diff --git a/COFF/Driver.cpp b/COFF/Driver.cpp
index c2fc048..bc24b91 100644
--- a/COFF/Driver.cpp
+++ b/COFF/Driver.cpp
@@ -830,6 +830,57 @@
   }
 }
 
+// link.exe replaces each %foo% in AltPath with the contents of environment
+// variable foo, and adds the two magic env vars _PDB (expands to the basename
+// of pdb's output path) and _EXT (expands to the extension of the output
+// binary).
+// lld only supports %_PDB% and %_EXT% and warns on references to all other env
+// vars.
+static void parsePDBAltPath(StringRef AltPath) {
+  SmallString<128> Buf;
+  StringRef PDBBasename =
+      sys::path::filename(Config->PDBPath, sys::path::Style::windows);
+  StringRef BinaryExtension =
+      sys::path::extension(Config->OutputFile, sys::path::Style::windows);
+  if (!BinaryExtension.empty())
+    BinaryExtension = BinaryExtension.substr(1); // %_EXT% does not include '.'.
+
+  // Invariant:
+  //   +--------- Cursor ('a...' might be the empty string).
+  //   |   +----- FirstMark
+  //   |   |   +- SecondMark
+  //   v   v   v
+  //   a...%...%...
+  size_t Cursor = 0;
+  while (Cursor < AltPath.size()) {
+    size_t FirstMark, SecondMark;
+    if ((FirstMark = AltPath.find('%', Cursor)) == StringRef::npos ||
+        (SecondMark = AltPath.find('%', FirstMark + 1)) == StringRef::npos) {
+      // Didn't find another full fragment, treat rest of string as literal.
+      Buf.append(AltPath.substr(Cursor));
+      break;
+    }
+
+    // Found a full fragment. Append text in front of first %, and interpret
+    // text between first and second % as variable name.
+    Buf.append(AltPath.substr(Cursor, FirstMark - Cursor));
+    StringRef Var = AltPath.substr(FirstMark, SecondMark - FirstMark + 1);
+    if (Var.equals_lower("%_pdb%"))
+      Buf.append(PDBBasename);
+    else if (Var.equals_lower("%_ext%"))
+      Buf.append(BinaryExtension);
+    else {
+      warn("only %_PDB% and %_EXT% supported in /pdbaltpath:, keeping " +
+           Var + " as literal");
+      Buf.append(Var);
+    }
+
+    Cursor = SecondMark + 1;
+  }
+
+  Config->PDBAltPath = Buf;
+}
+
 void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
   // If the first command line argument is "/lib", link.exe acts like lib.exe.
   // We call our own implementation of lib.exe that understands bitcode files.
@@ -1411,6 +1462,9 @@
       // tools won't work correctly if these assumptions are not held.
       sys::fs::make_absolute(Config->PDBAltPath);
       sys::path::remove_dots(Config->PDBAltPath);
+    } else {
+      // Don't do this earlier, so that Config->OutputFile is ready.
+      parsePDBAltPath(Config->PDBAltPath);
     }
   }
 
diff --git a/test/COFF/Inputs/empty.yaml b/test/COFF/Inputs/empty.yaml
new file mode 100644
index 0000000..6396f8a
--- /dev/null
+++ b/test/COFF/Inputs/empty.yaml
@@ -0,0 +1,67 @@
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_I386
+  Characteristics: [  ]
+sections:
+  - Name:            .text
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     31C0C3
+  - Name:            .data
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       4
+    SectionData:     ''
+  - Name:            .bss
+    Characteristics: [ IMAGE_SCN_CNT_UNINITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       4
+    SectionData:     ''
+symbols:
+  - Name:            .text
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          3
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        3963538403
+      Number:          1
+  - Name:            .data
+    Value:           0
+    SectionNumber:   2
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          0
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          2
+  - Name:            .bss
+    Value:           0
+    SectionNumber:   3
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          0
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          3
+  - Name:            '@feat.00'
+    Value:           1
+    SectionNumber:   -1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+  - Name:            _main
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_FUNCTION
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...
