From 18f153bdb9db52e7089a2d5293b96c45a3124a26 Mon Sep 17 00:00:00 2001 From: Ed Schouten Date: Sat, 4 Jul 2009 13:58:26 +0000 Subject: Import LLVM 74788. --- docs/CodingStandards.html | 65 ++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 64 insertions(+), 1 deletion(-) (limited to 'docs/CodingStandards.html') diff --git a/docs/CodingStandards.html b/docs/CodingStandards.html index 84ca8fa978ee..cf9111071f56 100644 --- a/docs/CodingStandards.html +++ b/docs/CodingStandards.html @@ -50,6 +50,8 @@
  • Do not use 'using namespace std'
  • Provide a virtual method anchor for classes in headers
    • +
  • Don't evaluate end() every time through a + loop
  • Prefer Preincrement
  • Avoid std::endl
    • @@ -661,6 +663,67 @@ increasing link times.

    + +
    + Don't evaluate end() every time through a loop +
    + +
    + +

    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:

    + +
    +
    +  BasicBlock *BB = ...
+  for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I)
+     ... use I ...
+
    +
    + +

    The problem with this construct is that it evaluates "BB->end()" +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:

    + +
    +
    +  BasicBlock *BB = ...
+  for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
+     ... use I ...
+
    +
    + +

    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 +"BB->end()" 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 first form and add a comment indicating +that you did it intentionally.

    + +

    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: "SomeMap[x]->end()" and map +lookups really aren't cheap. By writing it in the second form consistently, you +eliminate the issue entirely and don't even have to think about it.

    + +

    The second (even bigger) issue is that writing the loop in the first 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.

    + +

    While the second form of the loop is a few extra keystrokes, we do strongly +prefer it.

    + +
    +
    @@ -744,7 +807,7 @@ something.

    Chris Lattner
    LLVM Compiler Infrastructure
    - Last modified: $Date: 2009-03-23 05:53:34 +0100 (Mon, 23 Mar 2009) $ + Last modified: $Date: 2009-06-30 08:27:54 +0200 (Tue, 30 Jun 2009) $ -- cgit v1.2.3