Index: lib/Target/Mips/MipsTargetObjectFile.cpp
===================================================================
--- lib/Target/Mips/MipsTargetObjectFile.cpp
+++ lib/Target/Mips/MipsTargetObjectFile.cpp
@@ -136,6 +136,13 @@
     return false;
 
   Type *Ty = GVA->getValueType();
+
+  // It is possible that the target type is unsized. In this case don't
+  // attempt to put it in the small data section. This happens e.g. when
+  // building the FreeBSD kernel.
+  if (!Ty->isSized())
+    return false;
+
   return IsInSmallSection(
       GVA->getParent()->getDataLayout().getTypeAllocSize(Ty));
 }
Index: test/CodeGen/Mips/unsized-global.ll
===================================================================
--- /dev/null
+++ test/CodeGen/Mips/unsized-global.ll
@@ -0,0 +1,40 @@
+; Check that -mgpopt doesn't crash on unsized externals
+; RUN: llc -mtriple=mips64-unknown-freebsd -target-abi n64 -mgpopt -o - %s | FileCheck %s
+
+; ModuleID = 'uart_subr-fd65f8-reduce.test.c'
+source_filename = "uart_subr-fd65f8-reduce.test.c"
+target datalayout = "E-m:e-i8:8:32-i16:16:32-i64:64-n32:64-S128"
+target triple = "mips64-unknown-freebsd"
+
+%struct.a = type opaque
+
+@b = external global %struct.a, align 1
+
+; Function Attrs: norecurse nounwind readnone
+define i32 @d() local_unnamed_addr #0 {
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.cond, %entry
+  br i1 icmp ugt (%struct.a* @b, %struct.a* null), label %for.cond, label %for.end
+
+  ; we should use a sequence of HIGHEST/HIGHER/HI/LO here:
+  ; CHECK:      lui     $1, %highest(b)
+  ; CHECK-NEXT: daddiu  $1, $1, %higher(b)
+  ; CHECK-NEXT: dsll    $1, $1, 16
+  ; CHECK-NEXT: daddiu  $1, $1, %hi(b)
+  ; CHECK-NEXT: dsll    $1, $1, 16
+  ; CHECK-NEXT: daddiu  $2, $1, %lo(b)
+
+
+for.end:                                          ; preds = %for.cond
+  ret i32 undef
+}
+
+attributes #0 = { norecurse nounwind readnone "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="mips64" "target-features"="+mips64,+noabicalls" "unsafe-fp-math"="false" "use-soft-float"="false" }
+
+!llvm.module.flags = !{!0}
+!llvm.ident = !{!1}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{!"clang version 6.0.0 (https://github.com/llvm-mirror/clang.git 1131af2e23396b93b1004df1751bfa3fef7990be) (https://github.com/llvm-mirror/llvm.git a2788ab93c15f82a12423c907426f602a4fdc5a3)"}