RFR: 8286972: Support the new loop induction variable related PopulateIndex IR node on x86

[Vladimir Kozlov]

On Wed, 18 May 2022 17:25:38 GMT, Sandhya Viswanathan <sviswanathan at openjdk.org> wrote:

> This PR adds x86 backend support for the new loop induction variable related PopulateIndex IR node. 
> This IR node was added as part of [JDK-8280510](https://bugs.openjdk.java.net/browse/JDK-8280510).
> 
> The performance numbers are as follows:
> Before:
> Benchmark                   (count)   Mode  Cnt       Score       Error  Units
> IndexVector.exprWithIndex1    65536  thrpt    3   64556.552 ±  1126.396  ops/s
> IndexVector.exprWithIndex2    65536  thrpt    3   22117.050 ± 11452.098  ops/s
> IndexVector.indexArrayFill    65536  thrpt    3  117776.383 ±  1120.957  ops/s
> 
> After:
> Benchmark                   (count)   Mode  Cnt       Score       Error  Units
> IndexVector.exprWithIndex1    65536  thrpt    3  203180.290 ±  2147.807  ops/s
> IndexVector.exprWithIndex2    65536  thrpt    3  274132.756 ±  6853.393  ops/s
> IndexVector.indexArrayFill    65536  thrpt    3  374165.202 ± 46930.779  ops/s
> 
> Please review.
> 
> Best Regards,
> Sandhya

src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp line 2282:

> 2280:   bool is_bw_supported = VM_Version::supports_avx512bw();
> 2281:   if (is_bw && !is_bw_supported) {
> 2282:     assert(vlen_enc != Assembler::AVX_512bit, "required");

What are acceptable values of `vlen_enc`?

src/hotspot/cpu/x86/c2_MacroAssembler_x86.cpp line 2314:

> 2312:   } else {
> 2313:     assert(vlen_enc != Assembler::AVX_512bit, "required");
> 2314:     assert((dst->encoding() < 16),"XMM register should be 0-15");

The `} else {` case will be also executed on on KNL CPU. Did you tested with `-XX:+UseKNLSetting`?

src/hotspot/cpu/x86/x86.ad line 1475:

> 1473:         return false;
> 1474:       }
> 1475:       // fallthrough

Please, don't do `fallthrough` - `RoundVF` is not related to `PopulateIndex`. Why not `if (!is_LP64 || UseAVX < 2)`?
Are there limitations in 32 bits or you don't want spend time on not major platform (which is also understandable)?

src/hotspot/cpu/x86/x86.ad line 1821:

> 1819:     case Op_PopulateIndex:
> 1820:       if (size_in_bits > 256 && !VM_Version::supports_avx512bw())
> 1821:         return false;

Use `{}` according to our style.

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PR: https://git.openjdk.java.net/jdk/pull/8778