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prettify, no semantic changes
llvm-svn: 52460
This commit is contained in:
Gabor Greif
parent
b7e2818227
commit
41d941a446
@ -2235,82 +2235,96 @@ insert entries into the symbol table.</p>
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User</a></tt> class provides a base for expressing the ownership of <tt>User</tt>
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towards other <tt><a href="http://llvm.org/doxygen/classllvm_1_1Value.html">
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Value</a></tt>s. The <tt><a href="http://llvm.org/doxygen/classllvm_1_1Use.html">
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Use</a></tt> helper class is employed to do the bookkeeping and facilitate O(1)
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Use</a></tt> helper class is employed to do the bookkeeping and to facilitate <i>O(1)</i>
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addition and removal.</p>
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<pre>
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-----------------------------------------------------------------
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--- Interaction and relationship between User and Use objects ---
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-----------------------------------------------------------------
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<!-- ______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="PATypeHolder">Interaction and relationship between <tt>User</tt> and <tt>Use</tt> objects</a>
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</div>
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A subclass of User can choose between incorporating its Use objects
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<div class="doc_text">
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<p>
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A subclass of <tt>User</tt> can choose between incorporating its <tt>Use</tt> objects
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or refer to them out-of-line by means of a pointer. A mixed variant
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(some Uses inline others hung off) is impractical and breaks the invariant
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that the Use objects belonging to the same User form a contiguous array.
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(some <tt>Use</tt>s inline others hung off) is impractical and breaks the invariant
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that the <tt>Use</tt> objects belonging to the same <tt>User</tt> form a contiguous array.
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</p>
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</div>
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We have 2 different layouts in the User (sub)classes:
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Layout a)
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The Use object(s) are inside (resp. at fixed offset) of the User
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object and there are a fixed number of them.
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Layout b)
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The Use object(s) are referenced by a pointer to an
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array from the User object and there may be a variable
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number of them.
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<p>
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We have 2 different layouts in the <tt>User</tt> (sub)classes:
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<ul>
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<li><p>Layout a)
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The <tt>Use</tt> object(s) are inside (resp. at fixed offset) of the <tt>User</tt>
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object and there are a fixed number of them.</p>
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<li><p>Layout b)
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The <tt>Use</tt> object(s) are referenced by a pointer to an
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array from the <tt>User</tt> object and there may be a variable
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number of them.</p>
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</ul>
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<p>
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Initially each layout will possess a direct pointer to the
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start of the array of Uses. Though not mandatory for layout a),
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start of the array of <tt>Use</tt>s. Though not mandatory for layout a),
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we stick to this redundancy for the sake of simplicity.
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The User object will also store the number of Use objects it
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The <tt>User</tt> object will also store the number of <tt>Use</tt> objects it
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has. (Theoretically this information can also be calculated
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given the scheme presented below.)
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given the scheme presented below.)</p>
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<p>
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Special forms of allocation operators (<tt>operator new</tt>)
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will enforce the following memory layouts:</p>
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Special forms of allocation operators (operator new)
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will enforce the following memory layouts:
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<ul>
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<li><p>Layout a) will be modelled by prepending the <tt>User</tt> object by the <tt>Use[]</tt> array.</p>
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<pre>
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...---.---.---.---.-------...
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| P | P | P | P | User
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'''---'---'---'---'-------'''
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</pre>
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# Layout a) will be modelled by prepending the User object
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# by the Use[] array.
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#
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# ...---.---.---.---.-------...
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# | P | P | P | P | User
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# '''---'---'---'---'-------'''
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<li><p>Layout b) will be modelled by pointing at the Use[] array.</p>
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<pre>
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.-------...
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| User
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'-------'''
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v
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.---.---.---.---...
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| P | P | P | P |
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'---'---'---'---'''
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</pre>
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</ul>
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<i>(In the above figures '<tt>P</tt>' stands for the <tt>Use**</tt> that
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is stored in each <tt>Use</tt> object in the member <tt>Use::Prev</tt>)</i>
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<!-- ______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="PATypeHolder">The waymarking algorithm</a>
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</div>
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# Layout b) will be modelled by pointing at the Use[] array.
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#
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# .-------...
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# | User
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# '-------'''
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# |
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# v
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# .---.---.---.---...
