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Diffstat (limited to 'system/doc/efficiency_guide/functions.xml')
| -rw-r--r-- | system/doc/efficiency_guide/functions.xml | 120 |
1 files changed, 65 insertions, 55 deletions
diff --git a/system/doc/efficiency_guide/functions.xml b/system/doc/efficiency_guide/functions.xml index ec1a45eaa9..bd23c9d90d 100644 --- a/system/doc/efficiency_guide/functions.xml +++ b/system/doc/efficiency_guide/functions.xml @@ -18,7 +18,6 @@ basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License for the specific language governing rights and limitations under the License. - </legalnotice> <title>Functions</title> @@ -30,17 +29,18 @@ </header> <section> - <title>Pattern matching</title> - <p>Pattern matching in function head and in <c>case</c> and <c>receive</c> - clauses are optimized by the compiler. With a few exceptions, there is nothing - to gain by rearranging clauses.</p> + <title>Pattern Matching</title> + <p>Pattern matching in function head as well as in <c>case</c> and + <c>receive</c> clauses are optimized by the compiler. With a few + exceptions, there is nothing to gain by rearranging clauses.</p> <p>One exception is pattern matching of binaries. The compiler - will not rearrange clauses that match binaries. Placing the - clause that matches against the empty binary <em>last</em> will usually - be slightly faster than placing it <em>first</em>.</p> + does not rearrange clauses that match binaries. Placing the + clause that matches against the empty binary <em>last</em> is usually + slightly faster than placing it <em>first</em>.</p> - <p>Here is a rather contrived example to show another exception:</p> + <p>The following is a rather unnatural example to show another + exception:</p> <p><em>DO NOT</em></p> <code type="erl"> @@ -53,27 +53,30 @@ atom_map1(five) -> 5; atom_map1(six) -> 6.</code> <p>The problem is the clause with the variable <c>Int</c>. - Since a variable can match anything, including the atoms - <c>four</c>, <c>five</c>, and <c>six</c> that the following clauses - also will match, the compiler must generate sub-optimal code that will - execute as follows:</p> + As a variable can match anything, including the atoms + <c>four</c>, <c>five</c>, and <c>six</c>, which the following clauses + also match, the compiler must generate suboptimal code that + executes as follows:</p> - <p>First the input value is compared to <c>one</c>, <c>two</c>, and + <list type="bulleted"> + <item>First, the input value is compared to <c>one</c>, <c>two</c>, and <c>three</c> (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).</p> + one of the first three clauses to execute (if any).</item> + + <item>>If none of the first three clauses match, the fourth clause + match as a variable always matches.</item> - <p>If none of the first three clauses matched, the fourth clause - will match since a variable always matches. If the guard test - <c>is_integer(Int)</c> succeeds, the fourth clause will be - executed.</p> + <item>If the guard test <c>is_integer(Int)</c> succeeds, the fourth + clause is executed.</item> - <p>If the guard test failed, the input value is compared to + <item>If the guard test fails, the input value is compared to <c>four</c>, <c>five</c>, and <c>six</c>, and the appropriate clause - is selected. (There will be a <c>function_clause</c> exception if - none of the values matched.)</p> + is selected. (There is a <c>function_clause</c> exception if none of + the values matched.)</item> + </list> - <p>Rewriting to either</p> + <p>Rewriting to either:</p> <p><em>DO</em></p> <code type="erl"><![CDATA[ @@ -85,7 +88,7 @@ atom_map2(five) -> 5; atom_map2(six) -> 6; atom_map2(Int) when is_integer(Int) -> Int.]]></code> - <p>or</p> + <p>or:</p> <p><em>DO</em></p> <code type="erl"><![CDATA[ @@ -97,9 +100,9 @@ atom_map3(four) -> 4; atom_map3(five) -> 5; atom_map3(six) -> 6.]]></code> - <p>will give slightly more efficient matching code.</p> + <p>gives slightly more efficient matching code.</p> - <p>Here is a less contrived example:</p> + <p>Another example:</p> <p><em>DO NOT</em></p> <code type="erl"><![CDATA[ @@ -116,7 +119,8 @@ map_pairs1(Map, [X|Xs], [Y|Ys]) -> match anything, the compiler is not allowed to rearrange the clauses, but must generate code that matches them in the order written.</p> - <p>If the function is rewritten like this</p> + <p>If the function is rewritten as follows, the compiler is free to + rearrange the clauses:</p> <p><em>DO</em></p> <code type="erl"><![CDATA[ @@ -127,8 +131,7 @@ map_pairs2(_Map, [_|_]=Xs, [] ) -> map_pairs2(Map, [X|Xs], [Y|Ys]) -> [Map(X, Y)|map_pairs2(Map, Xs, Ys)].]]