RFR: 8324724: Add Stub routines for FP16 conversions on aarch64

[Bhavana Kilambi]

This commit - https://github.com/openjdk/jdk/commit/8cfd74f76afc9e5d50c52104fef9974784718dd4 adds stub routines for FP16 conversions for float/float16 constants on x86 to get an accurate compile time value of the nodes. This task adds similar stub routines for aarch64 as well.

With this patch, if the inputs to the conversion functions are constants, the stub routines are executed to determine the compile time values of the ConvHF2F/ConvF2HF nodes (in their respective Value() functions) and the ConvHF2F/ConvF2HF nodes are replaced with ConI/ConF nodes. This might help in further compiler optimizations like constant folding.

The following testcase was used to test the disassembly -

public class FloatConv {

    private static final short sconst;
    private static final float fconst;

    static {
        sconst = Short.MAX_VALUE;
        fconst = Float.MIN_VALUE;
    }

    @Benchmark
    public float hf2f() {
        return Float.float16ToFloat(sconst);
    }

    @Benchmark
    public short f2hf() {
        return Float.floatToFloat16(fconst);
    }
}

**Disassembly without patch :**

**FloatConv.f2hf() :-**

...
ldr   s17, 0x0000ffff918cec80
fcvt  h16, s17
smov  x0, v16.h[0]
...
ret


**FloatConv.hf2f() :-**

...
orr   w11, wzr, #0x7fff
mov   v16.h[0], w11
fcvt  s0, h16
...
ret


**Disassembly with patch :**

**FloatConv.hf2f() :-**

...
ldr   s0, 0x0000ffffb58ce880
...
ret


**FloatConv.f2hf() :-**

...
mov   w0, wzr
...
ret


With this patch, the conversion computation is done well in advance and the ConvHF2F/ConvF2HF nodes are replaced with the ConI/ConF nodes and thus the constant values are just loaded into registers and returned.

The tests in - "hotspot/jtreg/compiler/intrinsics/float16" pass on both aarch64 and x86.

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

Commit messages:
 - 8324724: Add Stub routines for FP16 conversions on aarch64

Changes: https://git.openjdk.org/jdk/pull/17706/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17706&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324724
  Stats: 33 lines in 1 file changed: 32 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17706.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17706/head:pull/17706

PR: https://git.openjdk.org/jdk/pull/17706