How JavaScript works: inside the V8 engine

Re:How JavaScript works: inside the V8 engine by foldr on Tuesday August 22, 2017 @05:36AM (#15070857)

>[1]: If you can read assembly, this one's hilarious: https://godbolt.org/g/81qWoq

I only barely read assembly, so I was just trying to figure this one out.

So, the compiler has figured out that it just needs to test v against 0 (since the array is immutable), but it does this 2 times, and with a couple of apparently redundant 'je' instructions. Is that what's weird about the output?

In case it changed, this is the output I saw:

  example::exists_in_table:
    push rbp
    mov rbp, rsp
    mov al, 1
    test edi, edi
    je .LBB0_5
    je .LBB0_5
    test edi, edi
    je .LBB0_5
    je .LBB0_5
    xor eax, eax
  .LBB0_5:
    pop rbp
    ret

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Re:How JavaScript works: inside the V8 engine by Veedrac on Tuesday August 22, 2017 @11:06AM (#15073106)

Basically, yes. The first test and je show the compiler has managed the hard part, but the next je obviously can't be hit, the test after that literally doesn't do anything, and the two jumps after that also can't be hit. It's funny because this is all so trivial.

If I write it as some C

    bool should_jump = edi == 0;
    if (should_jump) goto LBB0_5;
    if (should_jump) goto LBB0_5;
    bool should_jump = edi == 0;
    if (should_jump) goto LBB0_5;
    if (should_jump) goto LBB0_5;

I can guarantee the compiler is going to produce good code from it.

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