Port the global copy optimization from the SROA pass to InstCombine.

This optimization is really just replacing allocas wholesale with
globals, there is no scalarization.

The underlying motivation for this patch is to simplify the SROA pass
and focus it on splitting and promoting allocas.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162271 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 171 insertions, 1 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index c485844aae..6ecb4c52c4 100644
--- a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -20,7 +20,154 @@
 #include "llvm/ADT/Statistic.h"
 using namespace llvm;
 
-STATISTIC(NumDeadStore, "Number of dead stores eliminated");
+STATISTIC(NumDeadStore,    "Number of dead stores eliminated");
+STATISTIC(NumGlobalCopies, "Number of allocas copied from constant global");
+
+/// pointsToConstantGlobal - Return true if V (possibly indirectly) points to
+/// some part of a constant global variable.  This intentionally only accepts
+/// constant expressions because we can't rewrite arbitrary instructions.
+static bool pointsToConstantGlobal(Value *V) {
+  if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
+    return GV->isConstant();
+  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
+    if (CE->getOpcode() == Instruction::BitCast ||
+        CE->getOpcode() == Instruction::GetElementPtr)
+      return pointsToConstantGlobal(CE->getOperand(0));
+  return false;
+}
+
+/// isOnlyCopiedFromConstantGlobal - Recursively walk the uses of a (derived)
+/// pointer to an alloca.  Ignore any reads of the pointer, return false if we
+/// see any stores or other unknown uses.  If we see pointer arithmetic, keep
+/// track of whether it moves the pointer (with IsOffset) but otherwise traverse
+/// the uses.  If we see a memcpy/memmove that targets an unoffseted pointer to
+/// the alloca, and if the source pointer is a pointer to a constant global, we
+/// can optimize this.
+static bool
+isOnlyCopiedFromConstantGlobal(Value *V, MemTransferInst *&TheCopy,
+                               SmallVectorImpl<Instruction *> &ToDelete,
+                               bool IsOffset = false) {
+  // We track lifetime intrinsics as we encounter them.  If we decide to go
+  // ahead and replace the value with the global, this lets the caller quickly
+  // eliminate the markers.
+
+  for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI!=E; ++UI) {
+    User *U = cast<Instruction>(*UI);
+
+    if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
+      // Ignore non-volatile loads, they are always ok.
+      if (!LI->isSimple()) return false;
+      continue;
+    }
+
+    if (BitCastInst *BCI = dyn_cast<BitCastInst>(U)) {
+      // If uses of the bitcast are ok, we are ok.
+      if (!isOnlyCopiedFromConstantGlobal(BCI, TheCopy, ToDelete, IsOffset))
+        return false;
+      continue;
+    }
+    if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(U)) {
+      // If the GEP has all zero indices, it doesn't offset the pointer.  If it
+      // doesn't, it does.
+      if (!isOnlyCopiedFromConstantGlobal(GEP, TheCopy, ToDelete,
+                                          IsOffset || !GEP->hasAllZeroIndices()))
+        return false;
+      continue;
+    }
+
+    if (CallSite CS = U) {
+      // If this is the function being called then we treat it like a load and
+      // ignore it.
+      if (CS.isCallee(UI))
+        continue;
+
+      // If this is a readonly/readnone call site, then we know it is just a
+      // load (but one that potentially returns the value itself), so we can
+      // ignore it if we know that the value isn't captured.
+      unsigned ArgNo = CS.getArgumentNo(UI);
+      if (CS.onlyReadsMemory() &&
+          (CS.getInstruction()->use_empty() || CS.doesNotCapture(ArgNo)))
+        continue;
+
+      // If this is being passed as a byval argument, the caller is making a
+      // copy, so it is only a read of the alloca.
+      if (CS.isByValArgument(ArgNo))
+        continue;
+    }
+
+    // Lifetime intrinsics can be handled by the caller.
+    if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(U)) {
+      if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
+          II->getIntrinsicID() == Intrinsic::lifetime_end) {
+        assert(II->use_empty() && "Lifetime markers have no result to use!");
+        ToDelete.push_back(II);
+        continue;
+      }
+    }
+
+    // If this is isn't our memcpy/memmove, reject it as something we can't
+    // handle.
+    MemTransferInst *MI = dyn_cast<MemTransferInst>(U);
+    if (MI == 0)
+      return false;
+
+    // If the transfer is using the alloca as a source of the transfer, then
+    // ignore it since it is a load (unless the transfer is volatile).
+    if (UI.getOperandNo() == 1) {
+      if (MI->isVolatile()) return false;
+      continue;
+    }
+
+    // If we already have seen a copy, reject the second one.
+    if (TheCopy) return false;
+
+    // If the pointer has been offset from the start of the alloca, we can't
+    // safely handle this.
+    if (IsOffset) return false;
+
+    // If the memintrinsic isn't using the alloca as the dest, reject it.
+    if (UI.getOperandNo() != 0) return false;
+
+    // If the source of the memcpy/move is not a constant global, reject it.
+    if (!pointsToConstantGlobal(MI->getSource()))
+      return false;
+
+    // Otherwise, the transform is safe.  Remember the copy instruction.
+    TheCopy = MI;
+  }
+  return true;
+}
+
+/// isOnlyCopiedFromConstantGlobal - Return true if the specified alloca is only
+/// modified by a copy from a constant global.  If we can prove this, we can
+/// replace any uses of the alloca with uses of the global directly.
+static MemTransferInst *
+isOnlyCopiedFromConstantGlobal(AllocaInst *AI,
+                               SmallVectorImpl<Instruction *> &ToDelete) {
+  MemTransferInst *TheCopy = 0;
+  if (isOnlyCopiedFromConstantGlobal(AI, TheCopy, ToDelete))
+    return TheCopy;
+  return 0;
+}
+
+/// getPointeeAlignment - Compute the minimum alignment of the value pointed
+/// to by the given pointer.
+static unsigned getPointeeAlignment(Value *V, const TargetData &TD) {
+  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
+    if (CE->getOpcode() == Instruction::BitCast ||
+        (CE->getOpcode() == Instruction::GetElementPtr &&
+         cast<GEPOperator>(CE)->hasAllZeroIndices()))
+      return getPointeeAlignment(CE->getOperand(0), TD);
+
+  if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
+    if (!GV->isDeclaration())
+      return TD.getPreferredAlignment(GV);
+
+  if (PointerType *PT = dyn_cast<PointerType>(V->getType()))
+    return TD.getABITypeAlignment(PT->getElementType());
+
+  return 0;
+}
 
