diff --git a/llvm/include/llvm/DebugInfo/DWARF/DWARFDebugAddr.h b/llvm/include/llvm/DebugInfo/DWARF/DWARFDebugAddr.h
--- a/llvm/include/llvm/DebugInfo/DWARF/DWARFDebugAddr.h
+++ b/llvm/include/llvm/DebugInfo/DWARF/DWARFDebugAddr.h
@@ -31,7 +31,7 @@
   uint64_t Offset;
   /// The total length of the entries for this table, not including the length
   /// field itself.
-  uint32_t Length = 0;
+  uint64_t Length = 0;
   /// The DWARF version number.
   uint16_t Version;
   /// The size in bytes of an address on the target architecture. For
diff --git a/llvm/lib/DebugInfo/DWARF/DWARFDebugAddr.cpp b/llvm/lib/DebugInfo/DWARF/DWARFDebugAddr.cpp
--- a/llvm/lib/DebugInfo/DWARF/DWARFDebugAddr.cpp
+++ b/llvm/lib/DebugInfo/DWARF/DWARFDebugAddr.cpp
@@ -50,34 +50,48 @@
                              "section is not large enough to contain an "
                              "address table length at offset 0x%" PRIx64,
                              Offset);
-  // TODO: Add support for DWARF64.
   Format = dwarf::DwarfFormat::DWARF32;
   Length = Data.getU32(OffsetPtr);
   if (Length == dwarf::DW_LENGTH_DWARF64) {
+    // Check that we can read the extended unit length field.
+    if (!Data.isValidOffsetForDataOfSize(*OffsetPtr, 8)) {
+      invalidateLength();
+      return createStringError(
+          errc::invalid_argument,
+          "section is not large enough to contain an extended length field "
+          "of the address table at offset 0x%" PRIx64,
+          Offset);
+    }
+    Format = dwarf::DwarfFormat::DWARF64;
+    Length = Data.getU64(OffsetPtr);
+  } else if (Length >= dwarf::DW_LENGTH_lo_reserved) {
+    uint64_t DiagnosticLength = Length;
     invalidateLength();
     return createStringError(
         errc::not_supported,
-        "DWARF64 is not supported in .debug_addr at offset 0x%" PRIx64, Offset);
+        "address table at offset 0x%" PRIx64
+        " has unsupported reserved unit length of value 0x%" PRIx64,
+        Offset, DiagnosticLength);
   }
 
   if (!Data.isValidOffsetForDataOfSize(*OffsetPtr, Length)) {
-    uint32_t DiagnosticLength = Length;
+    uint64_t DiagnosticLength = Length;
     invalidateLength();
     return createStringError(
         errc::invalid_argument,
         "section is not large enough to contain an address table "
-        "at offset 0x%" PRIx64 " with a unit_length value of 0x%" PRIx32,
+        "at offset 0x%" PRIx64 " with a unit_length value of 0x%" PRIx64,
         Offset, DiagnosticLength);
   }
   uint64_t EndOffset = *OffsetPtr + Length;
   // Ensure that we can read the remaining header fields.
   if (Length < 4) {
-    uint32_t DiagnosticLength = Length;
+    uint64_t DiagnosticLength = Length;
     invalidateLength();
     return createStringError(
         errc::invalid_argument,
         "address table at offset 0x%" PRIx64
-        " has a unit_length value of 0x%" PRIx32
+        " has a unit_length value of 0x%" PRIx64
         ", which is too small to contain a complete header",
         Offset, DiagnosticLength);
   }
@@ -142,12 +156,14 @@
 
 void DWARFDebugAddrTable::dump(raw_ostream &OS, DIDumpOptions DumpOpts) const {
   if (DumpOpts.Verbose)
-    OS << format("0x%8.8" PRIx32 ": ", Offset);
-  if (Length)
-    OS << format("Address table header: length = 0x%8.8" PRIx32
+    OS << format("0x%8.8" PRIx64 ": ", Offset);
+  if (Length) {
+    int LengthFieldWidth = (Format == dwarf::DwarfFormat::DWARF64) ? 16 : 8;
+    OS << format("Address table header: length = 0x%0*" PRIx64
                  ", version = 0x%4.4" PRIx16 ", addr_size = 0x%2.2" PRIx8
                  ", seg_size = 0x%2.2" PRIx8 "\n",
-                 Length, Version, AddrSize, SegSize);
+                 LengthFieldWidth, Length, Version, AddrSize, SegSize);
+  }
 
