From c1da204c433459713cbf4c62f0addf458e6b4b9f Mon Sep 17 00:00:00 2001
From: Chris Lattner <sabre@nondot.org>
Date: Sun, 21 Nov 2010 08:39:01 +0000
Subject: [PATCH] apply Dan's fix for PR8268 which allows constant folding to
 handle indexes over zero sized elements.  This allows us to compile:

  #include <string>
  void foo() { std::string s; }

into an empty function.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119933 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/ConstantFolding.cpp | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/lib/Analysis/ConstantFolding.cpp b/lib/Analysis/ConstantFolding.cpp
index f2e89a773e7..3ef03569d4e 100644
--- a/lib/Analysis/ConstantFolding.cpp
+++ b/lib/Analysis/ConstantFolding.cpp
@@ -538,7 +538,7 @@ static Constant *CastGEPIndices(Constant *const *Ops, unsigned NumOps,
   for (unsigned i = 1; i != NumOps; ++i) {
     if ((i == 1 ||
          !isa<StructType>(GetElementPtrInst::getIndexedType(Ops[0]->getType(),
-                                                            reinterpret_cast<Value *const *>(Ops+1),
+                                        reinterpret_cast<Value *const *>(Ops+1),
                                                             i-1))) &&
         Ops[i]->getType() != IntPtrTy) {
       Any = true;
@@ -639,12 +639,19 @@ static Constant *SymbolicallyEvaluateGEP(Constant *const *Ops, unsigned NumOps,
         
       // Determine which element of the array the offset points into.
       APInt ElemSize(BitWidth, TD->getTypeAllocSize(ATy->getElementType()));
+      const IntegerType *IntPtrTy = TD->getIntPtrType(Ty->getContext());
       if (ElemSize == 0)
-        return 0;
-      APInt NewIdx = Offset.udiv(ElemSize);
-      Offset -= NewIdx * ElemSize;
-      NewIdxs.push_back(ConstantInt::get(TD->getIntPtrType(Ty->getContext()),
-                                         NewIdx));
+        // The element size is 0. This may be [0 x Ty]*, so just use a zero
+        // index for this level and procede to the next level to see if it can
+        // accomodate the offset.
+        NewIdxs.push_back(ConstantInt::get(IntPtrTy, 0));
+      else {
+        // The element size is non-zero divide the offset by the element
+        // size (rounding down), to compute the index at this level.
+        APInt NewIdx = Offset.udiv(ElemSize);
+        Offset -= NewIdx * ElemSize;
+        NewIdxs.push_back(ConstantInt::get(IntPtrTy, NewIdx));
+      }
       Ty = ATy->getElementType();
     } else if (const StructType *STy = dyn_cast<StructType>(Ty)) {
       // Determine which field of the struct the offset points into. The
@@ -743,7 +750,8 @@ Constant *llvm::ConstantFoldInstruction(Instruction *I, const TargetData *TD) {
 Constant *llvm::ConstantFoldConstantExpression(const ConstantExpr *CE,
                                                const TargetData *TD) {
   SmallVector<Constant*, 8> Ops;
-  for (User::const_op_iterator i = CE->op_begin(), e = CE->op_end(); i != e; ++i) {
+  for (User::const_op_iterator i = CE->op_begin(), e = CE->op_end();
+       i != e; ++i) {
     Constant *NewC = cast<Constant>(*i);
     // Recursively fold the ConstantExpr's operands.
     if (ConstantExpr *NewCE = dyn_cast<ConstantExpr>(NewC))