[llvm-commits] [llvm] r55391 - /llvm/trunk/lib/Transforms/Scalar/GVN.cpp

[Owen Anderson]

Author: resistor
Date: Tue Aug 26 17:07:42 2008
New Revision: 55391

URL: http://llvm.org/viewvc/llvm-project?rev=55391&view=rev
Log:
Put a heuristic in place to prevent GVN from falling into bad cases with massively complicated CFGs.
This speeds up a particular testcase from 12+ hours to 5 seconds with little perceptible loss of quality.

Modified:
    llvm/trunk/lib/Transforms/Scalar/GVN.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/GVN.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/GVN.cpp?rev=55391&r1=55390&r2=55391&view=diff

==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/GVN.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/GVN.cpp Tue Aug 26 17:07:42 2008
@@ -862,6 +862,12 @@
   DenseMap<BasicBlock*, Value*> deps;
   MD.getNonLocalDependency(L, deps);
   
+  // If we had to process more than one hundred blocks to find the
+  // dependencies, this load isn't worth worrying about.  Optimizing
+  // it will be too expensive.
+  if (deps.size() > 100)
+    return false;
+  
   DenseMap<BasicBlock*, Value*> repl;
   
   // Filter out useless results (non-locals, etc)