[LICM] Sink unused l-invariant loads in preheader. (#157559)

Unused loop invariant loads were not sunk from the preheader to the exit
block, increasing live range.

This commit moves the sinkUnusedInvariant logic from indvarsimplify to
LICM also adds functionality to sink unused load that's not
clobbered by the loop body.

Author: VigneshwarJ

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

@@ -162,8 +162,6 @@ class IndVarSimplify {
                                  const SCEV *ExitCount,
                                  PHINode *IndVar, SCEVExpander &Rewriter);
 
-  bool sinkUnusedInvariants(Loop *L);
-
 public:
   IndVarSimplify(LoopInfo *LI, ScalarEvolution *SE, DominatorTree *DT,
                  const DataLayout &DL, TargetLibraryInfo *TLI,
@@ -1079,85 +1077,6 @@ linearFunctionTestReplace(Loop *L, BasicBlock *ExitingBB,
   return true;
 }
 
-//===----------------------------------------------------------------------===//
-//  sinkUnusedInvariants. A late subpass to cleanup loop preheaders.
-//===----------------------------------------------------------------------===//
-
-/// If there's a single exit block, sink any loop-invariant values that
-/// were defined in the preheader but not used inside the loop into the
-/// exit block to reduce register pressure in the loop.
-bool IndVarSimplify::sinkUnusedInvariants(Loop *L) {
-  BasicBlock *ExitBlock = L->getExitBlock();
-  if (!ExitBlock) return false;
-
-  BasicBlock *Preheader = L->getLoopPreheader();
-  if (!Preheader) return false;
-
-  bool MadeAnyChanges = false;
-  for (Instruction &I : llvm::make_early_inc_range(llvm::reverse(*Preheader))) {
-
-    // Skip BB Terminator.
-    if (Preheader->getTerminator() == &I)
-      continue;
-
-    // New instructions were inserted at the end of the preheader.
-    if (isa<PHINode>(I))
-      break;
-
-    // Don't move instructions which might have side effects, since the side
-    // effects need to complete before instructions inside the loop.  Also don't
-    // move instructions which might read memory, since the loop may modify
-    // memory. Note that it's okay if the instruction might have undefined
-    // behavior: LoopSimplify guarantees that the preheader dominates the exit
-    // block.
-    if (I.mayHaveSideEffects() || I.mayReadFromMemory())
-      continue;
-
-    // Skip debug or pseudo instructions.
-    if (I.isDebugOrPseudoInst())
-      continue;
-
-    // Skip eh pad instructions.
-    if (I.isEHPad())
-      continue;
-
-    // Don't sink alloca: we never want to sink static alloca's out of the
-    // entry block, and correctly sinking dynamic alloca's requires
-    // checks for stacksave/stackrestore intrinsics.
-    // FIXME: Refactor this check somehow?
-    if (isa<AllocaInst>(&I))
-      continue;
-
-    // Determine if there is a use in or before the loop (direct or
-    // otherwise).
-    bool UsedInLoop = false;
-    for (Use &U : I.uses()) {
-      Instruction *User = cast<Instruction>(U.getUser());
-      BasicBlock *UseBB = User->getParent();
-      if (PHINode *P = dyn_cast<PHINode>(User)) {
-        unsigned i =
-          PHINode::getIncomingValueNumForOperand(U.getOperandNo());
-        UseBB = P->getIncomingBlock(i);
-      }
-      if (UseBB == Preheader || L->contains(UseBB)) {
-        UsedInLoop = true;
-        break;
-      }
-    }
-
-    // If there is, the def must remain in the preheader.
-    if (UsedInLoop)
-      continue;
-
-    // Otherwise, sink it to the exit block.
-    I.moveBefore(ExitBlock->getFirstInsertionPt());
-    SE->forgetValue(&I);
-    MadeAnyChanges = true;
-  }
-
-  return MadeAnyChanges;
-}
-
 static void replaceExitCond(BranchInst *BI, Value *NewCond,
                             SmallVectorImpl<WeakTrackingVH> &DeadInsts) {
   auto *OldCond = BI->getCondition();
@@ -2065,10 +1984,6 @@ bool IndVarSimplify::run(Loop *L) {
 
   // The Rewriter may not be used from this point on.
 
