fix PR8710 - teach global opt that some constantexprs are too complex to

put in a global variable's initializer.

llvm-svn: 121100

1 files changed, 85 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
index f66e70d43dc..366179563d6 100644
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -2046,14 +2046,82 @@ static GlobalVariable *InstallGlobalCtors(GlobalVariable *GCL,
 }
 
 
-static Constant *getVal(DenseMap<Value*, Constant*> &ComputedValues,
-                        Value *V) {
+static Constant *getVal(DenseMap<Value*, Constant*> &ComputedValues, Value *V) {
   if (Constant *CV = dyn_cast<Constant>(V)) return CV;
   Constant *R = ComputedValues[V];
   assert(R && "Reference to an uncomputed value!");
   return R;
 }
 
+static inline bool 
+isSimpleEnoughValueToCommit(Constant *C,
+                            SmallPtrSet<Constant*, 8> &SimpleConstants);
+
+
+/// isSimpleEnoughValueToCommit - Return true if the specified constant can be
+/// handled by the code generator.  We don't want to generate something like:
+///   void *X = &X/42;
+/// because the code generator doesn't have a relocation that can handle that.
+///
+/// This function should be called if C was not found (but just got inserted)
+/// in SimpleConstants to avoid having to rescan the same constants all the
+/// time.
+static bool isSimpleEnoughValueToCommitHelper(Constant *C,
+                                   SmallPtrSet<Constant*, 8> &SimpleConstants) {
+  // Simple integer, undef, constant aggregate zero, global addresses, etc are
+  // all supported.
+  if (C->getNumOperands() == 0 || isa<BlockAddress>(C) ||
+      isa<GlobalValue>(C))
+    return true;
+  
+  // Aggregate values are safe if all their elements are.
+  if (isa<ConstantArray>(C) || isa<ConstantStruct>(C) ||
+      isa<ConstantVector>(C)) {
+    for (unsigned i = 0, e = C->getNumOperands(); i != e; ++i) {
+      Constant *Op = cast<Constant>(C->getOperand(i));
+      if (!isSimpleEnoughValueToCommit(Op, SimpleConstants))
+        return false;
+    }
+    return true;
+  }
+  
+  // We don't know exactly what relocations are allowed in constant expressions,
+  // so we allow &global+constantoffset, which is safe and uniformly supported
+  // across targets.
+  ConstantExpr *CE = cast<ConstantExpr>(C);
+  switch (CE->getOpcode()) {
+  case Instruction::BitCast:
+  case Instruction::IntToPtr:
+  case Instruction::PtrToInt:
+    // These casts are always fine if the casted value is.
+    return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants);
+      
+  // GEP is fine if it is simple + constant offset.
+  case Instruction::GetElementPtr:
+    for (unsigned i = 1, e = CE->getNumOperands(); i != e; ++i)
+      if (!isa<ConstantInt>(CE->getOperand(i)))
+        return false;
+    return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants);
+      
+  case Instruction::Add:
+    // We allow simple+cst.
+    if (!isa<ConstantInt>(CE->getOperand(1)))
+      return false;
+    return isSimpleEnoughValueToCommit(CE->getOperand(0), SimpleConstants);
+  }
+  return false;
+}
+
+static inline bool 
+isSimpleEnoughValueToCommit(Constant *C,
+                            SmallPtrSet<Constant*, 8> &SimpleConstants) {
+  // If we already checked this constant, we win.
+  if (!SimpleConstants.insert(C)) return true;
+  // Check the constant.
+  return isSimpleEnoughValueToCommitHelper(C, SimpleConstants);
+}
+
+
 /// isSimpleEnoughPointerToCommit - Return true if this constant is simple
 /// enough for us to understand.  In particular, if it is a cast of something,
 /// we punt.  We basically just support direct accesses to globals and GEP's of
@@ -2219,7 +2287,8 @@ static bool EvaluateFunction(Function *F, Constant *&RetVal,
                              const SmallVectorImpl<Constant*> &ActualArgs,
                              std::vector<Function*> &CallStack,
                              DenseMap<Constant*, Constant*> &MutatedMemory,
-                             std::vector<GlobalVariable*> &AllocaTmps) {
+                             std::vector<GlobalVariable*> &AllocaTmps,
+                             SmallPtrSet<Constant*, 8> &SimpleConstants) {
   // Check to see if this function is already executing (recursion).  If so,
   // bail out.  TODO: we might want to accept limited recursion.
   if (std::find(CallStack.begin(), CallStack.end(), F) != CallStack.end())
@@ -2254,7 +2323,13 @@ static bool EvaluateFunction(Function *F, Constant *&RetVal,
       if (!isSimpleEnoughPointerToCommit(Ptr))
         // If this is too complex for us to commit, reject it.
         return false;
+      
       Constant *Val = getVal(Values, SI->getOperand(0));
+
+      // If this might be too difficult for the backend to handle (e.g. the addr
+      // of one global variable divided by another) then we can't commit it.
+      if (!isSimpleEnoughValueToCommit(Val, SimpleConstants))
+        return false;
       MutatedMemory[Ptr] = Val;
     } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(CurInst)) {
       InstResult = ConstantExpr::get(BO->getOpcode(),
@@ -2331,7 +2406,7 @@ static bool EvaluateFunction(Function *F, Constant *&RetVal,
         Constant *RetVal;
         // Execute the call, if successful, use the return value.
         if (!EvaluateFunction(Callee, RetVal, Formals, CallStack,
-                              MutatedMemory, AllocaTmps))
+                              MutatedMemory, AllocaTmps, SimpleConstants))
           return false;
         InstResult = RetVal;
       }
@@ -2417,11 +2492,16 @@ static bool EvaluateStaticConstructor(Function *F) {
   /// unbounded.
   std::vector<Function*> CallStack;
 
+  /// SimpleConstants - These are constants we have checked and know to be
+  /// simple enough to live in a static initializer of a global.
+  SmallPtrSet<Constant*, 8> SimpleConstants;
+  
   // Call the function.
   Constant *RetValDummy;
   bool EvalSuccess = EvaluateFunction(F, RetValDummy,
                                       SmallVector<Constant*, 0>(), CallStack,
-                                      MutatedMemory, AllocaTmps);
+                                      MutatedMemory, AllocaTmps,
+                                      SimpleConstants);
   if (EvalSuccess) {
     // We succeeded at evaluation: commit the result.
     DEBUG(dbgs() << "FULLY EVALUATED GLOBAL CTOR FUNCTION '"