diff --git a/llvm/test/tools/yaml2obj/ELF/emachine.yaml b/llvm/test/tools/yaml2obj/ELF/emachine.yaml
--- a/llvm/test/tools/yaml2obj/ELF/emachine.yaml
+++ b/llvm/test/tools/yaml2obj/ELF/emachine.yaml
@@ -2,7 +2,7 @@
 
 ## Test we can use an arbitrary value.
 
-# RUN: yaml2obj --docnum=1 %s -o %t1
+# RUN: yaml2obj -D MACHINE=0x1234 %s -o %t1
 # RUN: llvm-readelf %t1 --file-headers | FileCheck %s --check-prefix=UNKNOWN
 
 # UNKNOWN: Machine: 1234
@@ -12,44 +12,23 @@
   Class:   ELFCLASS64
   Data:    ELFDATA2MSB
   Type:    ET_REL
-  Machine: 0x1234
+  Machine: [[MACHINE]]
 
 ## Test we can't use a number that doesn't fit into 2 bytes.
 
-# RUN: not yaml2obj --docnum=2 %s 2>&1 | FileCheck %s --check-prefix=ERR
+# RUN: not yaml2obj -D MACHINE=0x12345 %s 2>&1 | FileCheck %s --check-prefix=ERR
 
 # ERR: error: out of range hex16 number
 
---- !ELF
-FileHeader:
-  Class:   ELFCLASS64
-  Data:    ELFDATA2MSB
-  Type:    ET_REL
-  Machine: 0x12345
-
 ## Test we can use a known named constant.
 
-# RUN: yaml2obj --docnum=3 %s -o %t3
+# RUN: yaml2obj -D MACHINE=EM_NONE %s -o %t3
 # RUN: llvm-readelf %t3 --file-headers | FileCheck %s --check-prefix=NONE
 
 # NONE: Machine: None
 
---- !ELF
-FileHeader:
-  Class:   ELFCLASS64
-  Data:    ELFDATA2MSB
-  Type:    ET_REL
-  Machine: EM_NONE
-
 ## Test we can't use an unknown string constant.
 
-# RUN: not yaml2obj --docnum=4 %s 2>&1 | FileCheck %s --check-prefix=ERR2
+# RUN: not yaml2obj -D MACHINE=EM_UNKNOWN_FOO %s 2>&1 | FileCheck %s --check-prefix=ERR2
 
 # ERR2: error: invalid hex16 number
-
---- !ELF
-FileHeader:
-  Class:   ELFCLASS64
-  Data:    ELFDATA2MSB
-  Type:    ET_REL
-  Machine: EM_UNKNOWN_FOO
diff --git a/llvm/test/tools/yaml2obj/preprocess.yaml b/llvm/test/tools/yaml2obj/preprocess.yaml
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/yaml2obj/preprocess.yaml
@@ -0,0 +1,41 @@
+# RUN: yaml2obj -D NAME= %s | llvm-nm - | FileCheck --check-prefix=FBAR %s
+# FBAR: U fbar
+
+# RUN: yaml2obj -D NAME=o %s | llvm-nm - | FileCheck --check-prefix=FOOBAR %s
+# FOOBAR: U foobar
+
+# RUN: not yaml2obj -D NAME %s 2>&1 | FileCheck --check-prefix=ERR1 %s
+# ERR1: error: bad override, missing field name: NAME
+
+# RUN: not yaml2obj -D =value %s 2>&1 | FileCheck --check-prefix=ERR2 %s
+# ERR2: error: bad override, missing field name: =value
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_X86_64
+Symbols:
+  - Name: f[[NAME]][[NAME]]bar
+
+## FileCheck uses [[ as a pattern. It cannot be used for [[ testing.
+## Test that undefined patterns are not substituted.
+
+# RUN: yaml2obj --docnum=2 %s | llvm-nm --just-symbol-name --no-sort - > %t.nosubst
+# RUN: echo -e 'a[[\n[[a\n[[a]\n[[a][[a]][[a]]' | cmp - %t.nosubst
+
+# RUN: yaml2obj --docnum=2 -D a=b %s | llvm-nm --just-symbol-name --no-sort - > %t.subst
+# RUN: echo -e 'a[[\n[[a\n[[a]\n[[a]bb' | cmp - %t.subst
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_X86_64
+Symbols:
+  - Name: "a[["
+  - Name: "[[a"
+  - Name: "[[a]"
+  - Name: "[[a][[a]][[a]]"
diff --git a/llvm/tools/yaml2obj/yaml2obj.cpp b/llvm/tools/yaml2obj/yaml2obj.cpp
--- a/llvm/tools/yaml2obj/yaml2obj.cpp
+++ b/llvm/tools/yaml2obj/yaml2obj.cpp
@@ -31,6 +31,9 @@
 static cl::opt<std::string>
   Input(cl::Positional, cl::desc("<input>"), cl::init("-"));
 
+static cl::list<std::string>
+    D("D", cl::desc("list of overridden fields: field=value"));
+
 static cl::opt<unsigned>
     DocNum("docnum", cl::init(1),
            cl::desc("Read specified document from input (default = 1)"));
@@ -38,6 +41,41 @@
 static cl::opt<std::string> OutputFilename("o", cl::desc("Output filename"),
                                            cl::value_desc("filename"));
 
+static Optional<std::string> preprocess(StringRef Buf,
+                                        yaml::ErrorHandler ErrHandler) {
+  DenseMap<StringRef, StringRef> Substitution;
+  for (StringRef Define : D) {
+    StringRef F, V;
+    std::tie(F, V) = Define.split('=');
+    if (!Define.count('=') || F.empty()) {
+      ErrHandler("bad override, missing field name: " + Define);
+      return {};
+    }
+    Substitution.try_emplace(F, V);
+  }
+
+  std::string Preprocessed;
+  while (!Buf.empty()) {
+    if (Buf.startswith("[[")) {
+      size_t I = Buf.find("]]", 2);
+      if (I != size_t(-1)) {
+        StringRef Macro = Buf.substr(2, I - 2);
+        auto It = Substitution.find(Macro);
+        if (It != Substitution.end()) {
+          Preprocessed += It->second;
+          Buf = Buf.substr(I + 2);
+          continue;
+        }
+      }
+    }
+
+    Preprocessed += Buf[0];
+    Buf = Buf.substr(1);
+  }
+
+  return Preprocessed;
+}
+
 int main(int argc, char **argv) {
   InitLLVM X(argc, argv);
   cl::ParseCommandLineOptions(argc, argv);
@@ -57,12 +95,15 @@
     return 1;
   }
 
-  ErrorOr<std::unique_ptr<MemoryBuffer>> Buf =
+  ErrorOr<std::unique_ptr<MemoryBuffer>> Buffer =
       MemoryBuffer::getFileOrSTDIN(Input);
+  if (!Buffer)
+    return 1;
+
+  Optional<std::string> Buf = preprocess(Buffer.get()->getBuffer(), ErrHandler);
   if (!Buf)
     return 1;
-
-  yaml::Input YIn(Buf.get()->getBuffer());
+  yaml::Input YIn(*Buf);
   if (!convertYAML(YIn, Out->os(), ErrHandler, DocNum))
     return 1;