diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -468,19 +468,16 @@
 /// Copy over the debug info for a variable to its SRA replacements.
 static void transferSRADebugInfo(GlobalVariable *GV, GlobalVariable *NGV,
                                  uint64_t FragmentOffsetInBits,
-                                 uint64_t FragmentSizeInBits) {
+                                 uint64_t FragmentSizeInBits,
+                                 uint64_t VarSize) {
   SmallVector<DIGlobalVariableExpression *, 1> GVs;
   GV->getDebugInfo(GVs);
   for (auto *GVE : GVs) {
     DIVariable *Var = GVE->getVariable();
-    Optional<uint64_t> VarSize = Var->getSizeInBits();
-
     DIExpression *Expr = GVE->getExpression();
     // If the FragmentSize is smaller than the variable,
     // emit a fragment expression.
-    // If the variable size is unknown a fragment must be
-    // emitted to be safe.
-    if (!VarSize || FragmentSizeInBits < *VarSize) {
+    if (FragmentSizeInBits < VarSize) {
       if (auto E = DIExpression::createFragmentExpression(
               Expr, FragmentOffsetInBits, FragmentSizeInBits))
         Expr = *E;
@@ -505,6 +502,7 @@
   assert(GV->hasLocalLinkage());
   Constant *Init = GV->getInitializer();
   Type *Ty = Init->getType();
+  uint64_t VarSize = DL.getTypeSizeInBits(Ty);
 
   std::map<unsigned, GlobalVariable *> NewGlobals;
 
@@ -560,7 +558,7 @@
       // Copy over the debug info for the variable.
       uint64_t Size = DL.getTypeAllocSizeInBits(NGV->getValueType());
       uint64_t FragmentOffsetInBits = Layout.getElementOffsetInBits(ElementIdx);
-      transferSRADebugInfo(GV, NGV, FragmentOffsetInBits, Size);
+      transferSRADebugInfo(GV, NGV, FragmentOffsetInBits, Size, VarSize);
     } else {
       uint64_t EltSize = DL.getTypeAllocSize(ElTy);
       Align EltAlign = DL.getABITypeAlign(ElTy);
@@ -573,7 +571,7 @@
       if (NewAlign > EltAlign)
         NGV->setAlignment(NewAlign);
       transferSRADebugInfo(GV, NGV, FragmentSizeInBits * ElementIdx,
-                           FragmentSizeInBits);
+                           FragmentSizeInBits, VarSize);
     }
   }
 
diff --git a/llvm/test/DebugInfo/Generic/global-sra-struct-fwd-decl.ll b/llvm/test/DebugInfo/Generic/global-sra-struct-fwd-decl.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/DebugInfo/Generic/global-sra-struct-fwd-decl.ll
@@ -0,0 +1,63 @@
+; RUN: opt -S -globalopt < %s | FileCheck %s
+; Generated at -O2 -g from:
+; typedef struct {} a;
+; static struct {
+;     long b;
+;     a c;
+; } d;
+; e() {
+;     long f = d.b + 1;
+;     d.b = f;
+; }
+; (IR is modified so that d's struct type is forward declared.)
+
+; Check that the global variable "d" is not
+; emitted as a fragment if its struct type is
+; forward declared but d.c has zero length, so
+; a fragment shouldn't be emitted.
+
+source_filename = "t.c"
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+%struct.anon = type { i64, %struct.a }
+%struct.a = type {}
+
+; CHECK: @d.0 = internal unnamed_addr global i64 0, align 8, !dbg ![[GVE:.*]]
+@d = internal global %struct.anon zeroinitializer, align 8, !dbg !0
+
+; Function Attrs: noinline nounwind uwtable
+define dso_local i32 @e() #0 !dbg !18 {
+entry:
+  %0 = load i64, i64* getelementptr inbounds (%struct.anon, %struct.anon* @d, i32 0, i32 0), align 8
+  %add = add nsw i64 %0, 1
+  call void @llvm.dbg.value(metadata i64 %add, metadata !24, metadata !DIExpression()), !dbg !25
+  store i64 %add, i64* getelementptr inbounds (%struct.anon, %struct.anon* @d, i32 0, i32 0), align 8
+  ret i32 undef
+}
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.declare(metadata, metadata, metadata)
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!2}
+!llvm.module.flags = !{!14, !15}
+
+; CHECK: ![[GVE]] = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!0 = !DIGlobalVariableExpression(var: !1, expr: !DIExpression())
+!1 = distinct !DIGlobalVariable(name: "d", scope: !2, file: !3, line: 6, type: !7, isLocal: true, isDefinition: true)
+!2 = distinct !DICompileUnit(language: DW_LANG_C99, file: !3, isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !{}, globals: !{!0}, splitDebugInlining: false, nameTableKind: None)
+!3 = !DIFile(filename: "t.c", directory: "/")
+!7 = distinct !DICompositeType(tag: DW_TAG_structure_type, file: !3, line: 3, flags: DIFlagFwdDecl)
+!10 = !DIBasicType(name: "long int", size: 64, encoding: DW_ATE_signed)
+!12 = !DIDerivedType(tag: DW_TAG_typedef, name: "a", file: !3, line: 2, baseType: !13)
+!13 = distinct !DICompositeType(tag: DW_TAG_structure_type, file: !3, line: 1, elements: !{})
+!14 = !{i32 7, !"Dwarf Version", i32 4}
+!15 = !{i32 2, !"Debug Info Version", i32 3}
+!18 = distinct !DISubprogram(name: "e", scope: !3, file: !3, line: 7, type: !19, scopeLine: 7, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !{})
+!19 = !DISubroutineType(types: !{!21})
+!21 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!24 = !DILocalVariable(name: "f", scope: !18, file: !3, line: 8, type: !10)
+!25 = !DILocation(line: 0, scope: !18)