RFR: 8347981: RISC-V: Add Zfa zli imm loads [v3]

[Fei Yang]

On Wed, 22 Jan 2025 14:29:13 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> Hi, please consider!
>> 
>> This patch the Zfa zli floating point immediate load.
>> As a bonus it adds fmv_?_x(Rd, zr); for loading fp/dp 0x0.
>> There are some more instruction in Zfa, but this was such a clear use-case so I only did fli as a start.
>> 
>> When using one of the 32 'popular' floats we can now materialze them without a load.
>> E.g.
>> `float f = f1 * 0.5 + f2 * 2.0;`
>> Only require 2 loads instead of 4: as '0.5' and '2.0' is such popular float values.
>> 
>> As Java is often memory bound we should also investigate doing lui+ssli+fmv for float/doubles instead of a load when materializing.
>> 
>> Note the _fli_s/_fli_d will be proper merged on the 8347794: RISC-V: Add Zfhmin - Float cleanup.
>> 
>> Passes:
>> ./test/jdk/java/lang/Math
>> ./test/hotspot/jtreg/compiler/floatingpoint/
>> ./test/jdk/java/util/Formatter/
>> ./test/jdk/java/lang/Float/
>> ./test/jdk/java/lang/Double/
>> ./test/hotspot/jtreg/compiler/c2/FloatingPointFoldingTest.java
>> ./test/hotspot/jtreg/compiler/eliminateAutobox/
>> ./test/hotspot/jtreg/vmTestbase/jit/
>> 
>> Running tier1
>> 
>> Thanks!
>
> Robbin Ehn has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains eight commits:
> 
>  - Merge branch 'master' into zfa
>  - Merge branch 'master' into zfa
>  - EXPERIMENTAL
>  - Merge branch 'master' into zfa
>  - Merge branch 'master' into zfa
>  - Flip bool static decl
>  - Merge branch 'master' into zfa
>  - Baseline

Thanks for the update. Seems fine to me modulo one minor comment.

src/hotspot/cpu/riscv/vm_version_riscv.hpp line 160:

> 158:   decl(ext_Zbs         , "Zbs"         , RV_NO_FLAG_BIT, true , UPDATE_DEFAULT(UseZbs))         \
> 159:   decl(ext_Zcb         , "Zcb"         , RV_NO_FLAG_BIT, true , UPDATE_DEFAULT(UseZcb))         \
> 160:   decl(ext_Zfa         , "Zfa"         , RV_NO_FLAG_BIT, true , UPDATE_DEFAULT(UseZfa))         \

Are we auto-enabling an experimental feature?

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

PR Review: https://git.openjdk.org/jdk/pull/23171#pullrequestreview-2568931169
PR Review Comment: https://git.openjdk.org/jdk/pull/23171#discussion_r1926407162