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add a blurb on const versions of chain traversals and a word of caution 

llvm-svn: 99638
This commit is contained in:
Gabor Greif 2010-03-26 19:30:47 +00:00
parent 6096d5a279
commit 6d6cf8ff00
1 changed files with 14 additions and 8 deletions
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22
llvm/docs/ProgrammersManual.html
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@ -1211,14 +1211,14 @@ and erasing, but does not support iteration.</p>
<div class="doc_text"> <div class="doc_text">
<p>SmallPtrSet has all the advantages of SmallSet (and a SmallSet of pointers is <p>SmallPtrSet has all the advantages of <tt>SmallSet</tt> (and a <tt>SmallSet</tt> of pointers is
transparently implemented with a SmallPtrSet), but also supports iterators. If transparently implemented with a <tt>SmallPtrSet</tt>), but also supports iterators. If
more than 'N' insertions are performed, a single quadratically more than 'N' insertions are performed, a single quadratically
probed hash table is allocated and grows as needed, providing extremely probed hash table is allocated and grows as needed, providing extremely
efficient access (constant time insertion/deleting/queries with low constant efficient access (constant time insertion/deleting/queries with low constant
factors) and is very stingy with malloc traffic.</p> factors) and is very stingy with malloc traffic.</p>
<p>Note that, unlike std::set, the iterators of SmallPtrSet are invalidated <p>Note that, unlike <tt>std::set</tt>, the iterators of <tt>SmallPtrSet</tt> are invalidated
whenever an insertion occurs. Also, the values visited by the iterators are not whenever an insertion occurs. Also, the values visited by the iterators are not
visited in sorted order.</p> visited in sorted order.</p>
@ -1960,6 +1960,10 @@ for (Value::use_iterator i = F-&gt;use_begin(), e = F-&gt;use_end(); i != e; ++i
errs() &lt;&lt; *Inst &lt;&lt; "\n"; errs() &lt;&lt; *Inst &lt;&lt; "\n";
} }
</pre> </pre>
Note that dereferencing a <tt>Value::use_iterator</tt is not a very cheap
operation. Instead of performing <tt>*i</tt> above several times, consider
doing it only once in the loop body and reusing its result.
</div> </div>
<p>Alternatively, it's common to have an instance of the <a <p>Alternatively, it's common to have an instance of the <a
@ -1981,11 +1985,13 @@ for (User::op_iterator i = pi-&gt;op_begin(), e = pi-&gt;op_end(); i != e; ++i)
</pre> </pre>
</div> </div>
<!-- <p>Declaring objects as <tt>const</tt> is an important tool of enforcing
def-use chains ("finding all users of"): Value::use_begin/use_end mutation free algorithms (such as analyses etc.). For this purpose above
use-def chains ("finding all values used"): User::op_begin/op_end [op=operand] iterators come in constant flavors as <tt>Value::const_use_iterator</tt>
--> and <tt>Value::const_op_iterator</tt>. They automatically arise when
calling <tt>use/op_begin()</tt> on <tt>const Value*</tt>s or
<tt>const User*</tt>s respectively. Upon dereferencing, they return
<tt>const Use*</tt>s. Otherwise the above patterns remain unchanged.
</div> </div>
<!--_______________________________________________________________________--> <!--_______________________________________________________________________-->