RFR(M) 8136406: Remove ZapDeadCompiledLocals code

[Vladimir Kozlov]

On 9/15/15 9:38 AM, Christian Thalinger wrote:
>
>> On Sep 14, 2015, at 9:22 PM, Vladimir Kozlov <vladimir.kozlov at oracle.com <mailto:vladimir.kozlov at oracle.com>> wrote:
>>
>> https://bugs.openjdk.java.net/browse/JDK-8136406
>> http://cr.openjdk.java.net/~kvn/8136406/webrev/
>
> Should we remove:
>
>    void OopMap::set_value(VMReg reg) {
>
> altogether?

No, it could be useful in a future and debugging.

Thanks,
Vladimir

>
>>
>> Dead code elimination.
>> I did not find any bugs since 2000 associated with ZapDeadCompilerLocals. There were few in 1999,2000 when that code
>> was added.
>>
>> I do not remember ever used ZapDeadCompilerLocals. I tried today and it does not work - corresponding calls into
>> runtime to check oopmap are not generated. The code has condition:
>>
>> (n->as_MachSafePoint()->oop_map() != NULL);
>>
>> But oop_map set only during BuildOopMaps() which is called after Insert_zap_nodes(). And we can't move
>> Insert_zap_nodes() call because calls insertion should happen before short branches are processed.
>> So it is dead code we always had in debug Server VM.
>>
>> As result of this cleaning we can remove OopMapValue::value_value used only with ZapDeadCompiledLocals - see
>> OopMap::set_value(). It will allow increase maximum allocatable spill area by factor of 2. When we generate oopmaps we
>> may bailout compilation when we hit this limit:
>>
>>      // Check for a legal reg name in the oopMap and bailout if it is not.
>>      if (!omap->legal_vm_reg_name(r)) {
>>        regalloc->C->record_method_not_compilable("illegal oopMap register name");
>>        continue;
>>      }
>>
>> Changes passed JPRT.
>>
>> Thanks,
>> Vladimir
>