></code> - <p>the compiler is free to rearrange the clauses. It will generate code - similar to this</p> + <p>The compiler will generate code similar to this:</p> <p><em>DO NOT (already done by the compiler)</em></p> <code type="erl"><![CDATA[ @@ -145,31 +148,35 @@ explicit_map_pairs(Map, Xs0, Ys0) -> Ys0 end.]]></code> - <p>which should be slightly faster for presumably the most common case + <p>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 is able to deduce a better type - for the variable <c>Xs</c>.)</p> + (Another advantage is that Dialyzer can deduce a better type + for the <c>Xs</c> variable.)</p> </section> <section> - <title>Function Calls </title> + <title>Function Calls</title> - <p>Here is an intentionally rough guide to the relative costs of - different kinds of calls. It is based on benchmark figures run on + <p>This is an intentionally rough guide to the relative costs of + different calls. It is based on benchmark figures run on Solaris/Sparc:</p> <list type="bulleted"> <item>Calls to local or external functions (<c>foo()</c>, <c>m:foo()</c>) - are the fastest kind of calls.</item> + are the fastest calls.</item> + <item>Calling or applying a fun (<c>Fun()</c>, <c>apply(Fun, [])</c>) - is about <em>three times</em> as expensive as calling a local function.</item> + is about <em>three times</em> as expensive as calling a local + function.</item> + <item>Applying an exported function (<c>Mod:Name()</c>, - <c>apply(Mod, Name, [])</c>) is about twice as expensive as calling a fun, - or about <em>six times</em> as expensive as calling a local function.</item> + <c>apply(Mod, Name, [])</c>) is about twice as expensive as calling + a fun or about <em>six times</em> as expensive as calling a local + function.</item> </list> <section> - <title>Notes and implementation details</title> + <title>Notes and Implementation Details</title> <p>Calling and applying a fun does not involve any hash-table lookup. A fun contains an (indirect) pointer to the function that implements @@ -178,42 +185,44 @@ explicit_map_pairs(Map, Xs0, Ys0) -> <warning><p><em>Tuples are not fun(s)</em>. A "tuple fun", <c>{Module,Function}</c>, is not a fun. The cost for calling a "tuple fun" is similar to that - of <c>apply/3</c> or worse. Using "tuple funs" is <em>strongly discouraged</em>, - as they may not be supported in a future release, - and because there exists a superior alternative since the R10B - release, namely the <c>fun Module:Function/Arity</c> syntax.</p></warning> + of <c>apply/3</c> or worse. + Using "tuple funs" is <em>strongly discouraged</em>, + as they might not be supported in a future Erlang/OTP release, + and because there exists a superior alternative from R10B, + namely the <c>fun Module:Function/Arity</c> syntax.</p></warning> <p><c>apply/3</c> must look up the code for the function to execute - in a hash table. Therefore, it will always be slower than a + in a hash table. It is therefore always slower than a direct call or a fun call.</p> <p>It no longer matters (from a performance point of view) - whether you write</p> + whether you write:</p> <code type="erl"> Module:Function(Arg1, Arg2)</code> - <p>or</p> + <p>or:</p> <code type="erl"> apply(Module, Function, [Arg1,Arg2])</code> - <p>(The compiler internally rewrites the latter code into the former.)</p> + <p>The compiler internally rewrites the latter code into the + former.</p> - <p>The following code</p> + <p>The following code is slightly slower because the shape of the + list of arguments is unknown at compile time.</p> <code type="erl"> apply(Module, Function, Arguments)</code> - <p>is slightly slower because the shape of the list of arguments - is not known at compile time.</p> </section> </section> <section> - <title>Memory usage in recursion</title> - <p>When writing recursive functions it is preferable to make them - tail-recursive so that they can execute in constant memory space.</p> + <title>Memory Usage in Recursion</title> + <p>When writing recursive functions, it is preferable to make them + tail-recursive so that they can execute in constant memory space:</p> + <p><em>DO</em></p> <code type="none"> list_length(List) -> @@ -224,13 +233,14 @@ list_length([], AccLen) -> list_length([_|Tail], AccLen) -> list_length(Tail, AccLen + 1). % Tail-recursive</code> + <p><em>DO NOT</em></p> + <code type="none"> list_length([]) -> 0. % Base case list_length([_ | Tail]) -> list_length(Tail) + 1. % Not tail-recursive</code> </section> - </chapter> |