RFR: 8351994: Enable Extended EVEX to REX2/REX demotion when src and dst are the same [v34]

[Jatin Bhateja]

On Wed, 21 May 2025 23:35:39 GMT, Srinivas Vamsi Parasa <sparasa at openjdk.org> wrote:

>> Intel APX NDD instructions are encoded using EVEX encoding. The goal of this PR is to enable optimized instruction encoding for Intel APX NDD instructions when the non-destructive destination is same as the first source.
>> 
>> For example:
>> 
>> `eaddl r18, r18, r25` can be encoded as `addl r18, r25` using APX REX2 encoding
>> `eaddl r2, r2, r7` can be encoded as `addl r2, r7` using non-APX legacy encoding
>
> Srinivas Vamsi Parasa has updated the pull request incrementally with five additional commits since the last revision:
> 
>  - refactor to use is_P6_or_later()
>  - rename byte1 to opcode_byte
>  - rename evex_opcode_prefix_and_encode as emit_eevex_or_demote
>  - rename evex to eevex in method names
>  - reset swap=false as default

src/hotspot/cpu/x86/assembler_x86.cpp line 12862:

> 12860:       emit_prefix_and_int8(get_prefixq(src2, dst, is_map1), opcode_byte);
> 12861:     }
> 12862:     else {

Suggestion:

    } else {

src/hotspot/cpu/x86/assembler_x86.cpp line 12968:

> 12966:       encode = is_prefixq ? prefixq_and_encode(dst_enc, src_enc, is_map1) : prefix_and_encode(dst_enc, src_enc, is_map1);
> 12967:     }
> 12968:     else {

Suggestion:

    } else {

src/hotspot/cpu/x86/assembler_x86.cpp line 12978:

> 12976:       encode = vex_prefix_and_encode(nds_enc, dst_enc, src_enc, pre, opc, &attributes, /* src_is_gpr */ true, /* nds_is_ndd */ true, no_flags);
> 12977:     }
> 12978:     else {

Suggestion:

    } else {

src/hotspot/cpu/x86/vm_version_x86.hpp line 680:

> 678:   static int  cpu_family()        { return _cpu;}
> 679:   static bool is_P6()             { return cpu_family() >= 6; }
> 680:   static bool is_P6_or_later()    { return cpu_family() == 6 || cpu_family() == 18 || cpu_family() == 19; }

Do we need cpu_family() == 18  check ?
19 is for Diamond Rapids and 6 for all Xerons before it, including E-core variants.

src/hotspot/cpu/x86/vm_version_x86.hpp line 680:

> 678:   static int  cpu_family()        { return _cpu;}
> 679:   static bool is_P6()             { return cpu_family() >= 6; }
> 680:   static bool is_P6_or_later()    { return cpu_family() == 6 || cpu_family() == 18 || cpu_family() == 19; }

``` suggestion
  static bool is_intel_server_family()    { return cpu_family() == 6 || cpu_family() == 18 || cpu_family() == 19; }

We already have is_P6(), which returns true for CPU family >=6, a minor name change suggestion.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/24431#discussion_r2103266719
PR Review Comment: https://git.openjdk.org/jdk/pull/24431#discussion_r2103267701
PR Review Comment: https://git.openjdk.org/jdk/pull/24431#discussion_r2103268316
PR Review Comment: https://git.openjdk.org/jdk/pull/24431#discussion_r2103252756
PR Review Comment: https://git.openjdk.org/jdk/pull/24431#discussion_r2103260881