RFR: 8294588: Auto vectorize half precision floating point conversion APIs

[Sandhya Viswanathan]

On Fri, 2 Dec 2022 04:22:39 GMT, Smita Kamath <svkamath at openjdk.org> wrote:

> Hi All, 
> 
> I have added changes for autovectorizing Float.float16ToFloat and Float.floatToFloat16 API's.
> Following are the performance numbers of JMH micro Fp16ConversionBenchmark:
> Before code changes:
> Benchmark | (size) | Mode | Cnt | Score | Error | Units
> Fp16ConversionBenchmark.float16ToFloat | 2048 | thrpt | 3 | 1044.653 | ±     0.041 | ops/ms
> Fp16ConversionBenchmark.float16ToFloatMemory | 2048 | thrpt | 3 | 2341529.9 | ± 11765.453 | ops/ms
> Fp16ConversionBenchmark.floatToFloat16 | 2048 | thrpt | 3 | 2156.662 | ±     0.653 | ops/ms
> Fp16ConversionBenchmark.floatToFloat16Memory | 2048 | thrpt | 3 | 2007988.1 | ±   361.696 | ops/ms
> 
> After:
> Benchmark | (size) | Mode |  Cnt | Score | Error |   Units
> Fp16ConversionBenchmark.float16ToFloat  | 2048 | thrpt | 3 |  20460.349 |±  372.327 |  ops/ms
> Fp16ConversionBenchmark.float16ToFloatMemory | 2048 |  thrpt | 3 | 2342125.200 |± 9250.899  |ops/ms
> Fp16ConversionBenchmark.floatToFloat16  |  2048 | thrpt  |  3 |   22553.977 |±  483.034 | ops/ms
> Fp16ConversionBenchmark.floatToFloat16Memory | 2048 | thrpt |  3 |  2007899.797 |±  150.296 | ops/ms
> 
> Kindly review and share your feedback.
> 
> Thanks.
> Smita

src/hotspot/cpu/x86/x86.ad line 1688:

> 1686:     case Op_HF2FV:
> 1687:     case Op_F2HFV:
> 1688:       if (!VM_Version::supports_f16c() && !VM_Version::supports_avx512vl()) {

We need different check for vector flavors (HF2FV/F2HV) vs the scalar flavors (ConvF2HF/ConvHF2F).
The check needed for vector flavors is:
if (!VM_Version::supports_f16c() && !VM_Version::supports_avx512()) {  return false; }

Also in vm_version_x86.cpp, the F16C features should be disabled when UseAVX is set to 0, i.e. the following 
 if (UseAVX < 1) {
    _features &= ~CPU_AVX;
    _features &= ~CPU_VZEROUPPER;
  }
should be updated to:
 if (UseAVX < 1) {
    _features &= ~CPU_AVX;
    _features &= ~CPU_VZEROUPPER;
    _features &= ~CPU_F16C;
  }

src/hotspot/cpu/x86/x86.ad line 2002:

> 2000:         return false;
> 2001:       }
> 2002:       break;

This can be removed as match_rule_supported() has previously happened.

src/hotspot/cpu/x86/x86.ad line 3710:

> 3708:     int src_size = Matcher::vector_length_in_bytes(this, $src);
> 3709:     int dst_size = src_size * 2;
> 3710:     int vlen_enc = vector_length_encoding(dst_size);

This could now be changed to:
int vlen_enc = Matcher::vector_length_encoding(this);

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

PR: https://git.openjdk.org/jdk/pull/11471