View Source Functions
Pattern Matching
Pattern matching in function head as well as in case
and receive
clauses are
optimized by the compiler. With a few exceptions, there is nothing to gain by
rearranging clauses.
One exception is pattern matching of binaries. The compiler does not rearrange clauses that match binaries. Placing the clause that matches against the empty binary last is usually slightly faster than placing it first.
The following is a rather unnatural example to show another exception where rearranging clauses is beneficial:
DO NOT
atom_map1(one) -> 1;
atom_map1(two) -> 2;
atom_map1(three) -> 3;
atom_map1(Int) when is_integer(Int) -> Int;
atom_map1(four) -> 4;
atom_map1(five) -> 5;
atom_map1(six) -> 6.
The problem is the clause with the variable Int
. As a variable can match
anything, including the atoms four
, five
, and six
, which the following
clauses also match, the compiler must generate suboptimal code that executes as
follows:
- First, the input value is compared to
one
,two
, andthree
(using a single instruction that does a binary search; thus, quite efficient even if there are many values) to select which one of the first three clauses to execute (if any). - If none of the first three clauses match, the fourth clause match as a variable always matches.
- If the guard test
is_integer(Int)
succeeds, the fourth clause is executed. - If the guard test fails, the input value is compared to
four
,five
, andsix
, and the appropriate clause is selected. (There is afunction_clause
exception if none of the values matched.)
Rewriting to either:
DO
atom_map2(one) -> 1;
atom_map2(two) -> 2;
atom_map2(three) -> 3;
atom_map2(four) -> 4;
atom_map2(five) -> 5;
atom_map2(six) -> 6;
atom_map2(Int) when is_integer(Int) -> Int.
or:
DO
atom_map3(Int) when is_integer(Int) -> Int;
atom_map3(one) -> 1;
atom_map3(two) -> 2;
atom_map3(three) -> 3;
atom_map3(four) -> 4;
atom_map3(five) -> 5;
atom_map3(six) -> 6.
gives slightly more efficient matching code.
Another example:
DO NOT
map_pairs1(_Map, [], Ys) ->
Ys;
map_pairs1(_Map, Xs, []) ->
Xs;
map_pairs1(Map, [X|Xs], [Y|Ys]) ->
[Map(X, Y)|map_pairs1(Map, Xs, Ys)].
The first argument is not a problem. It is variable, but it is a variable in
all clauses. The problem is the variable in the second argument, Xs
, in the
middle clause. Because the variable can match anything, the compiler is not
allowed to rearrange the clauses, but must generate code that matches them in
the order written.
If the function is rewritten as follows, the compiler is free to rearrange the clauses:
DO
map_pairs2(_Map, [], Ys) ->
Ys;
map_pairs2(_Map, [_|_]=Xs, [] ) ->
Xs;
map_pairs2(Map, [X|Xs], [Y|Ys]) ->
[Map(X, Y)|map_pairs2(Map, Xs, Ys)].
The compiler will generate code similar to this:
DO NOT (already done by the compiler)
explicit_map_pairs(Map, Xs0, Ys0) ->
case Xs0 of
[X|Xs] ->
case Ys0 of
[Y|Ys] ->
[Map(X, Y)|explicit_map_pairs(Map, Xs, Ys)];
[] ->
Xs0
end;
[] ->
Ys0
end.
This is slightly faster for probably the most common case that the input lists
are not empty or very short. (Another advantage is that Dialyzer can deduce a
better type for the Xs
variable.)
Function Calls
This is a rough hierarchy of the performance of the different types of function calls:
- Calls to local or external functions (
foo()
,m:foo()
) are the fastest calls. - Calling or applying a fun (
Fun()
,apply(Fun, [])
) is just a little slower than external calls. - Applying an exported function (
Mod:Name()
,apply(Mod, Name, [])
) where the number of arguments is known at compile time is next. - Applying an exported function (
apply(Mod, Name, Args)
) where the number of arguments is not known at compile time is the least efficient.
Notes and Implementation Details
Calling and applying a fun does not involve any hash-table lookup. A fun contains an (indirect) pointer to the function that implements the fun.
apply/3
must look up the code for the function to execute in a
hash table. It is therefore always slower than a direct call or a fun call.
Caching callback functions into funs may be more efficient in the long run than apply calls for frequently-used callbacks.