If an alloca only has two types of uses: 1) reads 2) a memcpy/memmove that

copies from a constant global, then we can change the reads to read from the
global instead of from the alloca.  This eliminates the alloca and the memcpy,
and promotes secondary optimizations (because the loads are now loads from
a constant global).

This is important for a common C idiom:

void foo() {
   int A[] = {1,2,3,4,5,6,7,8,9...};
   ... only reads of A ...
}

For some reason, people forget to mark the array static or const.

This triggers on these multisource benchmarks:
JM/ldecode: block_pos, [3 x [4 x [4 x i32]]]
FreeBench/mason: m, [18 x i32], inlined 4 times
MiBench/office-stringsearch: search_strings, [1332 x i8*]
MiBench/office-stringsearch: find_strings, [1333 x i8*]
Prolangs-C++/city: dirs, [9 x i8*], inlined 4 places

and these spec benchmarks:
177.mesa: message, [8 x [32 x i8]]
186.crafty: bias_rl45, [64 x i32]
186.crafty: diag_sq, [64 x i32]
186.crafty: empty, [9 x i8]
186.crafty: xlate, [15 x i8]
186.crafty: status, [13 x i8]
186.crafty: bdinfo, [25 x i8]
445.gobmk: routines, [16 x i8*]
458.sjeng: piece_rep, [14 x i8*]
458.sjeng: t, [13 x i32], inlined 4 places.
464.h264ref: block8x8_idx, [3 x [4 x [4 x i32]]]
464.h264ref: block_pos, [3 x [4 x [4 x i32]]]
464.h264ref: j_off_tab, [12 x i32]

This implements Transforms/ScalarRepl/memcpy-from-global.ll

llvm-svn: 36429

lib/Transforms/Scalar/ScalarReplAggregates.cpp

@@ -24,6 +24,7 @@
 #include "llvm/Constants.h"
 #include "llvm/DerivedTypes.h"
 #include "llvm/Function.h"
+#include "llvm/GlobalVariable.h"
 #include "llvm/Instructions.h"
 #include "llvm/IntrinsicInst.h"
 #include "llvm/Pass.h"
@@ -42,6 +43,7 @@ using namespace llvm;
 STATISTIC(NumReplaced,  "Number of allocas broken up");
 STATISTIC(NumPromoted,  "Number of allocas promoted");
 STATISTIC(NumConverted, "Number of aggregates converted to scalar");
+STATISTIC(NumGlobals,   "Number of allocas copied from constant global");
 
 namespace {
   struct VISIBILITY_HIDDEN SROA : public FunctionPass {
@@ -76,6 +78,7 @@ namespace {
     const Type *CanConvertToScalar(Value *V, bool &IsNotTrivial);
     void ConvertToScalar(AllocationInst *AI, const Type *Ty);
     void ConvertUsesToScalar(Value *Ptr, AllocaInst *NewAI, unsigned Offset);
+    static Instruction *isOnlyCopiedFromConstantGlobal(AllocationInst *AI);
   };
 
   RegisterPass<SROA> X("scalarrepl", "Scalar Replacement of Aggregates");
@@ -165,9 +168,10 @@ bool SROA::performScalarRepl(Function &F) {
         continue;
       }
 
-    // We cannot transform the allocation instruction if it is an array
+    // Check to see if we can perform the core SROA transformation.  We cannot
-    // allocation (allocations OF arrays are ok though), and an allocation of a
+    // transform the allocation instruction if it is an array allocation
-    // scalar value cannot be decomposed at all.
+    // (allocations OF arrays are ok though), and an allocation of a scalar
+    // value cannot be decomposed at all.
     if (!AI->isArrayAllocation() &&
         (isa<StructType>(AI->getAllocatedType()) ||
          isa<ArrayType>(AI->getAllocatedType()))) {
@@ -186,6 +190,23 @@ bool SROA::performScalarRepl(Function &F) {
         continue;
       }
     }
+    
+    // Check to see if this allocation is only modified by a memcpy/memmove from
+    // a constant global.  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.
+    if (Instruction *TheCopy = isOnlyCopiedFromConstantGlobal(AI)) {
+      DOUT << "Found alloca equal to global: " << *AI;
+      DOUT << "  memcpy = " << *TheCopy;
+      Constant *TheSrc = cast<Constant>(TheCopy->getOperand(2));
+      AI->replaceAllUsesWith(ConstantExpr::getBitCast(TheSrc, AI->getType()));
+      TheCopy->eraseFromParent();  // Don't mutate the global.
+      AI->eraseFromParent();
+      ++NumGlobals;
+      Changed = true;
+      continue;
+    }
         
     // Otherwise, couldn't process this.
   }
@@ -197,7 +218,7 @@ bool SROA::performScalarRepl(Function &F) {
 /// predicate, do SROA now.
 void SROA::DoScalarReplacement(AllocationInst *AI, 
                                std::vector<AllocationInst*> &WorkList) {
-  DOUT << "Found inst to xform: " << *AI;
+  DOUT << "Found inst to SROA: " << *AI;
   SmallVector<AllocaInst*, 32> ElementAllocas;
   if (const StructType *ST = dyn_cast<StructType>(AI->getAllocatedType())) {
     ElementAllocas.reserve(ST->getNumContainedTypes());
@@ -1116,3 +1137,83 @@ void SROA::ConvertUsesToScalar(Value *Ptr, AllocaInst *NewAI, unsigned Offset) {
     }
   }
 }
+
+
+/// PointsToConstantGlobal - Return true if V (possibly indirectly) points to
+/// some part of a constant global variable.  This intentionally only accepts
+/// constant expressions because we don't 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, Instruction *&TheCopy,
+                                           bool isOffset) {
+  for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI!=E; ++UI) {
+    if (isa<LoadInst>(*UI)) {
+      // Ignore loads, they are always ok.
+      continue;
+    }
+    if (BitCastInst *BCI = dyn_cast<BitCastInst>(*UI)) {
+      // If uses of the bitcast are ok, we are ok.
+      if (!isOnlyCopiedFromConstantGlobal(BCI, TheCopy, isOffset))
+        return false;
+      continue;
+    }
+    if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
+      // If the GEP has all zero indices, it doesn't offset the pointer.  If it
+      // doesn't, it does.
+      if (!isOnlyCopiedFromConstantGlobal(GEP, TheCopy,
+                                         isOffset || !GEP->hasAllZeroIndices()))
+        return false;
+      continue;
+    }
+    
+    // If this is isn't our memcpy/memmove, reject it as something we can't
+    // handle.
+    if (!isa<MemCpyInst>(*UI) && !isa<MemMoveInst>(*UI))
+      return false;
+
+    // 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() != 1) return false;
+    
+    MemIntrinsic *MI = cast<MemIntrinsic>(*UI);
+    
+    // If the source of the memcpy/move is not a constant global, reject it.
+    if (!PointsToConstantGlobal(MI->getOperand(2)))
+      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.
+Instruction *SROA::isOnlyCopiedFromConstantGlobal(AllocationInst *AI) {
+  Instruction *TheCopy = 0;
+  if (::isOnlyCopiedFromConstantGlobal(AI, TheCopy, false))
+    return TheCopy;
+  return 0;
+}