diff --git a/test/COFF/pdbaltpath.test b/test/COFF/pdbaltpath.test
new file mode 100644
index 0000000..952e208
--- /dev/null
+++ b/test/COFF/pdbaltpath.test
@@ -0,0 +1,39 @@
+# RUN: yaml2obj %p/Inputs/empty.yaml > %t.obj
+
+# RUN: lld-link /entry:main %t.obj /out:%t.exe /debug /pdbaltpath:hello.pdb
+# RUN: llvm-readobj -coff-debug-directory %t.exe | FileCheck --check-prefix HELLO %s
+# HELLO:       PDBFileName: hello.pdb
+
+# RUN: lld-link /entry:main %t.obj /out:%t.exe /debug /pdbaltpath:%_Pdb%
+# RUN: llvm-readobj -coff-debug-directory %t.exe | FileCheck --check-prefix PDBVAR %s
+# PDBVAR:       PDBFileName: pdbaltpath.test.tmp.pdb
+
+# RUN: lld-link /entry:main %t.obj /out:%t.exe /debug /pdbaltpath:foo%_ExT%.pdb
+# RUN: llvm-readobj -coff-debug-directory %t.exe | FileCheck --check-prefix EXTVAR %s
+# EXTVAR:       PDBFileName: fooexe.pdb
+
+# RUN: lld-link /entry:main %t.obj /out:%t.exe /debug /pdbaltpath:%_PDB
+# RUN: llvm-readobj -coff-debug-directory %t.exe | FileCheck --check-prefix NOCLOSE %s
+# NOCLOSE:       PDBFileName: %_PDB
+
+# RUN: lld-link /entry:main %t.obj /out:%t.exe /debug /pdbaltpath:foo%_PDB
+# RUN: llvm-readobj -coff-debug-directory %t.exe | FileCheck --check-prefix NOCLOSE2 %s
+# NOCLOSE2:       PDBFileName: foo%_PDB
+
+# RUN: lld-link /entry:main %t.obj /out:%t.exe /debug /pdbaltpath:foo%_PDB%bar%_EXT
+# RUN: llvm-readobj -coff-debug-directory %t.exe | FileCheck --check-prefix CLOSEONE %s
+# CLOSEONE:       PDBFileName: foopdbaltpath.test.tmp.pdbbar%_EXT
+
+# RUN: lld-link /entry:main %t.obj /out:%t.exe /debug /pdbaltpath:foo%_PDB%bar%_EXT%
+# RUN: llvm-readobj -coff-debug-directory %t.exe | FileCheck --check-prefix CLOSETWO %s
+# CLOSETWO:       PDBFileName: foopdbaltpath.test.tmp.pdbbarexe
+
+# RUN: lld-link /entry:main %t.obj /out:%t.exe /debug /pdbaltpath:foo%_PDB%bar%_EXT%a
+# RUN: llvm-readobj -coff-debug-directory %t.exe | FileCheck --check-prefix CLOSETWO2 %s
+# CLOSETWO2:       PDBFileName: foopdbaltpath.test.tmp.pdbbarexea
+
+# RUN: lld-link /entry:main %t.obj /out:%t.exe /debug /pdbaltpath:foo%FoO%bar%r%a 2>&1 | FileCheck --check-prefix UNKNOWN-WARN %s
+# RUN: llvm-readobj -coff-debug-directory %t.exe | FileCheck --check-prefix ENVVARS %s
+# UNKNOWN-WARN: only %_PDB% and %_EXT% supported in /pdbaltpath:, keeping %FoO% as literal
+# UNKNOWN-WARN: only %_PDB% and %_EXT% supported in /pdbaltpath:, keeping %r% as literal
+# ENVVARS:       PDBFileName: foo%FoO%bar%r%a