[BranchFolding] Merge MMOs during tail merge

Summary:
If we remove the MMOs from Load/Store instructions,
they are treated as volatile. This makes other optimization passes unhappy.
eg. Load/Store Optimization

So, it looks better to merge, not remove.

Reviewers: gberry, mcrosier

Subscribers: gberry, llvm-commits

Differential Revision: http://reviews.llvm.org/D14797

llvm-svn: 254694

1 files changed, 26 insertions, 16 deletions

diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index e41926a819c..3878281a4fe 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -744,24 +744,35 @@ bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
   return true;
 }
 
-static bool hasIdenticalMMOs(const MachineInstr *MI1, const MachineInstr *MI2) {
+// Add MI1's MMOs to MI2's MMOs while excluding any duplicates. The MI scheduler
+// currently doesn't handle multiple MMOs, so duplicates would likely pessimize
+// the scheduler.
+static void mergeMMOs(MachineInstr *MI1, MachineInstr *MI2) {
   auto I1 = MI1->memoperands_begin(), E1 = MI1->memoperands_end();
   auto I2 = MI2->memoperands_begin(), E2 = MI2->memoperands_end();
-  if ((E1 - I1) != (E2 - I2))
-    return false;
-  for (; I1 != E1; ++I1, ++I2) {
-    if (**I1 != **I2)
-      return false;
+  MachineFunction *MF = MI1->getParent()->getParent();
+
+  // Mostly, MI1's MMO count is 1 or zero. So we don't have to use
+  // SmallSet.
+  for (; I1 != E1; ++I1) {
+    bool IsDupMMO = false;
+    for (I2 = MI2->memoperands_begin(); I2 != E2; ++I2) {
+      if (**I1 == **I2) {
+        IsDupMMO = true;
+        break;
+      }
+    }
+    if (IsDupMMO == false) {
+      MI2->addMemOperand(*MF, *I1);
+      E2 = MI2->memoperands_end();
+    }
   }
-  return true;
 }
 
 static void
-removeMMOsFromMemoryOperations(MachineBasicBlock::iterator MBBIStartPos,
-                               MachineBasicBlock &MBBCommon) {
-  // Remove MMOs from memory operations in the common block
-  // when they do not match the ones from the block being tail-merged.
-  // This ensures later passes conservatively compute dependencies.
+mergeMMOsFromMemoryOperations(MachineBasicBlock::iterator MBBIStartPos,
+                              MachineBasicBlock &MBBCommon) {
+  // Merge MMOs from memory operations in the common block
   MachineBasicBlock *MBB = MBBIStartPos->getParent();
   // Note CommonTailLen does not necessarily matches the size of
   // the common BB nor all its instructions because of debug
@@ -792,8 +803,7 @@ removeMMOsFromMemoryOperations(MachineBasicBlock::iterator MBBIStartPos,
     assert(MBBICommon->isIdenticalTo(&*MBBI) && "Expected matching MIIs!");
 
     if (MBBICommon->mayLoad() || MBBICommon->mayStore())
-      if (!hasIdenticalMMOs(&*MBBI, &*MBBICommon))
-        MBBICommon->clearMemRefs();
+      mergeMMOs(&*MBBI, &*MBBICommon);
 
     ++MBBI;
     ++MBBICommon;
@@ -913,8 +923,8 @@ bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
         continue;
       DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber()
                    << (i == e-1 ? "" : ", "));
-      // Remove MMOs from memory operations as needed.
-      removeMMOsFromMemoryOperations(SameTails[i].getTailStartPos(), *MBB);
+      // Merge MMOs from memory operations as needed.
+      mergeMMOsFromMemoryOperations(SameTails[i].getTailStartPos(), *MBB);
       // Hack the end off BB i, making it jump to BB commonTailIndex instead.
       ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
       // BB i is no longer a predecessor of SuccBB; remove it from the worklist.