diff --git a/llvm/lib/Analysis/MemoryBuiltins.cpp b/llvm/lib/Analysis/MemoryBuiltins.cpp
--- a/llvm/lib/Analysis/MemoryBuiltins.cpp
+++ b/llvm/lib/Analysis/MemoryBuiltins.cpp
@@ -830,7 +830,9 @@
 }
 
 SizeOffsetType ObjectSizeOffsetVisitor::visitGlobalVariable(GlobalVariable &GV){
-  if (!GV.hasDefinitiveInitializer())
+  if (!GV.getValueType()->isSized() || GV.hasExternalWeakLinkage() ||
+      ((!GV.hasInitializer() || GV.isInterposable()) &&
+       Options.EvalMode != ObjectSizeOpts::Mode::Min))
     return unknown();
 
   APInt Size(IntTyBits, DL.getTypeAllocSize(GV.getValueType()));
diff --git a/llvm/test/Transforms/InstSimplify/gv-alloca-cmp.ll b/llvm/test/Transforms/InstSimplify/gv-alloca-cmp.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstSimplify/gv-alloca-cmp.ll
@@ -0,0 +1,60 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 2
+; RUN: opt < %s -passes=instsimplify -S | FileCheck %s
+
+@gv = global i64 zeroinitializer, align 16
+
+define i1 @cmp_gv_alloca() {
+; CHECK-LABEL: define i1 @cmp_gv_alloca() {
+; CHECK-NEXT:    ret i1 false
+;
+  %alloca = alloca i64, align 8
+  %cmp = icmp eq ptr %alloca, @gv
+  ret i1 %cmp
+}
+
+@gv_externally_init = externally_initialized global i64 zeroinitializer, align 16
+
+define i1 @cmp_gv_alloca_extern_init() {
+; CHECK-LABEL: define i1 @cmp_gv_alloca_extern_init() {
+; CHECK-NEXT:    ret i1 false
+;
+  %alloca = alloca i64, align 8
+  %cmp = icmp eq ptr %alloca, @gv_externally_init
+  ret i1 %cmp
+}
+
+@const_gv = protected addrspace(4) externally_initialized global [4096 x i64] zeroinitializer, align 16
+define i1 @cmp_gv_alloca_cast() {
+; CHECK-LABEL: define i1 @cmp_gv_alloca_cast() {
+; CHECK-NEXT:    ret i1 false
+;
+  %alloca = alloca i64, align 8, addrspace(5)
+  %cast.alloca = addrspacecast ptr addrspace(5) %alloca to ptr
+  %cmp = icmp eq ptr %cast.alloca, addrspacecast (ptr addrspace(4) @const_gv to ptr)
+  ret i1 %cmp
+}
+
+@gv_weak = weak global i64 zeroinitializer, align 16
+
+define i1 @cmp_gv_weak_alloca() {
+; CHECK-LABEL: define i1 @cmp_gv_weak_alloca() {
+; CHECK-NEXT:    ret i1 false
+;
+  %alloca = alloca i64, align 8
+  %cmp = icmp eq ptr %alloca, @gv_weak
+  ret i1 %cmp
+}
+
+%opaque = type opaque
+@gv_unsized = weak global %opaque zeroinitializer, align 16
+
+define i1 @cmp_gv_unsized_alloca() {
+; CHECK-LABEL: define i1 @cmp_gv_unsized_alloca() {
+; CHECK-NEXT:    [[ALLOCA:%.*]] = alloca i64, align 8
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr [[ALLOCA]], @gv_unsized
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %alloca = alloca i64, align 8
+  %cmp = icmp eq ptr %alloca, @gv_unsized
+  ret i1 %cmp
+}
diff --git a/llvm/test/Transforms/LowerConstantIntrinsics/objectsize_basic.ll b/llvm/test/Transforms/LowerConstantIntrinsics/objectsize_basic.ll
--- a/llvm/test/Transforms/LowerConstantIntrinsics/objectsize_basic.ll
+++ b/llvm/test/Transforms/LowerConstantIntrinsics/objectsize_basic.ll
@@ -146,3 +146,41 @@
   %objsize = call i64 @llvm.objectsize.i64(ptr %ptr, i1 false, i1 true, i1 true)
   ret i64 %objsize
 }
+
+@gv_weak = weak global i64 zeroinitializer, align 16
+
+define i32 @promote_with_objectsize_min_false() {
+; CHECK-LABEL: @promote_with_objectsize_min_false(
+; CHECK-NEXT:    ret i32 -1
+;
+  %size = call i32 @llvm.objectsize.i32.p0(ptr @gv_weak, i1 false, i1 false, i1 false)
+  ret i32 %size
+}
+
+define i32 @promote_with_objectsize_min_true() {
+; CHECK-LABEL: @promote_with_objectsize_min_true(
+; CHECK-NEXT:    ret i32 8
+;
+  %size = call i32 @llvm.objectsize.i32.p0(ptr @gv_weak, i1 true, i1 false, i1 false)
+  ret i32 %size
+}
+
+@gv_extern = extern_weak global i64, align 16
+
+define i32 @promote_with_objectsize_nullunknown_false() {
+; CHECK-LABEL: @promote_with_objectsize_nullunknown_false(
+; CHECK-NEXT:    ret i32 0
+;
+  %size = call i32 @llvm.objectsize.i32.p0(ptr @gv_extern, i1 true, i1 false, i1 false)
+  ret i32 %size
+}
+
+define i32 @promote_with_objectsize_nullunknown_true() {
+; CHECK-LABEL: @promote_with_objectsize_nullunknown_true(
+; CHECK-NEXT:    ret i32 0
+;
+  %size = call i32 @llvm.objectsize.i32.p0(ptr @gv_extern, i1 true, i1 true, i1 false)
+  ret i32 %size
+}
+
+declare i32 @llvm.objectsize.i32.p0(ptr, i1, i1, i1)