Re:Benchmarking JS
by mraleph on Wednesday May 13, 2015 @12:36PM
(#9540158)
Indeed there are some relatively well understood negative consequences to LICM and various other redundancy elimination optimizations: e.g. they increase life-time of values which can have negative impact on register allocation. Another thing is getting LICM right in the presence of conditional control flow within the loop is a non-trivial exercise, in dynamically typed languages this becomes a problem because JIT in general wants to hoist type guards aggressively but it has to account for conditional control flow to avoid weird/unnecessary deopts
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Yes, LCM is pretty much a disaster in practice, and people have been trying to fix it ever since.
The set of things you are worrying about it actually precisely the set of things CLZ solves :)
> The set of things you are worrying about it actually precisely the set of things CLZ solves :)
I think the set of things I worry about is slightly different, because they operate in a static environment and I operate in a dynamic one. For example, they don't seem to be worried about the transformation from:
loop {
if (pred) {
// o and o.f are loop invariants
Guard(o is X);
v = Load(o.f);
Use(v)
}
}
Guard(o is X);
v = Load(o.f);
loop {
if (pred) {
Use(v)
}
}