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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