Index: lld/ELF/Driver.cpp
===================================================================
--- lld/ELF/Driver.cpp
+++ lld/ELF/Driver.cpp
@@ -554,6 +554,8 @@
     return {BuildIdKind::Fast, {}};
 
   StringRef S = Arg->getValue();
+  if (S == "fast")
+    return {BuildIdKind::Fast, {}};
   if (S == "md5")
     return {BuildIdKind::Md5, {}};
   if (S == "sha1" || S == "tree")
Index: lld/test/ELF/build-id.s
===================================================================
--- lld/test/ELF/build-id.s
+++ lld/test/ELF/build-id.s
@@ -7,6 +7,8 @@
 
 # RUN: ld.lld --build-id %t -o %t2 -threads
 # RUN: llvm-objdump -s %t2 | FileCheck -check-prefix=DEFAULT %s
+# RUN: ld.lld --build-id=fast %t -o %t2 -threads
+# RUN: llvm-objdump -s %t2 | FileCheck -check-prefix=DEFAULT %s
 # RUN: ld.lld --build-id %t -o %t2 -no-threads
 # RUN: llvm-objdump -s %t2 | FileCheck -check-prefix=DEFAULT %s