Integrated: 8314618: RISC-V: -XX:MaxVectorSize does not work as expected

[Gui Cao]

On Mon, 21 Aug 2023 02:01:35 GMT, Gui Cao <gcao at openjdk.org> wrote:

> 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.
> 
> 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 at 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 at plct-c8:~/jdk-rvv/build/linux-riscv64-server-fastdebug/jdk/bin...

This pull request has now been integrated.

Changeset: a66b5df1
Author:    Gui Cao <gcao at openjdk.org>
Committer: Fei Yang <fyang at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/a66b5df14a163d2990e6dd746906942367e4fdb2
Stats:     4 lines in 2 files changed: 1 ins; 1 del; 2 mod

8314618: RISC-V: -XX:MaxVectorSize does not work as expected

Reviewed-by: fyang, dzhang

-------------

PR: https://git.openjdk.org/jdk/pull/15356