[GVN] Introduce loop load PRE

This patch allows PRE of the following type of loads:

```
preheader:
  br label %loop

loop:
  br i1 ..., label %merge, label %clobber

clobber:
  call foo() // Clobbers %p
  br label %merge

merge:
  ...
  br i1 ..., label %loop, label %exit

```

Into
```
preheader:
  %x0 = load %p
  br label %loop

loop:
  %x.pre = phi(x0, x2)
  br i1 ..., label %merge, label %clobber

clobber:
  call foo() // Clobbers %p
  %x1 = load %p
  br label %merge

merge:
  x2 = phi(x.pre, x1)
  ...
  br i1 ..., label %loop, label %exit

```

So instead of loading from %p on every iteration, we load only when the actual clobber happens.
The typical pattern which it is trying to address is: hot loop, with all code inlined and
provably having no side effects, and some side-effecting calls on cold path.

The worst overhead from it is, if we always take clobber block, we make 1 more load
overall (in preheader). It only matters if loop has very few iteration. If clobber block is not taken
at least once, the transform is neutral or profitable.

There are several improvements prospect open up:
- We can sometimes be smarter in loop-exiting blocks via split of critical edges;
- If we have block frequency info, we can handle multiple clobbers. The only obstacle now is that
  we don't know if their sum is colder than the header.

Differential Revision: https://reviews.llvm.org/D99926
Reviewed By: reames

include/llvm/Transforms/Scalar/GVN.h

@@ -328,6 +328,12 @@ private:
   bool PerformLoadPRE(LoadInst *Load, AvailValInBlkVect &ValuesPerBlock,
                       UnavailBlkVect &UnavailableBlocks);
 
+  /// Try to replace a load which executes on each loop iteraiton with Phi
+  /// translation of load in preheader and load(s) in conditionally executed
+  /// paths.
+  bool performLoopLoadPRE(LoadInst *Load, AvailValInBlkVect &ValuesPerBlock,
+                          UnavailBlkVect &UnavailableBlocks);
+
   /// Eliminates partially redundant \p Load, replacing it with \p
   /// AvailableLoads (connected by Phis if needed).
   void eliminatePartiallyRedundantLoad(

lib/Transforms/Scalar/GVN.cpp

@@ -98,6 +98,7 @@ STATISTIC(NumGVNBlocks, "Number of blocks merged");
 STATISTIC(NumGVNSimpl, "Number of instructions simplified");
 STATISTIC(NumGVNEqProp, "Number of equalities propagated");
 STATISTIC(NumPRELoad, "Number of loads PRE'd");
+STATISTIC(NumPRELoopLoad, "Number of loop loads PRE'd");
 
 STATISTIC(IsValueFullyAvailableInBlockNumSpeculationsMax,
           "Number of blocks speculated as available in "
@@ -1447,6 +1448,84 @@ bool GVN::PerformLoadPRE(LoadInst *Load, AvailValInBlkVect &ValuesPerBlock,
   return true;
 }
 
