RFR: 8240772: x86_64: Pre-generate Assembler::popa, pusha and vzeroupper

[Claes Redestad]

Hi Ioi,

good suggestions. I will rework this tomorrow, along with Vladimir's
suggestion to add an explicit call to precompute_instructions from
the stubGenerator.

Thanks!

/Claes

On 2020-03-10 19:40, Ioi Lam wrote:
> Hi Claes,
> 
> This is a really good optimization! Small bang for big bucks!
> 
> I have a suggestion code coding style:
> 
> Rename Assembler::popa to Assembler::popa_slow();
> 
> void Assembler::popa() { // 64bit
>    if (!precomputed) {
>      precompute_instructions();
>    }
>    copy_precomputed_instructions(popa_code, popa_len);
> }
> 
> static void precompute_instructions() {
>    ...
>    MacroAssembler masm(&buffer);
> 
>    address begin_popa  = masm.code_section()->end();
>    masm.popa_slow();
>    address end_popa    = masm.code_section()->end();
>    ...
> }
> 
> ----
> 
> Also, maybe you can add this assert after generating the code for all 3 
> macros:
> 
>    assert(masm->code()->total_relocation_size() == 0 &&
> masm->code()->total_oop_size() == 0 &&
> masm->code()->total_metadata_size() == 0,
>           "precomputed code cannot have any of these");
> 
> 
> Thanks!
> - Ioi
> 
> 
> 
> On 3/10/20 6:46 AM, Claes Redestad wrote:
>> Hi,
>>
>> calculate some invariant Assembler routines at bootstrap, copy on
>> subsequent invocations.
>>
>> For popa and pusha this means an overhead reduction of around 98% (from
>> ~2500 instructions to emit a pusha to ~50). For vzeroupper an overhead
>> reduction of ~65% (117 -> 42). Together these add up to about a 1%
>> reduction of instructions executed on a Hello World - with some
>> (smaller) scaling impact on larger applications.
>>
>> The initialization is very simple/naive, i.e., lacks any kind of 
>> synchronization protocol. But as this setup is guaranteed to happen very
>> early during bootstrap this should be fine. Thanks Ioi for some helpful
>> suggestions here!
>>
>> Bug:    https://bugs.openjdk.java.net/browse/JDK-8240772
>> Webrev: http://cr.openjdk.java.net/~redestad/8240772/open.00/
>>
>> Testing: tier1-3
>>
>> Thanks!
>>
>> /Claes
>