RFR: 8347981: RISC-V: Add Zfa zli imm loads

[Ludovic Henry]

On Fri, 17 Jan 2025 08:47:09 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!

src/hotspot/cpu/riscv/assembler_riscv.hpp line 335:

> 333: private:
> 334: 
> 335:   bool static zfa_zli_lookup_double(uint64_t value, int* Rs) {

Suggestion:

  static bool zfa_zli_lookup_double(uint64_t value, int* Rs) {

src/hotspot/cpu/riscv/assembler_riscv.hpp line 376:

> 374: 
> 375: 
> 376:   bool static zfa_zli_lookup_float(uint32_t value, int* Rs = nullptr) {

Suggestion:

  static bool zfa_zli_lookup_float(uint32_t value, int* Rs = nullptr) {

src/hotspot/cpu/riscv/assembler_riscv.hpp line 431:

> 429:     }
> 430:     uint64_t d_bits = (uint64_t)julong_cast(d);
> 431:     return zfa_zli_lookup_double(d_bits, Rs);

Would it better to inline the switch here directly? Is it used anywhere else?

src/hotspot/cpu/riscv/macroAssembler_riscv.cpp line 2634:

> 2632:   }
> 2633:   int Rs = -1;
> 2634:   can_zfa_zli_float(imm, &Rs);

should that be a `guarantee(can_zfa_zli_float(imm, &Rs));`?

src/hotspot/cpu/riscv/macroAssembler_riscv.cpp line 2645:

> 2643:   }
> 2644:   int Rs = -1;
> 2645:   can_zfa_zli_double(imm, &Rs);

should that be a `guarantee(can_zfa_zli_double(imm, &Rs));`?

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

PR Review Comment: https://git.openjdk.org/jdk/pull/23171#discussion_r1920394378
PR Review Comment: https://git.openjdk.org/jdk/pull/23171#discussion_r1920393982
PR Review Comment: https://git.openjdk.org/jdk/pull/23171#discussion_r1920393473
PR Review Comment: https://git.openjdk.org/jdk/pull/23171#discussion_r1920392131
PR Review Comment: https://git.openjdk.org/jdk/pull/23171#discussion_r1920388910