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# | P | P | P | P |
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# '---'---'---'---'''
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<div class="doc_text">
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<p>
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Since the <tt>Use</tt> objects will be deprived of the direct pointer to
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their <tt>User</tt> objects, there must be a fast and exact method to
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recover it. This is accomplished by the following scheme:</p>
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</div>
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(In the above figures 'P' stands for the Use** that
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is stored in each Use object in the member Use::Prev)
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Since the Use objects will be deprived of the direct pointer to
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their User objects, there must be a fast and exact method to
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recover it. This is accomplished by the following scheme:
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A bit-encoding in the 2 LSBits of the Use::Prev will allow to find the
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start of the User object:
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00 --> binary digit 0
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01 --> binary digit 1
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10 --> stop and calc (s)
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11 --> full stop (S)
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Given a Use*, all we have to do is to walk till we get
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a stop and we either have a User immediately behind or
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A bit-encoding in the 2 LSBits (least significant bits) of the <tt>Use::Prev</tt> will allow to find the
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start of the <tt>User</tt> object:
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<ul>
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<li><tt>00</tt> —> binary digit 0</li>
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<li><tt>01</tt> —> binary digit 1</li>
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<li><tt>10</tt> —> stop and calculate (<tt>s</tt>)</li>
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<li><tt>11</tt> —> full stop (<tt>S</tt>)</li>
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</ul>
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<p>
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Given a <tt>Use*</tt>, all we have to do is to walk till we get
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a stop and we either have a <tt>User</tt> immediately behind or
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we have to walk to the next stop picking up digits
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and calculating the offset:
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and calculating the offset:</p>
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<pre>
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.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.----------------
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| 1 | s | 1 | 0 | 1 | 0 | s | 1 | 1 | 0 | s | 1 | 1 | s | 1 | S | User (or User*)
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'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'----------------
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@ -2320,14 +2334,24 @@ and calculating the offset:
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| | |______________________>
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| |______________________________________>
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|__________________________________________________________>
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</pre>
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<p>
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Only the significant number of bits need to be stored between the
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stops, so that the worst case is 20 memory accesses when there are
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1000 Use objects.
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stops, so that the <i>worst case is 20 memory accesses</i> when there are
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1000 <tt>Use</tt> objects associated with a <tt>User</tt>.</p>
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The following literate Haskell fragment demonstrates the concept:
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<!-- ______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="PATypeHolder">Reference implementation</a>
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</div>
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<div class="doc_text">
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<p>
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The following literate Haskell fragment demonstrates the concept:</p>
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</div>
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<div class="doc_code">
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<pre>
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> import Test.QuickCheck
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>
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> digits :: Int -> [Char] -> [Char]
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@ -2345,13 +2369,16 @@ The following literate Haskell fragment demonstrates the concept:
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>
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> test = takeLast 40 $ dist 20 []
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>
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</pre>
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</div>
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<p>
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Printing <test> gives: <tt>"1s100000s11010s10100s1111s1010s110s11s1S"</tt></p>
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<p>
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The reverse algorithm computes the length of the string just by examining
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a certain prefix:</p>
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Printing <test> gives: "1s100000s11010s10100s1111s1010s110s11s1S"
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The reverse algorithm computes the
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length of the string just by examining
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a certain prefix:
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<div class="doc_code">
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<pre>
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> pref :: [Char] -> Int
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> pref "S" = 1
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> pref ('s':'1':rest) = decode 2 1 rest
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@ -2361,45 +2388,64 @@ a certain prefix:
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> decode walk acc ('1':rest) = decode (walk + 1) (acc * 2 + 1) rest
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> decode walk acc _ = walk + acc
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>
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</pre>
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</div>
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<p>
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Now, as expected, printing <pref test> gives <tt>40</tt>.</p>
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<p>
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We can <i>quickCheck</i> this with following property:</p>
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Now, as expected, printing <pref test> gives 40.
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We can quickCheck this with following property:
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<div class="doc_code">
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<pre>
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> testcase = dist 2000 []
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> testcaseLength = length testcase
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>
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> identityProp n = n > 0 && n <= testcaseLength ==> length arr == pref arr
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> where arr = takeLast n testcase
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>
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</pre>
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</div>
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<p>
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As expected <quickCheck identityProp> gives:</p>
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As expected <quickCheck identityProp> gives:
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<pre>
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*Main> quickCheck identityProp
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OK, passed 100 tests.
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</pre>
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<p>
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Let's be a bit more exhaustive:</p>
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Let's be a bit more exhaustive:
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<div class="doc_code">
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<pre>
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>
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> deepCheck p = check (defaultConfig { configMaxTest = 500 }) p
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>
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</pre>
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</div>
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<p>
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And here is the result of <deepCheck identityProp>:</p>
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And here is the result of <deepCheck identityProp>:
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<pre>
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*Main> deepCheck identityProp
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OK, passed 500 tests.
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To maintain the invariant that the 2 LSBits of each Use** in Use
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never change after being set up, setters of Use::Prev must re-tag the
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new Use** on every modification. Accordingly getters must strip the
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tag bits.
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For layout b) instead of the User we will find a pointer (User* with LSBit set).
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Following this pointer brings us to the User. A portable trick will ensure
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that the first bytes of User (if interpreted as a pointer) will never have
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the LSBit set.
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</pre>
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<!-- ______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="PATypeHolder">Tagging considerations</a>
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</div>
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<p>
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To maintain the invariant that the 2 LSBits of each <tt>Use**</tt> in <tt>Use</tt>
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never change after being set up, setters of <tt>Use::Prev</tt> must re-tag the
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new <tt>Use**</tt> on every modification. Accordingly getters must strip the
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tag bits.</p>
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<p>
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For layout b) instead of the <tt>User</tt> we will find a pointer (<tt>User*</tt> with LSBit set).
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Following this pointer brings us to the <tt>User</tt>. A portable trick will ensure
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that the first bytes of <tt>User</tt> (if interpreted as a pointer) will never have
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the LSBit set.</p>
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</div>
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<!-- *********************************************************************** -->
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