Add a test for deduplication.

Author: jcranmer-intel

lld/test/COFF/common-dedup.ll

@@ -0,0 +1,38 @@
+; REQUIRES: x86
+; RUN: split-file %s %t.dir
+; RUN: llc %t.dir/t1.ll -o %t.t1.obj --filetype=obj
+; RUN: llc %t.dir/t2.ll -o %t.t2.obj --filetype=obj
+; RUN: lld-link %t.t1.obj %t.t2.obj  -entry:main -out:%t.exe
+; RUN: llvm-readobj --section-headers %t.exe | FileCheck %s
+
+; Make sure that the data section contains just one copy of @a, not two.
+; CHECK: Name: .data
+; CHECK-NEXT: VirtualSize: 0x1000
+
+;--- t1.ll
+target triple = "x86_64-pc-windows-msvc"
+@a = common global [4096 x i8] zeroinitializer
+
+define i32 @usea() {
+  %ref_common = load i32, ptr @a
+  ret i32 %ref_common
+}
+
+;--- t2.ll
+target triple = "x86_64-pc-windows-msvc"
+@a = common global [4096 x i8] zeroinitializer
+
+define i32 @useb() {
+  %ref_common = load i32, ptr @a
+  ret i32 %ref_common
+}
+
+declare i32 @usea()
+
+define dso_local i32 @main() local_unnamed_addr {
+entry:
+  %a = tail call i32 @usea()
+  %b = tail call i32 @useb()
+  %add = add nsw i32 %a, %b
+  ret i32 %add
+}