ConstantFold: Zero-sized globals might land on top of another global

A zero sized array is zero sized and might share its address with
another global.

llvm-svn: 223684

2 files changed, 21 insertions, 3 deletions

diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
index 719a3a4b4c8..4cb22bc97a7 100644
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -1348,12 +1348,24 @@ static FCmpInst::Predicate evaluateFCmpRelation(Constant *V1, Constant *V2) {
 
 static ICmpInst::Predicate areGlobalsPotentiallyEqual(const GlobalValue *GV1,
                                                       const GlobalValue *GV2) {
-  auto isLinkageUnsafeForEquality = [](const GlobalValue *GV) {
-    return GV->hasExternalWeakLinkage() || GV->hasWeakAnyLinkage();
+  auto isGlobalUnsafeForEquality = [](const GlobalValue *GV) {
+    if (GV->hasExternalWeakLinkage() || GV->hasWeakAnyLinkage())
+      return true;
+    if (const auto *GVar = dyn_cast<GlobalVariable>(GV)) {
+      Type *Ty = GVar->getType()->getPointerElementType();
+      // A global with opaque type might end up being zero sized.
+      if (!Ty->isSized())
+        return true;
+      // A global with an empty type might lie at the address of any other
+      // global.
+      if (Ty->isEmptyTy())
+        return true;
+    }
+    return false;
   };
   // Don't try to decide equality of aliases.
   if (!isa<GlobalAlias>(GV1) && !isa<GlobalAlias>(GV2))
-    if (!isLinkageUnsafeForEquality(GV1) && !isLinkageUnsafeForEquality(GV2))
+    if (!isGlobalUnsafeForEquality(GV1) && !isGlobalUnsafeForEquality(GV2))
       return ICmpInst::ICMP_NE;
   return ICmpInst::BAD_ICMP_PREDICATE;
 }
diff --git a/llvm/test/Assembler/ConstantExprNoFold.ll b/llvm/test/Assembler/ConstantExprNoFold.ll
index bed58c52ad9..83236d5793b 100644
--- a/llvm/test/Assembler/ConstantExprNoFold.ll
+++ b/llvm/test/Assembler/ConstantExprNoFold.ll
@@ -36,6 +36,12 @@ target datalayout = "p:32:32"
 @F = global i1 icmp eq (i32* @weakany, i32* @glob)
 @weakany = weak global i32 0
 
+; Empty globals might end up anywhere, even on top of another global.
+; CHECK: @empty.cmp = global i1 icmp eq ([0 x i8]* @empty.1, [0 x i8]* @empty.2)
+@empty.1 = external global [0 x i8], align 1
+@empty.2 = external global [0 x i8], align 1
+@empty.cmp = global i1 icmp eq ([0 x i8]* @empty.1, [0 x i8]* @empty.2)
+
 ; Don't add an inbounds on @glob.a3, since it's not inbounds.
 ; CHECK: @glob.a3 = alias getelementptr (i32* @glob.a2, i32 1)
 @glob = global i32 0