-  // Loop-invariant instructions in the preheader that aren't used in the
-  // loop may be sunk below the loop to reduce register pressure.
-  Changed |= sinkUnusedInvariants(L);
-
   // rewriteFirstIterationLoopExitValues does not rely on the computation of
   // trip count and therefore can further simplify exit values in addition to
   // rewriteLoopExitValues.

llvm/lib/Transforms/Scalar/LICM.cpp

@@ -211,9 +211,15 @@ static Instruction *cloneInstructionInExitBlock(
 static void eraseInstruction(Instruction &I, ICFLoopSafetyInfo &SafetyInfo,
                              MemorySSAUpdater &MSSAU);
 
-static void moveInstructionBefore(Instruction &I, BasicBlock::iterator Dest,
-                                  ICFLoopSafetyInfo &SafetyInfo,
-                                  MemorySSAUpdater &MSSAU, ScalarEvolution *SE);
+static void moveInstructionBefore(
+    Instruction &I, BasicBlock::iterator Dest, ICFLoopSafetyInfo &SafetyInfo,
+    MemorySSAUpdater &MSSAU, ScalarEvolution *SE,
+    MemorySSA::InsertionPlace Point = MemorySSA::BeforeTerminator);
+
+static bool sinkUnusedInvariantsFromPreheaderToExit(
+    Loop *L, AAResults *AA, ICFLoopSafetyInfo *SafetyInfo,
+    MemorySSAUpdater &MSSAU, ScalarEvolution *SE, DominatorTree *DT,
+    SinkAndHoistLICMFlags &SinkFlags, OptimizationRemarkEmitter *ORE);
 
 static void foreachMemoryAccess(MemorySSA *MSSA, Loop *L,
                                 function_ref<void(Instruction *)> Fn);
@@ -471,6 +477,12 @@ bool LoopInvariantCodeMotion::runOnLoop(Loop *L, AAResults *AA, LoopInfo *LI,
                                     TLI, TTI, L, MSSAU, &SafetyInfo, Flags, ORE)
             : sinkRegion(DT->getNode(L->getHeader()), AA, LI, DT, TLI, TTI, L,
                          MSSAU, &SafetyInfo, Flags, ORE);
+
+  // sink pre-header defs that are unused in-loop into the unique exit to reduce
+  // pressure.
+  Changed |= sinkUnusedInvariantsFromPreheaderToExit(L, AA, &SafetyInfo, MSSAU,
+                                                     SE, DT, Flags, ORE);
+
   Flags.setIsSink(false);
   if (Preheader)
     Changed |= hoistRegion(DT->getNode(L->getHeader()), AA, LI, DT, AC, TLI, L,
@@ -1456,19 +1468,80 @@ static void eraseInstruction(Instruction &I, ICFLoopSafetyInfo &SafetyInfo,
 
 static void moveInstructionBefore(Instruction &I, BasicBlock::iterator Dest,
                                   ICFLoopSafetyInfo &SafetyInfo,
-                                  MemorySSAUpdater &MSSAU,
-                                  ScalarEvolution *SE) {
+                                  MemorySSAUpdater &MSSAU, ScalarEvolution *SE,
+                                  MemorySSA::InsertionPlace Point) {
   SafetyInfo.removeInstruction(&I);
   SafetyInfo.insertInstructionTo(&I, Dest->getParent());
   I.moveBefore(*Dest->getParent(), Dest);
   if (MemoryUseOrDef *OldMemAcc = cast_or_null<MemoryUseOrDef>(
           MSSAU.getMemorySSA()->getMemoryAccess(&I)))
-    MSSAU.moveToPlace(OldMemAcc, Dest->getParent(),
-                      MemorySSA::BeforeTerminator);
+    MSSAU.moveToPlace(OldMemAcc, Dest->getParent(), Point);
   if (SE)
     SE->forgetBlockAndLoopDispositions(&I);
 }
 
