RFR: 8372641: [s390x] Test failure TestMergeStores.java

[Manuel Hässig]

On Thu, 27 Nov 2025 07:09:01 GMT, Harshit470250 <duke at openjdk.org> wrote:

> [JDK-8347405](https://bugs.openjdk.org/browse/JDK-8347405) introduced a mergeStores optimisation which requires ReverseBytesS opcode and as it was not implemented for s390 the test case is failing.
> I also implemented ReverseBytesUS.

Thank you for fixing this, @Harshit470250. The new instructions look good to me. I am only unsure about the cost (see comments below).

src/hotspot/cpu/s390/s390.ad line 11619:

> 11617:   match(Set dst (ReverseBytesS src));
> 11618:   predicate(UseByteReverseInstruction);
> 11619:   ins_cost(DEFAULT_COST);

Would this not be twice the `DEFAULT_COST` since it uses two instructions?

Suggestion:

  ins_cost(2 * DEFAULT_COST);

src/hotspot/cpu/s390/s390.ad line 11635:

> 11633:   match(Set dst (ReverseBytesUS src));
> 11634:   predicate(UseByteReverseInstruction);
> 11635:   ins_cost(DEFAULT_COST);

Suggestion:

  ins_cost(2 * DEFAULT_COST);

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

Changes requested by mhaessig (Committer).

PR Review: https://git.openjdk.org/jdk/pull/28523#pullrequestreview-3513961005
PR Review Comment: https://git.openjdk.org/jdk/pull/28523#discussion_r2567535924
PR Review Comment: https://git.openjdk.org/jdk/pull/28523#discussion_r2567536919