Changes in directory llvm/docs:
Passes.html updated: 1.1 -> 1.2 --- Log message: For PR601: http://llvm.org/PR601 : Add a little flesh to the document skeleton. --- Diffs of the changes: (+49 -7) Passes.html | 56 +++++++++++++++++++++++++++++++++++++++++++++++++------- 1 files changed, 49 insertions(+), 7 deletions(-) Index: llvm/docs/Passes.html diff -u llvm/docs/Passes.html:1.1 llvm/docs/Passes.html:1.2 --- llvm/docs/Passes.html:1.1 Mon Mar 26 04:32:31 2007 +++ llvm/docs/Passes.html Mon Mar 26 21:49:31 2007 @@ -477,7 +477,10 @@ <a name="adce">Aggressive Dead Code Elimination</a> </div> <div class="doc_text"> - <p>Yet to be written.</p> + <p>ADCE aggressively tries to eliminate code. This pass is similar to + <a href="#dce">DCE</a> but it assumes that values are dead until proven + otherwise. This is similar to <a href="#sccp">SCCP</a>, except applied to + the liveness of values.</p> </div> <!-------------------------------------------------------------------------- --> @@ -493,7 +496,21 @@ <a name="block-placement">Profile Guided Basic Block Placement</a> </div> <div class="doc_text"> - <p>Yet to be written.</p> + <p>This pass implements a very simple profile guided basic block placement + algorithm. The idea is to put frequently executed blocks together at the + start of the function, and hopefully increase the number of fall-through + conditional branches. If there is no profile information for a particular + function, this pass basically orders blocks in depth-first order.</p> + <p>The algorithm implemented here is basically "Algo1" from "Profile Guided + Code Positioning" by Pettis and Hansen, except that it uses basic block + counts instead of edge counts. This could be improved in many ways, but is + very simple for now.</p> + <p>Basically we "place" the entry block, then loop over all successors in a + DFO, placing the most frequently executed successor until we run out of + blocks. Did we mention that this was <b>extremely</b> simplistic? This is + also much slower than it could be. When it becomes important, this pass + will be rewritten to use a better algorithm, and then we can worry about + efficiency.</p> </div> <!-------------------------------------------------------------------------- --> @@ -509,7 +526,23 @@ <a name="cee">Correlated Expression Elimination</a> </div> <div class="doc_text"> - <p>Yet to be written.</p> + <p>Correlated Expression Elimination propagates information from conditional + branches to blocks dominated by destinations of the branch. It propagates + information from the condition check itself into the body of the branch, + allowing transformations like these for example: + <pre> + if (i == 7) + ... 4*i; // constant propagation + + M = i+1; N = j+1; + if (i == j) + X = M-N; // = M-M == 0; + </pre></p> + + <p>This is called Correlated Expression Elimination because we eliminate or + simplify expressions that are correlated with the direction of a branch. In + this way we use static information to give us some information about the + dynamic value of a variable.</p> </div> <!-------------------------------------------------------------------------- --> @@ -517,7 +550,8 @@ <a name="condprop">Conditional Propagation</a> </div> <div class="doc_text"> - <p>Yet to be written.</p> + <p>This pass propagates information about conditional expressions through the + program, allowing it to eliminate conditional branches in some cases.</p> </div> <!-------------------------------------------------------------------------- --> @@ -530,10 +564,18 @@ <!-------------------------------------------------------------------------- --> <div class="doc_subsection"> - <a name="constprop">Simple constant propagation</a> + <a name="constprop">Constant Propagation</a> </div> <div class="doc_text"> - <p>Yet to be written.</p> + <p>This file implements constant propagation and merging. It looks for + instructions involving only constant operands and replaces them with a + constant value instead of an instruction. For example: + <pre>add i32 1, 2</pre><br/> + becomes + <pre>i32 3</pre></p> + <p>NOTE: this pass has a habit of making definitions be dead. It is a good + idea to to run a <a href="#die">DIE</a> (Dead Instruction Elimination) pass + sometime after running this pass.</p> </div> <!-------------------------------------------------------------------------- --> @@ -977,7 +1019,7 @@ <a href="mailto:[EMAIL PROTECTED]">Reid Spencer</a><br> <a href="http://llvm.org";>LLVM Compiler Infrastructure</a><br> - Last modified: $Date: 2007/03/26 09:32:31 $ + Last modified: $Date: 2007/03/27 02:49:31 $ </address> </body> _______________________________________________ llvm-commits mailing list llvm-commits@cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
