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add a note about re-evaluating end() every time through a loop.

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llvm-svn: 74511
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Chris Lattner 2009-06-30 06:13:23 +00:00
parent 4b4ba9f06c
commit f6944ea787
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@ -50,6 +50,8 @@
<li><a href="#ll_ns_std">Do not use 'using namespace std'</a></li>
<li><a href="#ll_virtual_anch">Provide a virtual method anchor for
classes in headers</a></li>
<li><a href="#ll_end">Don't evaluate end() every time through a
loop</a></li>
<li><a href="#ll_preincrement">Prefer Preincrement</a></li>
<li><a href="#ll_avoidendl">Avoid <tt>std::endl</tt></a></li>
</ol></li>
@ -661,6 +663,67 @@ increasing link times.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="ll_end">Don't evaluate end() every time through a loop</a>
</div>
<div class="doc_text">
<p>Because C++ doesn't have a standard "foreach" loop (though it can be emulated
with macros and may be coming in C++'0x) we end up writing a lot of loops that
manually iterate from begin to end on a variety of containers or through other
data structures. One common mistake is to write a loop in this style:</p>
<div class="doc_code">
<pre>
BasicBlock *BB = ...
for (BasicBlock::iterator I = BB->begin(); I != <b>BB->end()</b>; ++I)
... use I ...
</pre>
</div>
<p>The problem with this construct is that it evaluates "<tt>BB->end()</tt>"
every time through the loop. Instead of writing the loop like this, we strongly
prefer loops to be written so that they evaluate it once before the loop starts.
A convenient way to do this is like so:</p>
<div class="doc_code">
<pre>
BasicBlock *BB = ...
for (BasicBlock::iterator I = BB->begin(), E = <b>BB->end()</b>; I != E; ++I)
... use I ...
</pre>
</div>
<p>The observant may quickly point out that these two loops may have different
semantics: if the container (a basic block in this case) is being mutated, then
"<tt>BB->end()</tt>" may change its value every time through the loop and the
second loop may not in fact be correct. If you actually do depend on this
behavior, please write the loop in the second form and add a comment indicating
that you did it intentionally.</p>
<p>Why do we prefer the second form (when correct)? Writing the loop in the
first form has two problems: First it may be less efficient than evaluating it
at the start of the loop. In this case, the cost is probably minor: a few extra
loads every time through the loop. However, if the base expression is more
complex, then the cost can rise quickly. I've seen loops where the end
expression was actually something like: "<tt>SomeMap[x]->end()</tt>" and map
lookups really aren't cheap. By writing it in the first form consistently, you
eliminate the issue entirely and don't even have to think about it.</p>
<p>The second (even bigger) issue is that writing the loop in the second form
hints to the reader that the loop is mutating the container (a fact that a
comment would handily confirm!). If you write the loop in the second form, it
is immediately obvious without even looking at the body of the loop that the
container isn't being modified, which makes it easier to read the code and
understand what it does.</p>
<p>While the second form of the loop is a few extra keystrokes, we do strongly
prefer it.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">