commit cd2deacac6dc8c65288ce293f3e62067f94e2bf2
author Daniel Berlin <dberlin@dberlin.org> Thu Oct 20 20:13:45 2016 +0000
committer Daniel Berlin <dberlin@dberlin.org> Thu Oct 20 20:13:45 2016 +0000
tree b834364c405ec9085157ce1921ee372507026d1f
parent 0a2cd96e0527a1eb647016b32b13e9b98de6791a

[MSSA] Avoid unnecessary use walks when calling getClobberingMemoryAccess

Summary:
This allows us to mark when uses have been optimized.
This lets us avoid rewalking (IE when people call getClobberingAccess on everything), and also
enables us to later relax the requirement of use optimization during updates with less cost.

Reviewers: george.burgess.iv

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25172

llvm-svn: 284771

llvm/lib/Transforms/Utils/MemorySSA.cpp

diff --git a/llvm/lib/Transforms/Utils/MemorySSA.cpp b/llvm/lib/Transforms/Utils/MemorySSA.cpp
index b02a3f5..5c3ca75 100644
--- a/llvm/lib/Transforms/Utils/MemorySSA.cpp
+++ b/llvm/lib/Transforms/Utils/MemorySSA.cpp
@@ -1229,7 +1229,7 @@
 
 MemorySSA::MemorySSA(Function &Func, AliasAnalysis *AA, DominatorTree *DT)
     : AA(AA), DT(DT), F(Func), LiveOnEntryDef(nullptr), Walker(nullptr),
-      NextID(0) {
+      NextID(INVALID_MEMORYACCESS_ID) {
   buildMemorySSA();
 }
 
@@ -1332,7 +1332,7 @@
     }
 
     if (isUseTriviallyOptimizableToLiveOnEntry(*AA, MU->getMemoryInst())) {
-      MU->setDefiningAccess(MSSA->getLiveOnEntryDef());
+      MU->setDefiningAccess(MSSA->getLiveOnEntryDef(), true);
       continue;
     }
 
@@ -1428,13 +1428,13 @@
     // At the end of this loop, UpperBound is either a clobber, or lower bound
     // PHI walking may cause it to be < LowerBound, and in fact, < LastKill.
     if (FoundClobberResult || UpperBound < LocInfo.LastKill) {
-      MU->setDefiningAccess(VersionStack[UpperBound]);
+      MU->setDefiningAccess(VersionStack[UpperBound], true);
       // We were last killed now by where we got to
       LocInfo.LastKill = UpperBound;
     } else {
       // Otherwise, we checked all the new ones, and now we know we can get to
       // LastKill.
-      MU->setDefiningAccess(VersionStack[LocInfo.LastKill]);
+      MU->setDefiningAccess(VersionStack[LocInfo.LastKill], true);
     }
     LocInfo.LowerBound = VersionStack.size() - 1;
     LocInfo.LowerBoundBlock = BB;
@@ -1754,8 +1754,27 @@
   }
 
   // Re-point the uses at our defining access
-  if (!MA->use_empty())
-    MA->replaceAllUsesWith(NewDefTarget);
+  if (!MA->use_empty()) {
+    // Reset optimized on users of this store, and reset the uses.
+    // A few notes:
+    // 1. This is a slightly modified version of RAUW to avoid walking the
+    // uses twice here.
+    // 2. If we wanted to be complete, we would have to reset the optimized
+    // flags on users of phi nodes if doing the below makes a phi node have all
+    // the same arguments. Instead, we prefer users to removeMemoryAccess those
+    // phi nodes, because doing it here would be N^3.
+    if (MA->hasValueHandle())
+      ValueHandleBase::ValueIsRAUWd(MA, NewDefTarget);
+    // Note: We assume MemorySSA is not used in metadata since it's not really
+    // part of the IR.
+
+    while (!MA->use_empty()) {
+      Use &U = *MA->use_begin();
+      if (MemoryUse *MU = dyn_cast<MemoryUse>(U.getUser()))
+        MU->resetOptimized();
+      U.set(NewDefTarget);
+    }
+  }
 
   // The call below to erase will destroy MA, so we can't change the order we
   // are doing things here
@@ -2115,6 +2134,7 @@
   if (MemoryUse *MU = dyn_cast<MemoryUse>(MA)) {
     UpwardsMemoryQuery Q(MU->getMemoryInst(), MU);
     Cache.remove(MU, Q.StartingLoc, Q.IsCall);
+    MU->resetOptimized();
   } else {
     // If it is not a use, the best we can do right now is destroy the cache.
     Cache.clear();
@@ -2188,6 +2208,13 @@
   if (!StartingAccess)
     return MA;
 
+  // If this is an already optimized use or def, return the optimized result.
+  // Note: Currently, we do not store the optimized def result because we'd need
+  // a separate field, since we can't use it as the defining access.
+  if (MemoryUse *MU = dyn_cast<MemoryUse>(StartingAccess))
+    if (MU->isOptimized())
+      return MU->getDefiningAccess();
+
   const Instruction *I = StartingAccess->getMemoryInst();
   UpwardsMemoryQuery Q(I, StartingAccess);
   // We can't sanely do anything with a fences, they conservatively
@@ -2202,6 +2229,8 @@
   if (isUseTriviallyOptimizableToLiveOnEntry(*MSSA->AA, I)) {
     MemoryAccess *LiveOnEntry = MSSA->getLiveOnEntryDef();
     Cache.insert(StartingAccess, LiveOnEntry, Q.StartingLoc, Q.IsCall);
+    if (MemoryUse *MU = dyn_cast<MemoryUse>(StartingAccess))
+      MU->setDefiningAccess(LiveOnEntry, true);
     return LiveOnEntry;
   }
 
@@ -2218,6 +2247,8 @@
   DEBUG(dbgs() << *DefiningAccess << "\n");
   DEBUG(dbgs() << "Final Memory SSA clobber for " << *I << " is ");
   DEBUG(dbgs() << *Result << "\n");
+  if (MemoryUse *MU = dyn_cast<MemoryUse>(StartingAccess))
+    MU->setDefiningAccess(Result, true);
 
   return Result;
 }