Index: ELF/InputSection.cpp
===================================================================
--- ELF/InputSection.cpp
+++ ELF/InputSection.cpp
@@ -241,6 +241,13 @@
   case R_RELAX_TLS_IE_TO_LE:
   case R_RELAX_TLS_GD_TO_LE:
   case R_TLS:
+    // A weak undefined TLS symbol resolves to the base of the TLS
+    // block, i.e. gets a value of zero. If we pass --gc-sections to
+    // lld and .tbss is not referenced, it gets reclaimed and we don't
+    // create a TLS program header. Therefore, we resolve this
+    // statically to zero.
+    if (Body.isTls() && Body.isLazy() && Body.symbol()->isWeak())
+      return 0;
     if (Target->TcbSize)
       return Body.getVA<ELFT>(A) +
              alignTo(Target->TcbSize, Out<ELFT>::TlsPhdr->p_align);
Index: test/ELF/tls-weak-undef.s
===================================================================
--- /dev/null
+++ test/ELF/tls-weak-undef.s
@@ -0,0 +1,11 @@
+// REQUIRES: x86
+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+// RUN: ld.lld %t.o %t.a -o %t --gc-sections
+
+// Check that lld doesn't crash because we don't reference
+// the TLS phdr when it's not created.
+        .globl  _start
+_start:
+        movq    foo@gottpoff(%rip), %rax
+        .section        .tbss,"awT",@nobits
+        .weak   foo