   if (Addrs.size() > 0) {
     const char *AddrFmt =
@@ -171,7 +187,6 @@
 Optional<uint64_t> DWARFDebugAddrTable::getFullLength() const {
   if (Length == 0)
     return None;
-  // TODO: DWARF64 support.
-  return Length + sizeof(uint32_t);
+  return Length + dwarf::getUnitLengthFieldByteSize(Format);
 }
 
diff --git a/llvm/test/tools/llvm-dwarfdump/X86/debug_addr_dwarf64.s b/llvm/test/tools/llvm-dwarfdump/X86/debug_addr_dwarf64.s
--- a/llvm/test/tools/llvm-dwarfdump/X86/debug_addr_dwarf64.s
+++ b/llvm/test/tools/llvm-dwarfdump/X86/debug_addr_dwarf64.s
@@ -1,19 +1,25 @@
-# RUN: llvm-mc %s -filetype obj -triple i386-pc-linux -o - | \
-# RUN: llvm-dwarfdump -debug-addr - 2> %t.err | FileCheck %s
-# RUN: FileCheck %s -input-file %t.err -check-prefix=ERR
+# RUN: llvm-mc %s -filetype obj -triple x86_64-pc-linux -o - | \
+# RUN:   llvm-dwarfdump -debug-addr - | \
+# RUN:   FileCheck %s
 
-# CHECK: .debug_addr contents:
-# CHECK-NOT: {{.}}
-# ERR: DWARF64 is not supported in .debug_addr at offset 0x0
-# ERR-NOT: {{.}}
+# CHECK:      .debug_addr contents:
+# CHECK-NEXT: Address table header:
+# CHECK-SAME: length = 0x000000000000000c,
+# CHECK-SAME: version = 0x0005,
+# CHECK-SAME: addr_size = 0x04,
+# CHECK-SAME: seg_size = 0x00
+# CHECK-NEXT: Addrs: [
+# CHECK-NEXT: 0x00000000
+# CHECK-NEXT: 0x00001000
+# CHECK-NEXT: ]
 
-# DWARF64 table
-  .section  .debug_addr,"",@progbits
-.Ldebug_addr0:
-  .long 0xffffffff # unit_length DWARF64 mark
-  .quad 12         # unit_length
-  .short 5         # version
-  .byte 3          # address_size
-  .byte 0          # segment_selector_size
-  .long 0x00000000
-  .long 0x00000001
+    .section .debug_addr,"",@progbits
+    .long 0xffffffff                    # DWARF64 mark
+    .quad .LAddr0end-.LAddr0version     # Length
+.LAddr0version:
+    .short 5                            # Version
+    .byte 4                             # Address size
+    .byte 0                             # Segment selector size
+    .long 0x00000000
+    .long 0x00001000
+.LAddr0end:
diff --git a/llvm/test/tools/llvm-dwarfdump/X86/debug_addr_reserved_length.s b/llvm/test/tools/llvm-dwarfdump/X86/debug_addr_reserved_length.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/llvm-dwarfdump/X86/debug_addr_reserved_length.s
@@ -0,0 +1,8 @@
+# RUN: llvm-mc %s -filetype obj -triple x86_64-pc-linux -o - | \
+# RUN:   llvm-dwarfdump --debug-addr - 2>&1 | \
+# RUN:   FileCheck %s --implicit-check-not=error
+
+# CHECK: error: address table at offset 0x0 has unsupported reserved unit length of value 0xfffffff0
+
+.section .debug_addr,"",@progbits
+.long 0xfffffff0
diff --git a/llvm/test/tools/llvm-dwarfdump/X86/debug_addr_too_small_for_extended_length_field.s b/llvm/test/tools/llvm-dwarfdump/X86/debug_addr_too_small_for_extended_length_field.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/llvm-dwarfdump/X86/debug_addr_too_small_for_extended_length_field.s
@@ -0,0 +1,13 @@
+# RUN: llvm-mc %s -filetype obj -triple i386-pc-linux -o - | \
+# RUN: llvm-dwarfdump -debug-addr - 2> %t.err | FileCheck %s
+# RUN: FileCheck %s -input-file %t.err -check-prefix=ERR
+
+# CHECK: .debug_addr contents:
+# CHECK-NOT: {{.}}
+# ERR: section is not large enough to contain an extended length field of the address table at offset 0x0
+# ERR-NOT: {{.}}
+
+# too small section to contain an extended length field of a DWARF64 address table.
+  .section  .debug_addr,"",@progbits
+  .long 0xffffffff      # DWARF64 mark
+  .space 7              # a DWARF64 unit length field takes 8 bytes.