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));
     }
```