
                System.out.println("maxLaneCount:" + maxLaneCount);
            }
        }
        ```

        The compile command is as follows:

        ```
        javac --add-exports java.base/jdk.internal.vm.vector=ALL-UNNAMED GetMaxVectorSizeTest.java
        ```

        RISC-V without the -XX:MaxVectorSize=16 has the following execution results(risc-v rvv vector length is set 256 bit):
        ```
        zifeihan@plct-c8:~/jdk-rvv/build/linux-riscv64-server-fastdebug/jdk/bin$ ./java -XX:+UnlockExperimentalVMOptions -XX:+UseRVV --add-exports java.base/jdk.internal.vm.vector=ALL-UNNAMED GetMaxVectorSizeTest
        maxLaneCount:32
        ```

        RISC-V using the -XX:MaxVectorSize=16 results in the following(risc-v rvv vector length is set 256 bit):

        ```
        zifeihan@plct-c8:~/jdk-rvv/build/linux-riscv64-server-fastdebug/jdk/bin$ ./java -XX:MaxVectorSize=16 -XX:+UnlockExperimentalVMOptions -XX:+UseRVV --add-exports java.base/jdk.internal.vm.vector=ALL-UNNAMED GetMaxVectorSizeTest
        maxLaneCount:32
        ```


        AArch64 without the -XX:MaxVectorSize=16 has the following execution results(aarch64 sve vector length is set 256 bit):
        ```
        zifeihan@d915263bc793:~/jdk/build/linux-aarch64-server-fastdebug/jdk/bin$ ./java --add-exports java.base/jdk.internal.vm.vector=ALL-UNNAMED GetMaxVectorSizeTest
        maxLaneCount:32
        ```

        AArch64 using the -XX:MaxVectorSize=16 results in the following(aarch64 sve vector length is set 256 bit):
        ```
        zifeihan@d915263bc793:~/jdk/build/linux-aarch64-server-fastdebug/jdk/bin$ ./java -XX:MaxVectorSize=16 --add-exports java.base/jdk.internal.vm.vector=ALL-UNNAMED GetMaxVectorSizeTest
        maxLaneCount:16
        ```

        X86 without the -XX:MaxVectorSize=16 has the following execution results(x86 avx512, vector length is set 512 bit):

        ```
        zifeihan@plct-c8:~/jdk/build/linux-riscv64-server-fastdebug/jdk/bin$ java --add-exports java.base/jdk.internal.vm.vector=ALL-UNNAMED GetMaxVectorSizeTest
        maxLaneCount:64
        ```

        X86 using the -XX:MaxVectorSize=16 results in the following(x86 avx512, vector length is set 512 bit):

        ```
        zifeihan@plct-c8:~/jdk/build/linux-riscv64-server-fastdebug/jdk/bin$ java -XX:MaxVectorSize=16 --add-exports java.base/jdk.internal.vm.vector=ALL-UNNAMED GetMaxVectorSizeTest
        maxLaneCount:16
        ```



        The fix code is as follows:

        ```
        diff --git a/src/hotspot/cpu/riscv/vm_version_riscv.cpp b/src/hotspot/cpu/riscv/vm_version_riscv.cpp
        index 83f6f38d253..cf64e08ebc8 100644
        --- a/src/hotspot/cpu/riscv/vm_version_riscv.cpp
        +++ b/src/hotspot/cpu/riscv/vm_version_riscv.cpp
        @@ -267,8 +267,8 @@ void VM_Version::c2_initialize() {
               if (MaxVectorSize > _initial_vector_length) {
                 warning("Current system only supports max RVV vector length %d. Set MaxVectorSize to %d",
                         _initial_vector_length, _initial_vector_length);
        + MaxVectorSize = _initial_vector_length;
               }
        - MaxVectorSize = _initial_vector_length;
             } else {
               vm_exit_during_initialization(err_msg("Unsupported MaxVectorSize: %d", (int)MaxVectorSize));
             }
        ```

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                gcao Gui Cao
                gcao Gui Cao
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