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Diffstat (limited to 'system/doc/efficiency_guide')
| -rw-r--r-- | system/doc/efficiency_guide/myths.xml | 54 |
1 files changed, 16 insertions, 38 deletions
diff --git a/system/doc/efficiency_guide/myths.xml b/system/doc/efficiency_guide/myths.xml index d6cb27ddf0..168aa3d35c 100644 --- a/system/doc/efficiency_guide/myths.xml +++ b/system/doc/efficiency_guide/myths.xml @@ -46,44 +46,22 @@ Than Recursive Functions</title> <p><marker id="tail_recursive"></marker>According to the myth, - recursive functions leave references - to dead terms on the stack and the garbage collector has to copy - all those dead terms, while tail-recursive functions immediately - discard those terms.</p> - - <p>That used to be true before R7B. In R7B, the compiler started - to generate code that overwrites references to terms that will never - be used with an empty list, so that the garbage collector would not - keep dead values any longer than necessary.</p> - - <p>Even after that optimization, a tail-recursive function is - still most of the times faster than a body-recursive function. Why?</p> - - <p>It has to do with how many words of stack that are used in each - recursive call. In most cases, a recursive function uses more words - on the stack for each recursion than the number of words a tail-recursive - would allocate on the heap. As more memory is used, the garbage - collector is invoked more frequently, and it has more work traversing - the stack.</p> - - <p>In R12B and later releases, there is an optimization that - in many cases reduces the number of words used on the stack in - body-recursive calls. A body-recursive list function and a - tail-recursive function that calls <seealso - marker="stdlib:lists#reverse/1">lists:reverse/1</seealso> at - the end will use the same amount of memory. - <c>lists:map/2</c>, <c>lists:filter/2</c>, list comprehensions, - and many other recursive functions now use the same amount of space - as their tail-recursive equivalents.</p> - - <p>So, which is faster? - It depends. On Solaris/Sparc, the body-recursive function seems to - be slightly faster, even for lists with a lot of elements. On the x86 - architecture, tail-recursion was up to about 30% faster.</p> - - <p>So, the choice is now mostly a matter of taste. If you really do need - the utmost speed, you must <em>measure</em>. You can no longer be - sure that the tail-recursive list function always is the fastest.</p> + using a tail-recursive function that builds a list in reverse + followed by a call to <c>lists:reverse/1</c> is faster than + a body-recursive function that builds the list in correct order; + the reason being that body-recursive functions use more memory than + tail-recursive functions.</p> + + <p>That was true to some extent before R12B. It was even more true + before R7B. Today, not so much. A body-recursive function + generally uses the same amount of memory as a tail-recursive + function. It is generally not possible to predict whether the + tail-recursive or the body-recursive version will be + faster. Therefore, use the version that makes your code cleaner + (hint: it is usually the body-recursive version).</p> + + <p>For a more thorough discussion about tail and body recursion, + see <url href="http://ferd.ca/erlang-s-tail-recursion-is-not-a-silver-bullet.html">Erlang's Tail Recursion is Not a Silver Bullet</url>.</p> <note><p>A tail-recursive function that does not need to reverse the list at the end is faster than a body-recursive function, |