[llvm-commits] [llvm] r133111 - /llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp

[Eli Friedman]

Author: efriedma
Date: Wed Jun 15 18:59:25 2011
New Revision: 133111

URL: http://llvm.org/viewvc/llvm-project?rev=133111&view=rev
Log:
Add a limit to the number of instructions memdep will scan in a single block.  This prevents (at least in some cases) O(N^2) runtime in passes like DSE.

The limit in this patch is probably too high, but it is enough to stop DSE from going completely insane on a testcase I have (which has a single block with around 50,000 non-aliasing stores in it).

rdar://9471075


Modified:
    llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp

Modified: llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp?rev=133111&r1=133110&r2=133111&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp (original)
+++ llvm/trunk/lib/Analysis/MemoryDependenceAnalysis.cpp Wed Jun 15 18:59:25 2011
@@ -47,6 +47,11 @@
 STATISTIC(NumCacheCompleteNonLocalPtr,
           "Number of block queries that were completely cached");
 
+// Limit for the number of instructions to scan in a block.
+// FIXME: Figure out what a sane value is for this.
+//        (500 is relatively insane.)
+static const int BlockScanLimit = 500;
+
 char MemoryDependenceAnalysis::ID = 0;
   
 // Register this pass...
@@ -180,8 +185,16 @@
 MemDepResult MemoryDependenceAnalysis::
 getCallSiteDependencyFrom(CallSite CS, bool isReadOnlyCall,
                           BasicBlock::iterator ScanIt, BasicBlock *BB) {
+  unsigned Limit = BlockScanLimit;
+
   // Walk backwards through the block, looking for dependencies
   while (ScanIt != BB->begin()) {
+    // Limit the amount of scanning we do so we don't end up with quadratic
+    // running time on extreme testcases. 
+    --Limit;
+    if (!Limit)
+      return MemDepResult::getUnknown();
+
     Instruction *Inst = --ScanIt;
     
     // If this inst is a memory op, get the pointer it accessed
@@ -322,9 +335,17 @@
 
   const Value *MemLocBase = 0;
   int64_t MemLocOffset = 0;
-  
+
+  unsigned Limit = BlockScanLimit;
+
   // Walk backwards through the basic block, looking for dependencies.
   while (ScanIt != BB->begin()) {
+    // Limit the amount of scanning we do so we don't end up with quadratic
+    // running time on extreme testcases.
+    --Limit;
+    if (!Limit)
+      return MemDepResult::getUnknown();
+
     Instruction *Inst = --ScanIt;
 
     if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst)) {