Constant fold: getelementptr (i8* inttoptr (i64 1 to i8*), i32 -1)

Into: inttoptr (i64 0 to i8*)  -> null

This occurs in the example in PR1602.  With this fixed, we now compile
the example in PR1602 into fully "devirtualized" code:

define void @_Z1g1S(%struct.S* noalias  %s) {
entry:        %tmp131415 = getelementptr %struct.S* %s, i32 0, i32 0          ; <i32 (...)***> [#uses=1]        %tmp16 = load i32 (...)*** %tmp131415, align 4          ; <i32 (...)**> [#uses=1]
        %tmp26277 = load i32 (...)** %tmp16             ; <i32 (...)*> [#uses=1]
        %tmp2829 = bitcast i32 (...)* %tmp26277 to void (%struct.S*)*           ; <void (%struct.S*)*> [#uses=1]
        tail call void %tmp2829( %struct.S* %s )
        ret void
}

This still has the vtable dispatch (as required) but does not have any pointer
to method cruft left.

llvm-svn: 41046

1 files changed, 23 insertions, 1 deletions

diff --git a/llvm/lib/VMCore/ConstantFold.cpp b/llvm/lib/VMCore/ConstantFold.cpp
index a7662fd6a25..c950e8d07ff 100644
--- a/llvm/lib/VMCore/ConstantFold.cpp
+++ b/llvm/lib/VMCore/ConstantFold.cpp
@@ -1427,7 +1427,7 @@ Constant *llvm::ConstantFoldGetElementPtr(const Constant *C,
     //                        long 0, long 0)
     // To: int* getelementptr ([3 x int]* %X, long 0, long 0)
     //
-    if (CE->isCast() && NumIdx > 1 && Idx0->isNullValue())
+    if (CE->isCast() && NumIdx > 1 && Idx0->isNullValue()) {
       if (const PointerType *SPT =
           dyn_cast<PointerType>(CE->getOperand(0)->getType()))
         if (const ArrayType *SAT = dyn_cast<ArrayType>(SPT->getElementType()))
@@ -1436,6 +1436,28 @@ Constant *llvm::ConstantFoldGetElementPtr(const Constant *C,
             if (CAT->getElementType() == SAT->getElementType())
               return ConstantExpr::getGetElementPtr(
                       (Constant*)CE->getOperand(0), Idxs, NumIdx);
+    }
+    
+    // Fold: getelementptr (i8* inttoptr (i64 1 to i8*), i32 -1)
+    // Into: inttoptr (i64 0 to i8*)
+    // This happens with pointers to member functions in C++.
+    if (CE->getOpcode() == Instruction::IntToPtr && NumIdx == 1 &&
+        isa<ConstantInt>(CE->getOperand(0)) && isa<ConstantInt>(Idxs[0]) &&
+        cast<PointerType>(CE->getType())->getElementType() == Type::Int8Ty) {
+      Constant *Base = CE->getOperand(0);
+      Constant *Offset = Idxs[0];
+      
+      // Convert the smaller integer to the larger type.
+      if (Offset->getType()->getPrimitiveSizeInBits() < 
+          Base->getType()->getPrimitiveSizeInBits())
+        Offset = ConstantExpr::getSExt(Offset, Base->getType());
+      else if (Base->getType()->getPrimitiveSizeInBits() <
+               Offset->getType()->getPrimitiveSizeInBits())
+        Base = ConstantExpr::getZExt(Base, Base->getType());
+      
+      Base = ConstantExpr::getAdd(Base, Offset);
+      return ConstantExpr::getIntToPtr(Base, CE->getType());
+    }
   }
   return 0;
 }