 Instruction *InstCombiner::visitAllocaInst(AllocaInst &AI) {
   // Ensure that the alloca array size argument has type intptr_t, so that
@@ -113,6 +260,29 @@ Instruction *InstCombiner::visitAllocaInst(AllocaInst &AI) {
     }
   }
 
+  // Check to see if this allocation is only modified by a memcpy/memmove from
+  // a constant global whose alignment is equal to or exceeds that of the
+  // allocation.  If this is the case, we can change all users to use
+  // the constant global instead.  This is commonly produced by the CFE by
+  // constructs like "void foo() { int A[] = {1,2,3,4,5,6,7,8,9...}; }" if 'A'
+  // is only subsequently read.
+  SmallVector<Instruction *, 4> ToDelete;
+  if (MemTransferInst *Copy = isOnlyCopiedFromConstantGlobal(&AI, ToDelete)) {
+    if (AI.getAlignment() <= getPointeeAlignment(Copy->getSource(), *TD)) {
+      DEBUG(dbgs() << "Found alloca equal to global: " << AI << '\n');
+      DEBUG(dbgs() << "  memcpy = " << *Copy << '\n');
+      for (unsigned i = 0, e = ToDelete.size(); i != e; ++i)
+        EraseInstFromFunction(*ToDelete[i]);
+      Constant *TheSrc = cast<Constant>(Copy->getSource());
+      Instruction *NewI
+        = ReplaceInstUsesWith(AI, ConstantExpr::getBitCast(TheSrc,
+                                                           AI.getType()));
+      EraseInstFromFunction(*Copy);
+      ++NumGlobalCopies;
+      return NewI;
+    }
+  }
+
   // At last, use the generic allocation site handler to aggressively remove
   // unused allocas.
   return visitAllocSite(AI);