RFR: JDK-8314890: Reduce number of loads for Klass decoding in static code [v2]

[Andrew Haley]

On Thu, 24 Aug 2023 06:22:57 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> Small change that reduces the number of loads generated by the C++ compiler for a narrow Klass decoding operation (`CompressedKlassPointers::decode_xxx()`.
>> 
>> Stock: three loads (with two probably sharing a cache line) - UseCompressedClassPointers, encoding base and shift.
>> 
>> 
>>   8b7b62:   48 8d 05 7f 1b c3 00    lea    0xc31b7f(%rip),%rax        # 14e96e8 <UseCompressedClassPointers>
>>   8b7b69:   0f b6 00                movzbl (%rax),%eax
>>   8b7b6c:   84 c0                   test   %al,%al
>>   8b7b6e:   0f 84 9c 00 00 00       je     8b7c10 <_ZN10HeapRegion14object_iterateEP13ObjectClosure+0x260>
>>   8b7b74:   48 8d 15 05 62 c6 00    lea    0xc66205(%rip),%rdx        # 151dd80 <_ZN23CompressedKlassPointers6_shiftE>
>>   8b7b7b:   8b 7b 08                mov    0x8(%rbx),%edi
>>   8b7b7e:   8b 0a                   mov    (%rdx),%ecx
>>   8b7b80:   48 8d 15 01 62 c6 00    lea    0xc66201(%rip),%rdx        # 151dd88 <_ZN23CompressedKlassPointers5_baseE>
>>   8b7b87:   48 d3 e7                shl    %cl,%rdi
>>   8b7b8a:   48 03 3a                add    (%rdx),%rdi
>> 
>> 
>>   Patched: one load loads all three. Since shift occupies the lowest 8 bits, compiled code uses 8bit register; ditto the UseCompressedOops flag.
>> 
>> 
>>   8ba302:   48 8d 05 97 9c c2 00    lea    0xc29c97(%rip),%rax        # 14e3fa0 <_ZN23CompressedKlassPointers6_comboE>
>>   8ba309:   48 8b 08                mov    (%rax),%rcx
>>   8ba30c:   f6 c5 01                test   $0x1,%ch        # use compressed klass pointers?
>>   8ba30f:   0f 84 9b 00 00 00       je     8ba3b0 <_ZN10HeapRegion14object_iterateEP13ObjectClosure+0x260>
>>   8ba315:   8b 7b 08                mov    0x8(%rbx),%edi
>>   8ba318:   48 d3 e7                shl    %cl,%rdi        # shift 
>>   8ba31b:   66 31 c9                xor    %cx,%cx         # zero out lower 16 bits of base
>>   8ba31e:   48 01 cf                add    %rcx,%rdi       # add base 
>>   8ba321:   8b 4f 08                mov    0x8(%rdi),%ecx
>> 
>> ---
>> 
>> Performance measurements: 
>> 
>> G1, doing a full GC over a heap filled with 256 mio life j.l.Object instances.
>> 
>> I see a reduction of Full Pause times between 1.2% and 5%. I am unsure how reliable these numbers are since, despite my efforts (running tests on isolated CPUs etc.), the standard deviation was quite high at ˜4%. Still, in general, numbers seemed to go down rather than up.
>> 
>> ---
>> 
>> Future extensions: 
>> 
>> This patch uses the fact that the encoding base is aligned to metaspace reser...
>
> Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix -UseCCP case

src/hotspot/share/oops/compressedKlass.hpp line 74:

> 72:   static constexpr int bitpos_useccp = shift_bitlen;
> 73:   static address  base_from_combo()  { return  (address)(_combo & mask_base); }
> 74:   static int      shift_from_combo() { return  (int)(_combo & right_n_bits(shift_bitlen)); }

Suggestion:

  static int      shift() { return  (int)(_combo & right_n_bits(shift_bitlen)); }


What's the point of the `_from_combo` functions?

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PR Review Comment: https://git.openjdk.org/jdk/pull/15389#discussion_r1308447308