[erlang-questions] BEAM micro-optimization: term comparison (> 0) versus pattern-matching

[John Doe]

Just've checked, the first one is approx 2 times faster on 19.1.6
1> L = lists:seq(1, 1000000).
2> N = -1.
3> T = {error, -1}.
4> F1 = fun() -> lists:foreach(fun(_) -> case T of {success, Z} -> Z;
{error, Z} -> Z end end, L) end.
5> F2 = fun() -> lists:foreach(fun(_) -> case N of X when X > 0 -> X; _ ->
N end end, L) end.
6> timer:tc(F1).
{1089950,ok}
7>
7> timer:tc(F2).
{1870456,ok}

2017-04-15 21:13 GMT+03:00 Mikael Pettersson <mikpelinux@REDACTED>:

> Suppose a frequently called function returns a positive integer (always a
> fixnum)
> and a boolean "success" or "failure" indicator (even in the failure case
> the return
> value is significant and will be used).  Which is better from a performance
> perspective:
>
> 1. Tag the return value, {success, X} or {failure, Y}, and have callers
> pattern-match
>    on that
>
> or,
>
> 2. Indicate failure by negating Y, and have callers match on
>
>       X when X > 0 -> success case;
>       MinusY -> failure case % MinusY =< 0 is implicit
>
> Option 2 avoids the consing overheads of option 1, but I'm worried that
> the X > 0
> guard may be less optimized than a plain structural pattern-match.  The
> last time
> I checked, term-comparisons would check inline for identity, and otherwise
> call
> the general cmp() function which would then do a lot of type-casing.
>
> In my pure Erlang code I'm using option 1, but I'm reimplementing the
> function
> as a NIF, and would like to avoid the consing overheads is that's indeed
> better.
>
>
> /Mikael
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