Comment #6 on issue 2513 by [email protected]: life benchmark slowness
http://code.google.com/p/v8/issues/detail?id=2513
Let's look at the concrete code:
-- bool.js -----------------------------
function toInt(b) { return b ? 1 : 0; }
toInt(true);
toInt(false);
%OptimizeFunctionOnNextCall(toInt);
toInt(true);
----------------------------------------
One can see the code for toInt with a build of d8 containing a disassembler
(e.g. a debug build):
d8 --allow-natives-syntax --print-opt-code --code-comments ~/bool.js
Ignoring some function prologue/epilogue code, the interesting part is:
0x3d62a77a 26 3db1808059 cmp eax, 0x598080b1 ;; object: 0x598080b1
<true>
0x3d62a77f 31 0f8417000000 jz 60 (0x3d62a79c)
0x3d62a785 37 3dc1808059 cmp eax, 0x598080c1 ;; object: 0x598080c1
<false>
0x3d62a78a 42 0f8405000000 jz 53 (0x3d62a795)
0x3d62a790 48 e975f88d0d jmp 0x4af0a00a ;; deoptimization bailout
1
0x3d62a795 53 33c0 xor eax,eax
0x3d62a797 55 e905000000 jmp 65 (0x3d62a7a1)
0x3d62a79c 60 b802000000 mov eax,0x2
Given the current way how v8 collects and uses type information, this is as
good
as it can get. Even if we somehow handle 'boolean | 0' specially, the
resulting
code would not be any different. The only way this could be better is making
sure that the code above is only ever called with true/false, invalidating
that
code if that is not the case anymore. If we could guarantee that, a
comparison
and two conditional moves would suffice. But this is not the way v8
currently
works, and this won't change very soon, so the ternary is *the* efficient
way to
convert booleans to numbers in v8. Thinking about it, if we change the way
we
handle assumptions in the future, expressions like b?1:0 should be easily
compiled into the branch-free code, just like b|0.
In a nutshell: Use b?1:0, it will result in the best code you can get, and
probably this will stay the case in the forseeable future.
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