Luke Wagner wrote:
It seems to me that we*must* have Math.clz perform ToUint32() on its input. Otherwise (if ToInt32()
is used), even if the expression is written "Math.clz(x>>> 0)",
asm.js/type-inference won't be able to optimize away the<0 branch since the ToInt32() in the
semantics of Math.clz will convert a large unsigned integer argument into a negative integer.
Secondly, I think clz(0) should be 32, not 0:
- for x86's bsr, since the result is undefined if the input is zero, we need
a branch or cmov*anyway* to deal with zero.
- to make up for bsr's shortcomings, newer x86 chips have lzcnt (which JITs
can emit after testing the right cpuid bit), which defines clz(0) == 32.
- ARM's clz also defines clz(0) == 32.
The only reason not to do this I could imagine is if there was a de facto
standard among all x86 chips (such that the JIT could rely on it) to have
bsr(0) == 0, but I couldn't find any evidence of this.
Thanks, I just presented this pretty much verbatim, and Math.clz32 it
is, by TC39 consensus. ToUint32 on parameter, 0 => 32 and all.
/be
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