+bool GVN::performLoopLoadPRE(LoadInst *Load, AvailValInBlkVect &ValuesPerBlock,
+                             UnavailBlkVect &UnavailableBlocks) {
+  if (!LI)
+    return false;
+
+  const Loop *L = LI->getLoopFor(Load->getParent());
+  // TODO: Generalize to other loop blocks that dominate the latch.
+  if (!L || L->getHeader() != Load->getParent())
+    return false;
+
+  BasicBlock *Preheader = L->getLoopPreheader();
+  BasicBlock *Latch = L->getLoopLatch();
+  if (!Preheader || !Latch)
+    return false;
+
+  Value *LoadPtr = Load->getPointerOperand();
+  // Must be available in preheader.
+  if (!L->isLoopInvariant(LoadPtr))
+    return false;
+
+  // We plan to hoist the load to preheader without introducing a new fault.
+  // In order to do it, we need to prove that we cannot side-exit the loop
+  // once loop header is first entered before execution of the load.
+  if (ICF->isDominatedByICFIFromSameBlock(Load))
+    return false;
+
+  BasicBlock *LoopBlock = nullptr;
+  for (auto *Blocker : UnavailableBlocks) {
+    // Blockers from outside the loop are handled in preheader.
+    if (!L->contains(Blocker))
+      continue;
+
+    // Only allow one loop block. Loop header is not less frequently executed
+    // than each loop block, and likely it is much more frequently executed. But
+    // in case of multiple loop blocks, we need extra information (such as block
+    // frequency info) to understand whether it is profitable to PRE into
+    // multiple loop blocks.
+    if (LoopBlock)
+      return false;
+
+    // Do not sink into inner loops. This may be non-profitable.
+    if (L != LI->getLoopFor(Blocker))
+      return false;
+
+    // Blocks that dominate the latch execute on every single iteration, maybe
+    // except the last one. So PREing into these blocks doesn't make much sense
+    // in most cases. But the blocks that do not necessarily execute on each
+    // iteration are sometimes much colder than the header, and this is when
+    // PRE is potentially profitable.
+    if (DT->dominates(Blocker, Latch))
+      return false;
+
+    // Make sure that the terminator itself doesn't clobber.
+    if (Blocker->getTerminator()->mayWriteToMemory())
+      return false;
+
+    LoopBlock = Blocker;
+  }
+
+  if (!LoopBlock)
+    return false;
+
+  // Make sure the memory at this pointer cannot be freed, therefore we can
+  // safely reload from it after clobber.
+  if (LoadPtr->canBeFreed())
+    return false;
+
+  // TODO: Support critical edge splitting if blocker has more than 1 successor.
+  MapVector<BasicBlock *, Value *> AvailableLoads;
+  AvailableLoads[LoopBlock] = LoadPtr;
+  AvailableLoads[Preheader] = LoadPtr;
+
+  LLVM_DEBUG(dbgs() << "GVN REMOVING PRE LOOP LOAD: " << *Load << '\n');
+  eliminatePartiallyRedundantLoad(Load, ValuesPerBlock, AvailableLoads);
+  ++NumPRELoopLoad;
+  return true;
+}
+
 static void reportLoadElim(LoadInst *Load, Value *AvailableValue,
                            OptimizationRemarkEmitter *ORE) {
   using namespace ore;
@@ -1544,7 +1623,8 @@ bool GVN::processNonLocalLoad(LoadInst *Load) {
   if (!isLoadInLoopPREEnabled() && LI && LI->getLoopFor(Load->getParent()))
     return Changed;
 
-  return Changed || PerformLoadPRE(Load, ValuesPerBlock, UnavailableBlocks);
+  return Changed || PerformLoadPRE(Load, ValuesPerBlock, UnavailableBlocks) ||
+         performLoopLoadPRE(Load, ValuesPerBlock, UnavailableBlocks);
 }
 
 static bool impliesEquivalanceIfTrue(CmpInst* Cmp) {

test/Transforms/GVN/PRE/pre-loop-load.ll

@@ -7,22 +7,25 @@ declare void @may_free_memory()
 
 declare i32 @personality_function()
 
-; TODO: We can PRE the load from gc-managed memory away from the hot path.
+; We can PRE the load from gc-managed memory away from the hot path.
 define i32 @test_load_on_cold_path_gc(i32 addrspace(1)* %p) gc "statepoint-example" personality i32 ()* @"personality_function" {
 ; CHECK-LABEL: @test_load_on_cold_path_gc(
 ; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[X_PRE1:%.*]] = load i32, i32 addrspace(1)* [[P:%.*]], align 4
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[BACKEDGE:%.*]] ]
-; CHECK-NEXT:    [[X:%.*]] = load i32, i32 addrspace(1)* [[P:%.*]], align 4
+; CHECK-NEXT:    [[X:%.*]] = phi i32 [ [[X_PRE1]], [[ENTRY:%.*]] ], [ [[X2:%.*]], [[BACKEDGE:%.*]] ]
+; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ 0, [[ENTRY]] ], [ [[IV_NEXT:%.*]], [[BACKEDGE]] ]
 ; CHECK-NEXT:    [[COND:%.*]] = icmp ne i32 [[X]], 0
 ; CHECK-NEXT:    br i1 [[COND]], label [[HOT_PATH:%.*]], label [[COLD_PATH:%.*]]
 ; CHECK:       hot_path:
 ; CHECK-NEXT:    br label [[BACKEDGE]]
 ; CHECK:       cold_path:
 ; CHECK-NEXT:    call void @may_free_memory()
+; CHECK-NEXT:    [[X_PRE:%.*]] = load i32, i32 addrspace(1)* [[P]], align 4
 ; CHECK-NEXT:    br label [[BACKEDGE]]
 ; CHECK:       backedge:
+; CHECK-NEXT:    [[X2]] = phi i32 [ [[X_PRE]], [[COLD_PATH]] ], [ [[X]], [[HOT_PATH]] ]
 ; CHECK-NEXT:    [[IV_NEXT]] = add i32 [[IV]], [[X]]
 ; CHECK-NEXT:    [[LOOP_COND:%.*]] = icmp ult i32 [[IV_NEXT]], 1000
 ; CHECK-NEXT:    br i1 [[LOOP_COND]], label [[LOOP]], label [[EXIT:%.*]]