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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