Thanks a lot for your answer
El domingo, 3 de abril de 2016, 10:38:57 (UTC+2), Gem Dot Artigas escribió:
>
> 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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