RFR: JDK-8297660: x86: Redundant test+jump in C1 allocateArray [v2]

[Yi Yang]

On Sat, 26 Nov 2022 10:47:11 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> In `C1_MacroAssembler::initialize_body()` we test the input array length for 0. We do this again in `MacroAssembler::zero_memory()`. This results in a redundant test+jump instruction:
>> 
>> 
>>  13442   0x00007f58a8ca2f02:   sub    $0x10,%rsi                 
>>  13443   0x00007f58a8ca2f06:   je     0x00007f58a8ca2f26   <<
>>  13444   0x00007f58a8ca2f0c:   test   %rsi,%rsi                     
>>  13445   0x00007f58a8ca2f0f:   je     0x00007f58a8ca2f26    <<
>>  13446   0x00007f58a8ca2f15:   xor    %rbx,%rbx
>>  13447   0x00007f58a8ca2f18:   shr    $0x3,%rsi
>>  13448   0x00007f58a8ca2f1c:   mov    %rbx,0x8(%rax,%rsi,8)
>>  13449   0x00007f58a8ca2f21:   dec    %rsi
>>  13450   0x00007f58a8ca2f24:   jne    0x00007f58a8ca2f1c           ;*anewarray {reexecute=0 rethrow=0 return_oop=0}
>>  13451                                                             ; - java.lang.invoke.MethodHandles::<clinit>@24 (line 5109)
>> 
>> 
>> Since `MacroAssembler::zero_memory()` is only ever called from `C1_MacroAssembler::initialize_body()`, it does not need to test for len=0, since its caller already does.
>> 
>> ---
>> 
>> Patch removes one test+jump and adds an assertion for len>0 to zero_memory.
>> 
>> Patch ran through SAP nightlies and GHAs.
>
> Thomas Stuefe has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - remove outer conditional jump
>  - Revert "remove-redundant-test-from-zeromemory"
>    
>    This reverts commit 4f95969d4d3026ce2310230c37f469579dc32e88.

Marked as reviewed by yyang (Committer).

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PR: https://git.openjdk.org/jdk/pull/11372