Index: lld/trunk/ELF/LinkerScript.h
===================================================================
--- lld/trunk/ELF/LinkerScript.h
+++ lld/trunk/ELF/LinkerScript.h
@@ -195,6 +195,7 @@
   virtual bool isAbsolute(StringRef S) = 0;
   virtual const OutputSectionBase *getSymbolSection(StringRef S) = 0;
   virtual const OutputSectionBase *getOutputSection(StringRef S) = 0;
+  virtual uint64_t getOutputSectionSize(StringRef S) = 0;
 };
 
 // ScriptConfiguration holds linker script parse results.
@@ -245,6 +246,7 @@
   bool isAbsolute(StringRef S) override;
   const OutputSectionBase *getSymbolSection(StringRef S) override;
   const OutputSectionBase *getOutputSection(StringRef S) override;
+  uint64_t getOutputSectionSize(StringRef S) override;
 
   std::vector<OutputSectionBase *> *OutputSections;
 
Index: lld/trunk/ELF/LinkerScript.cpp
===================================================================
--- lld/trunk/ELF/LinkerScript.cpp
+++ lld/trunk/ELF/LinkerScript.cpp
@@ -868,6 +868,20 @@
   return &FakeSec;
 }
 
+// This function is essentially the same as getOutputSection(Name)->Size,
+// but it won't print out an error message if a given section is not found.
+//
+// Linker script does not create an output section if its content is empty.
+// We want to allow SIZEOF(.foo) where .foo is a section which happened to
+// be empty. That is why this function is different from getOutputSection().
+template <class ELFT>
+uint64_t LinkerScript<ELFT>::getOutputSectionSize(StringRef Name) {
+  for (OutputSectionBase *Sec : *OutputSections)
+    if (Sec->getName() == Name)
+      return Sec->Size;
+  return 0;
+}
+
 template <class ELFT> uint64_t LinkerScript<ELFT>::getHeaderSize() {
   return elf::getHeaderSize<ELFT>();
 }
@@ -1719,8 +1733,7 @@
   }
   if (Tok == "SIZEOF") {
     StringRef Name = readParenLiteral();
-    return
-        [=](uint64_t Dot) { return ScriptBase->getOutputSection(Name)->Size; };
+    return [=](uint64_t Dot) { return ScriptBase->getOutputSectionSize(Name); };
   }
   if (Tok == "ALIGNOF") {
     StringRef Name = readParenLiteral();
Index: lld/trunk/test/ELF/linkerscript/sizeof.s
===================================================================
--- lld/trunk/test/ELF/linkerscript/sizeof.s
+++ lld/trunk/test/ELF/linkerscript/sizeof.s
@@ -24,17 +24,17 @@
 # CHECK-NEXT:  0000000000000010 *ABS* 00000000 _bbb
 # CHECK-NEXT:  0000000000000018 *ABS* 00000000 _ccc
 
-## Check that we error out if trying to get size of
-## section that does not exist.
+## SIZEOF(.nonexistent_section) should return 0.
 # RUN: echo "SECTIONS { \
 # RUN:   .aaa         : { *(.aaa) } \
 # RUN:   .bbb         : { *(.bbb) } \
 # RUN:   .ccc         : { *(.ccc) } \
 # RUN:   _aaa = SIZEOF(.foo); \
 # RUN: }" > %t.script
-# RUN: not ld.lld -o %t1 --script %t.script %t 2>&1 \
-# RUN:  | FileCheck -check-prefix=ERR %s
-# ERR: undefined section .foo
+# RUN: ld.lld -o %t1 --script %t.script %t
+# RUN: llvm-objdump -t -section-headers %t1 | FileCheck -check-prefix=CHECK2 %s
+
+# CHECK2: 0000000000000000 *ABS* 00000000 _aaa
 
 .global _start
 _start: