Thanks a lot for your answer.
But, There is any reason for this? In other words, why they have been
disabed it by defect? Usually, What do the current browsers do? I would
like to know which are the the most correct flags for achademic research.
thanks in advance
El domingo, 3 de abril de 2016, 18:17:15 (UTC+2), Yang escribió:
>
> For Crankshaft, bounds check hoisting optimization has been disabled. You
> can try to turn the flag on and try your test case.
> Not sure if TurboFan already has similar optimization implemented or
> enabled.
>
> On Sunday, April 3, 2016 at 1:38:57 AM UTC-7, Gem Dot Artigas wrote:
>>
>> Hi,
>>
>>
>> I have an important question about check bounds. Why, in some very clear
>> cases, V8 put the instruction to obtain the length of the array out of the
>> loop, but the check bounds is still inside? (I refer the scenarios when the
>> loop control variable is clear to be incremented by 1 at each loop
>> iteration and it is not manipulated by no function)
>>
>>
>>
>> I put an example to clarify it:
>>
>> Given this JS function from Kraken suite:
>>
>> 1 findGraphNode = function(node) {
>> 2 for(var i=0;i<this.length;i++) {
>> 3 if(this[i].position == node.position) {
>> 4 return true;
>> 5 }
>> 6 }
>> 7 return false;
>> 8 }
>>
>>
>> The optimized generated x86 code is the following:
>>
>> I1 movq rax, this // load this
>> I2 test rax, 1 // check non-smi
>> I3 jz code_deoptimization // check non-smi
>> I4 movq r10, nodeList // check maps
>> I5 cmpq (rax-1), r10 // check maps
>> I6 jnz code_deoptimization // check maps
>> I7 movl rbx, (rax+27) // load elements length
>> I8 movq rdx,(rax+15) // load elements array
>> I9 movq rcx, (rbp+16) // load node
>> I10 testb rcx, 1 // check non-smi
>> I11 jz code_deoptimization // check non-smi
>> I12 movq r10, GraphNode // check maps
>> I13 cmpq (rcx-1), r10 // check maps
>> I14 jnz code_deoptimization // check maps
>> I15 movq rsi,(rcx+47) // load node.position
>> loop:
>> I16 cmpl rdi, rbx // compare i to this.length
>> I17 jge return_false // node not found: return false
>> I18 cmpq rsp, (stack_limit) // check stack
>> I19 jc external_exception // check stack
>> I20 cmpl rbx,rdi // check bounds
>> I21 jna code_deoptimization // check bounds
>> I22 movq r8, (rdx+rdi*8) // load this[i]
>> I23 testb r8, 1 // check non-smi
>> I24 jz code_deoptimization // check non-smi
>> I25 movq r10, GraphNode // check maps
>> I26 cmpq (r8-1), r10 // check maps
>> I27 jnz code_deoptimization // check maps
>> I28 movq r9,(r8+47) // load this[i].position
>> I29 cmpq r9,rsi // compare this[i].position to
>> node.position
>> I30 jz return_true // node found: return true
>> I31 addl rdi,0x1 // i++
>> I32 jmp loop // loop back edge
>>
>>
>> So, Why check bounds can not be put out of the loop? You can put a check
>> bounds out of the loop and compare it with this.length variable directly...
>>
>> Thanks in Avdance
>>
>
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