[llvm] r260014 - Compute live-in for MemorySSA

[Daniel Berlin via llvm-commits]

Author: dannyb
Date: Sat Feb  6 19:52:19 2016
New Revision: 260014

URL: http://llvm.org/viewvc/llvm-project?rev=260014&view=rev
Log:
Compute live-in for MemorySSA

Modified:
    llvm/trunk/lib/Transforms/Utils/MemorySSA.cpp

Modified: llvm/trunk/lib/Transforms/Utils/MemorySSA.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/MemorySSA.cpp?rev=260014&r1=260013&r2=260014&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/MemorySSA.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/MemorySSA.cpp Sat Feb  6 19:52:19 2016
@@ -249,10 +249,11 @@ MemorySSAWalker *MemorySSA::buildMemoryS
   // could just look up the memory access for every possible instruction in the
   // stream.
   SmallPtrSet<BasicBlock *, 32> DefiningBlocks;
-
+  SmallPtrSet<BasicBlock *, 32> DefUseBlocks;
   // Go through each block, figure out where defs occur, and chain together all
   // the accesses.
   for (BasicBlock &B : F) {
+    bool InsertIntoDefUse = false;
     bool InsertIntoDef = false;
     AccessListType *Accesses = nullptr;
     for (Instruction &I : B) {
@@ -261,17 +262,56 @@ MemorySSAWalker *MemorySSA::buildMemoryS
         continue;
       if (isa<MemoryDef>(MA))
         InsertIntoDef = true;
+      else if (isa<MemoryUse>(MA))
+        InsertIntoDefUse = true;
+
       if (!Accesses)
         Accesses = getOrCreateAccessList(&B);
       Accesses->push_back(MA);
     }
     if (InsertIntoDef)
       DefiningBlocks.insert(&B);
+    if (InsertIntoDefUse)
+      DefUseBlocks.insert(&B);
+  }
+
+  // Compute live-in.
+  // Live in is normally defined as "all the blocks on the path from each def to
+  // each of it's uses".
+  // MemoryDef's are implicit uses of previous state, so they are also uses.
+  // This means we don't really have def-only instructions.  The only
+  // MemoryDef's that are not really uses are those that are of the LiveOnEntry
+  // variable (because LiveOnEntry can reach anywhere, and every def is a
+  // must-kill of LiveOnEntry).
+  // In theory, you could precisely compute live-in by using alias-analysis to
+  // disambiguate defs and uses to see which really pair up with which.
+  // In practice, this would be really expensive and difficult. So we simply
+  // assume all defs are also uses that need to be kept live.
+  // Because of this, the end result of this live-in computation will be "the
+  // entire set of basic blocks that reach any use".
+
+  SmallPtrSet<BasicBlock *, 32> LiveInBlocks;
+  SmallVector<BasicBlock *, 64> LiveInBlockWorklist(DefUseBlocks.begin(),
+                                                    DefUseBlocks.end());
+  // Now that we have a set of blocks where a value is live-in, recursively add
+  // predecessors until we find the full region the value is live.
+  while (!LiveInBlockWorklist.empty()) {
+    BasicBlock *BB = LiveInBlockWorklist.pop_back_val();
+
+    // The block really is live in here, insert it into the set.  If already in
+    // the set, then it has already been processed.
+    if (!LiveInBlocks.insert(BB).second)
+      continue;
+
+    // Since the value is live into BB, it is either defined in a predecessor or
+    // live into it to.
+    LiveInBlockWorklist.append(pred_begin(BB), pred_end(BB));
   }
 
   // Determine where our MemoryPhi's should go
   IDFCalculator IDFs(*DT);
   IDFs.setDefiningBlocks(DefiningBlocks);
+  IDFs.setLiveInBlocks(LiveInBlocks);
   SmallVector<BasicBlock *, 32> IDFBlocks;
   IDFs.calculate(IDFBlocks);