+// If there's a single exit block, sink any loop-invariant values that were
+// defined in the preheader but not used inside the loop into the exit block
+// to reduce register pressure in the loop.
+static bool sinkUnusedInvariantsFromPreheaderToExit(
+    Loop *L, AAResults *AA, ICFLoopSafetyInfo *SafetyInfo,
+    MemorySSAUpdater &MSSAU, ScalarEvolution *SE, DominatorTree *DT,
+    SinkAndHoistLICMFlags &SinkFlags, OptimizationRemarkEmitter *ORE) {
+  BasicBlock *ExitBlock = L->getExitBlock();
+  if (!ExitBlock)
+    return false;
+
+  BasicBlock *Preheader = L->getLoopPreheader();
+  if (!Preheader)
+    return false;
+
+  bool MadeAnyChanges = false;
+
+  for (Instruction &I : llvm::make_early_inc_range(llvm::reverse(*Preheader))) {
+
+    // Skip terminator.
+    if (Preheader->getTerminator() == &I)
+      continue;
+
+    // New instructions were inserted at the end of the preheader.
+    if (isa<PHINode>(I))
+      break;
+
+    // Don't move instructions which might have side effects, since the side
+    // effects need to complete before instructions inside the loop. Note that
+    // it's okay if the instruction might have undefined behavior: LoopSimplify
+    // guarantees that the preheader dominates the exit block.
+    if (I.mayHaveSideEffects())
+      continue;
+
+    if (!canSinkOrHoistInst(I, AA, DT, L, MSSAU, true, SinkFlags, nullptr))
+      continue;
+
+    // Determine if there is a use in or before the loop (direct or
+    // otherwise).
+    bool UsedInLoopOrPreheader = false;
+    for (Use &U : I.uses()) {
+      auto *UserI = cast<Instruction>(U.getUser());
+      BasicBlock *UseBB = UserI->getParent();
+      if (auto *PN = dyn_cast<PHINode>(UserI)) {
+        UseBB = PN->getIncomingBlock(U);
+      }
+      if (UseBB == Preheader || L->contains(UseBB)) {
+        UsedInLoopOrPreheader = true;
+        break;
+      }
+    }
+    if (UsedInLoopOrPreheader)
+      continue;
+
+    moveInstructionBefore(I, ExitBlock->getFirstInsertionPt(), *SafetyInfo,
+                          MSSAU, SE, MemorySSA::Beginning);
+    MadeAnyChanges = true;
+  }
+
+  return MadeAnyChanges;
+}
+
 static Instruction *sinkThroughTriviallyReplaceablePHI(
     PHINode *TPN, Instruction *I, LoopInfo *LI,
     SmallDenseMap<BasicBlock *, Instruction *, 32> &SunkCopies,

llvm/test/CodeGen/AMDGPU/schedule-amdgpu-trackers.ll

@@ -73,10 +73,10 @@ define amdgpu_kernel void @constant_zextload_v64i16_to_v64i32(ptr addrspace(1) %
 }
 
 ; CHECK-LABEL: {{^}}excess_soft_clause_reg_pressure:
-; GFX908:    NumSgprs: 64
-; GFX908-GCNTRACKERS:    NumSgprs: 64
+; GFX908:    NumSgprs: 56
+; GFX908-GCNTRACKERS:    NumSgprs: 56
 ; GFX908:    NumVgprs: 43
-; GFX908-GCNTRACKERS:    NumVgprs: 39
+; GFX908-GCNTRACKERS:    NumVgprs: 40
 ; GFX908:    Occupancy: 5
 ; GFX908-GCNTRACKERS:    Occupancy: 6
 

llvm/test/CodeGen/AMDGPU/vgpr-spill-emergency-stack-slot.ll

@@ -22,8 +22,6 @@
 ; GFX9-DAG: s_mov_b32 s[[DESC3:[0-9]+]], 0xe00000
 
 ; OFFREG is offset system SGPR
-; GCN: buffer_store_dword {{v[0-9]+}}, off, s[[[DESC0]]:[[DESC3]]], 0 offset:{{[0-9]+}} ; 4-byte Folded Spill
-; GCN: buffer_load_dword v{{[0-9]+}}, off, s[[[DESC0]]:[[DESC3]]], 0 offset:{{[0-9]+}} ; 4-byte Folded Reload
 ; GCN: NumVgprs: 256
 ; GCN: ScratchSize: 640