Teach loop rotate to hoist trivially invariant instructions

in the duplicated block instead of duplicating them.  

Duplicating them into the end of the loop and the preheader 
means that we got a phi node in the header of the loop, 
which prevented LICM from hoisting them.  GVN would
usually come around later and merge the duplicated 
instructions so we'd get reasonable output... except that
anything dependent on the shoulda-been-hoisted value can't
be hoisted.  In PR5319 (which this fixes), a memory value
didn't get promoted.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113134 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 27 insertions, 10 deletions

diff --git a/lib/Transforms/Scalar/LoopRotation.cpp b/lib/Transforms/Scalar/LoopRotation.cpp
index 65acc1d925..a0d07726fe 100644
--- a/lib/Transforms/Scalar/LoopRotation.cpp
+++ b/lib/Transforms/Scalar/LoopRotation.cpp
@@ -143,11 +143,11 @@ bool LoopRotate::rotateLoop(Loop *Lp, LPPassManager &LPM) {
   // FIXME: Use common api to estimate size.
   for (BasicBlock::const_iterator OI = OrigHeader->begin(), 
          OE = OrigHeader->end(); OI != OE; ++OI) {
-      if (isa<PHINode>(OI)) 
-        continue;           // PHI nodes don't count.
-      if (isa<DbgInfoIntrinsic>(OI))
-        continue;  // Debug intrinsics don't count as size.
-      ++Size;
+    if (isa<PHINode>(OI)) 
+      continue;           // PHI nodes don't count.
+    if (isa<DbgInfoIntrinsic>(OI))
+      continue;  // Debug intrinsics don't count as size.
+    ++Size;
   }
 
   if (Size > MAX_HEADER_SIZE)
@@ -187,13 +187,30 @@ bool LoopRotate::rotateLoop(Loop *Lp, LPPassManager &LPM) {
   for (; PHINode *PN = dyn_cast<PHINode>(I); ++I)
     ValueMap[PN] = PN->getIncomingValue(PN->getBasicBlockIndex(OrigPreHeader));
 
-  // For the rest of the instructions, create a clone in the OldPreHeader.
+  // For the rest of the instructions, either hoist to the OrigPreheader if
+  // possible or create a clone in the OldPreHeader if not.
   TerminatorInst *LoopEntryBranch = OrigPreHeader->getTerminator();
-  for (; I != E; ++I) {
-    Instruction *C = I->clone();
-    C->setName(I->getName());
+  while (I != E) {
+    Instruction *Inst = I++;
+    
+    // If the instruction's operands are invariant and it doesn't read or write
+    // memory, then it is safe to hoist.  Doing this doesn't change the order of
+    // execution in the preheader, but does prevent the instruction from
+    // executing in each iteration of the loop.  This means it is safe to hoist
+    // something that might trap, but isn't safe to hoist something that reads
+    // memory (without proving that the loop doesn't write).
+    if (L->hasLoopInvariantOperands(Inst) &&
+        !Inst->mayReadFromMemory() && !Inst->mayWriteToMemory() &&
+        !isa<TerminatorInst>(Inst)) {
+      Inst->moveBefore(LoopEntryBranch);
+      continue;
+    }
+    
+    // Otherwise, create a duplicate of the instruction.
+    Instruction *C = Inst->clone();
+    C->setName(Inst->getName());
     C->insertBefore(LoopEntryBranch);
-    ValueMap[I] = C;
+    ValueMap[Inst] = C;
   }
 
   // Along with all the other instructions, we just cloned OrigHeader's