RFR: 8305524: AArch64: Fix arraycopy issue on SVE caused by matching rule vmask_gen_sub

[Andrew Haley]

On Fri, 7 Apr 2023 07:21:05 GMT, Pengfei Li <pli at openjdk.org> wrote:

> From recent tests, we find that `System.arraycopy()` call with a negated variable as its length argument does not perform the copy. This issue is reproducible by below test case on AArch64 platforms with SVE.
> 
> 
> public class Test {
>   static char[] src = {'A', 'A', 'A', 'A', 'A'};
>   static char[] dst = {'B', 'B', 'B', 'B', 'B'};
> 
>   static void copy(int nlen) {
>     System.arraycopy(src, 0, dst, 0, -nlen);
>   }
> 
>   public static void main(String[] args) {
>     for (int i = 0; i < 25000; i++) {
>       copy(0);
>     }
>     copy(-5);
>     for (char c : dst) {
>       if (c != 'A') {
>         throw new RuntimeException("Wrong value!");
>       }
>     }
>     System.out.println("PASS");
>   }
> }
> 
> /*
> $ java -Xint Test
> PASS
> $ java -Xbatch Test
> Exception in thread "main" java.lang.RuntimeException: Wrong value!
>         at Test.main(Test.java:16)
> */
> 
> 
> Cause of this is a new AArch64 matching rule `vmask_gen_sub` introduced by JDK-8293198. It matches `VectorMaskGen (SubL src1 src2)` on AArch64 platforms with SVE and generates SVE `whilelo` instructions. Current C2 compiler uses a technique called "partial inlining" to vectorize small array copy operations by generating vector masks. In above test case, a negated variable `-nlen` is used as the length argument of the call and `-nlen` has a small positive value, so it is a "partial inlining" case. C2 will transform the ideal graph to `VectorMaskGen (SubL 0 nlen)` and eventually output an instruction of `whilelo p0, nlen, zr` which always generates an all-false vector mask. That's why arraycopy does nothing.
> 
> The problem of that matching rule is that it regards inputs `src1` and `src2` as unsigned long integers but they can be signed in use cases of arraycopy. To fix the issue, this patch replaces `whilelo` instruction by `whilelt` in that rule as well as some other places.
> 
> We tested tier1~3 on SVE and found no new failure. A jtreg math library test jdk/internal/math/FloatingDecimal/TestFloatingDecimal.java which fails on SVE before can pass now.

Marked as reviewed by aph (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/13382#pullrequestreview-1378748623