RFR: 8282764: AArch64: compiler/vectorapi/reshape/TestVectorCastNeon.java failed with incorrect result

[Paul Sandoz]

On Thu, 17 Mar 2022 04:33:59 GMT, Ningsheng Jian <njian at openjdk.org> wrote:

> Vector API long to float conversion operation on NEON converts vector of 2 longs to 2 floats. In current implementation, we convert 2 longs to 2 doubles first and then converts 2 doubles to 2 floats. However, this two-steps conversion may have two roundings, while the expected behavior, conversion from long to float directly, has only one rounding. This will result in inconsistent result. E.g. for the failure test case:
> 
> 
> jshell> long l = 0x561a524000000001L;
> l ==> 6204361871487664129
> 
> jshell> float l2f = (float)l;
> l2f ==> 6.2043621E18
> 
> jshell> float l2d2f = (float)((double)l);
> l2d2f ==> 6.2043616E18
> 
> 
> Since we don't have NEON instruction to support vector long to float conversion, we fix the codegen by doing it in scalar operation one element by one element.
> 
> TEST: vector api jtreg tests passed.

Thanks for updating the copyright year. Java changes look good. Just one comment related to an additional value added for testing.

test/hotspot/jtreg/compiler/vectorapi/VectorCastShape128Test.java line 117:

> 115:         Long.MAX_VALUE,
> 116:         Long.MIN_VALUE,
> 117:         0x561a524000000001L,

A comment would be useful as to why this constant is important

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PR: https://git.openjdk.java.net/jdk/pull/7850