[memcpyopt] Restructure store(load src, dest) form of callslotopt for compile time

The search for the clobbering call is fairly expensive if uses are not optimized at construction.  Defer the clobber walk to the point in the implementation we need it; there are a bunch of bailouts before that point.  (e.g. If the source pointer is not an alloca, we can't do callslotopt.)

On a test case which involves a bunch of copies from argument pointers, this switches memcpyopt from > 1/2 second to < 10ms.

Author: preames

llvm/include/llvm/Transforms/Scalar/MemCpyOptimizer.h

@@ -61,7 +61,7 @@ class MemCpyOptPass : public PassInfoMixin<MemCpyOptPass> {
   bool processMemMove(MemMoveInst *M);
   bool performCallSlotOptzn(Instruction *cpyLoad, Instruction *cpyStore,
                             Value *cpyDst, Value *cpySrc, TypeSize cpyLen,
-                            Align cpyAlign, CallInst *C);
+                            Align cpyAlign, std::function<CallInst *()> GetC);
   bool processMemCpyMemCpyDependence(MemCpyInst *M, MemCpyInst *MDep);
   bool processMemSetMemCpyDependence(MemCpyInst *MemCpy, MemSetInst *MemSet);
   bool performMemCpyToMemSetOptzn(MemCpyInst *MemCpy, MemSetInst *MemSet);

llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp

@@ -761,27 +761,25 @@ bool MemCpyOptPass::processStore(StoreInst *SI, BasicBlock::iterator &BBI) {
       // Detect cases where we're performing call slot forwarding, but
       // happen to be using a load-store pair to implement it, rather than
       // a memcpy.
-      CallInst *C = nullptr;
-      if (auto *LoadClobber = dyn_cast<MemoryUseOrDef>(
-              MSSA->getWalker()->getClobberingMemoryAccess(LI))) {
-        // The load most post-dom the call. Limit to the same block for now.
-        // TODO: Support non-local call-slot optimization?
-        if (LoadClobber->getBlock() == SI->getParent())
-          C = dyn_cast_or_null<CallInst>(LoadClobber->getMemoryInst());
-      }
-
-      if (C) {
-        bool changed = performCallSlotOptzn(
-            LI, SI, SI->getPointerOperand()->stripPointerCasts(),
-            LI->getPointerOperand()->stripPointerCasts(),
-            DL.getTypeStoreSize(SI->getOperand(0)->getType()),
-            commonAlignment(SI->getAlign(), LI->getAlign()), C);
-        if (changed) {
-          eraseInstruction(SI);
-          eraseInstruction(LI);
-          ++NumMemCpyInstr;
-          return true;
-        }
+      auto GetCall = [&]() -> CallInst * {
+        // We defer this expensive clobber walk until the cheap checks
+        // have been done on the source inside performCallSlotOptzn.
+        if (auto *LoadClobber = dyn_cast<MemoryUseOrDef>(
+              MSSA->getWalker()->getClobberingMemoryAccess(LI)))
+          return dyn_cast_or_null<CallInst>(LoadClobber->getMemoryInst());
+        return nullptr;
+      };
+
+      bool changed = performCallSlotOptzn(
+          LI, SI, SI->getPointerOperand()->stripPointerCasts(),
+          LI->getPointerOperand()->stripPointerCasts(),
+          DL.getTypeStoreSize(SI->getOperand(0)->getType()),
+          commonAlignment(SI->getAlign(), LI->getAlign()), GetCall);
+      if (changed) {
+        eraseInstruction(SI);
+        eraseInstruction(LI);
+        ++NumMemCpyInstr;
+        return true;
       }
     }
   }
@@ -856,7 +854,8 @@ bool MemCpyOptPass::processMemSet(MemSetInst *MSI, BasicBlock::iterator &BBI) {
 bool MemCpyOptPass::performCallSlotOptzn(Instruction *cpyLoad,
                                          Instruction *cpyStore, Value *cpyDest,
                                          Value *cpySrc, TypeSize cpySize,
-                                         Align cpyAlign, CallInst *C) {
+                                         Align cpyAlign,
+                                         std::function<CallInst *()> GetC) {
   // The general transformation to keep in mind is
   //
   //   call @func(..., src, ...)
@@ -875,11 +874,6 @@ bool MemCpyOptPass::performCallSlotOptzn(Instruction *cpyLoad,
   if (cpySize.isScalable())
     return false;
 
-  // Lifetime marks shouldn't be operated on.
-  if (Function *F = C->getCalledFunction())
-    if (F->isIntrinsic() && F->getIntrinsicID() == Intrinsic::lifetime_start)
-      return false;
-
   // Require that src be an alloca.  This simplifies the reasoning considerably.
   auto *srcAlloca = dyn_cast<AllocaInst>(cpySrc);
   if (!srcAlloca)
@@ -896,6 +890,16 @@ bool MemCpyOptPass::performCallSlotOptzn(Instruction *cpyLoad,
   if (cpySize < srcSize)
     return false;
 
+  CallInst *C = GetC();
+  if (!C)
+    return false;
+
+  // Lifetime marks shouldn't be operated on.
+  if (Function *F = C->getCalledFunction())
+    if (F->isIntrinsic() && F->getIntrinsicID() == Intrinsic::lifetime_start)
+      return false;
+
+
   if (C->getParent() != cpyStore->getParent()) {
     LLVM_DEBUG(dbgs() << "Call Slot: block local restriction\n");
     return false;
@@ -1459,7 +1463,7 @@ bool MemCpyOptPass::processMemCpy(MemCpyInst *M, BasicBlock::iterator &BBI) {
           if (performCallSlotOptzn(
                   M, M, M->getDest(), M->getSource(),
                   TypeSize::getFixed(CopySize->getZExtValue()), Alignment,
-                  C)) {
+                  [C]() -> CallInst * { return C; })) {
             LLVM_DEBUG(dbgs() << "Performed call slot optimization:\n"
                               << "    call: " << *C << "\n"
                               << "    memcpy: " << *M << "\n");