diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -125,20 +125,85 @@
   return {};
 }
 
-static TargetInfo *createTargetInfo(opt::InputArgList &args) {
-  // TODO: should unspecified arch be an error rather than defaulting?
-  // Jez: ld64 seems to make unspecified arch an error when LTO is
-  // being used. I'm not sure why though. Feels like we should be able
-  // to infer the arch from our input files regardless
-  StringRef archName = args.getLastArgValue(OPT_arch, "x86_64");
-  config->arch = MachO::getArchitectureFromName(archName);
-  switch (MachO::getCPUTypeFromArchitecture(config->arch).first) {
-  case MachO::CPU_TYPE_X86_64:
+// Read the first few bytes of \p path and try to determine its type.
+static MachO::Architecture inferMachineType(StringRef path) {
+  auto mbOrErr = MemoryBuffer::getFileSlice(
+      path, /*MapSize=*/sizeof(mach_header_64), /*Offset=*/0);
+  if (auto ec = mbOrErr.getError()) {
+    error("cannot open " + path + ": " + ec.message());
+    return AK_unknown;
+  }
+
+  std::unique_ptr<MemoryBuffer> &mb = *mbOrErr;
+  MemoryBufferRef mbref = mb->getMemBufferRef();
+
+  const char *buf = mbref.getBufferStart();
+  auto *hdr = reinterpret_cast<const MachO::fat_header *>(buf);
+  if (support::endian::read32be(&hdr->magic) == MachO::FAT_MAGIC)
+    return AK_unknown;
+
+  if (identify_magic(mbref.getBuffer()) == file_magic::macho_object) {
+    // It doesn't matter whether we cast to the 32- or 64-bit versions of the
+    // mach header, the two fields we care about have the same size & layout.
+    auto *hdr = reinterpret_cast<const mach_header_64 *>(buf);
+    return MachO::getArchitectureFromCpuType(hdr->cputype, hdr->cpusubtype);
+  }
+
+  return AK_unknown;
+}
+
+// We only consider non-fat object files when inferring the machine type.
+static MachO::Architecture inferMachineType(const opt::InputArgList &args) {
+  for (const auto &arg : args) {
+    const auto &opt = arg->getOption();
+    switch (opt.getID()) {
+    case OPT_INPUT: {
+      auto arch = inferMachineType(arg->getValue());
+      if (arch != AK_unknown)
+        return arch;
+      break;
+    }
+    case OPT_filelist:
+      Optional<MemoryBufferRef> buffer = readFile(arg->getValue());
+      if (!buffer)
+        continue;
+      for (StringRef path : args::getLines(*buffer)) {
+        auto arch = inferMachineType(path);
+        if (arch != AK_unknown)
+          return arch;
+      }
+      break;
+    }
+  }
+  return AK_unknown;
+}
+
+static TargetInfo *createTargetInfo(const opt::InputArgList &args) {
+  if (args.hasArg(OPT_arch)) {
+    StringRef archName = args.getLastArgValue(OPT_arch);
+    config->arch = MachO::getArchitectureFromName(archName);
+    if (config->arch == AK_unknown)
+      error("invalid -arch " + archName);
+  } else {
+    config->arch = inferMachineType(args);
+    if (config->arch == AK_unknown)
+      error("unable to infer architecture from input files, please use -arch "
+            "to specify");
+  }
+
+  switch (config->arch) {
+  case MachO::AK_x86_64:
+  case MachO::AK_x86_64h:
     return createX86_64TargetInfo();
-  case MachO::CPU_TYPE_ARM64:
+  case MachO::AK_arm64:
+  case MachO::AK_arm64e:
     return createARM64TargetInfo();
+  case MachO::AK_unknown:
+    return createUnknownTargetInfo();
   default:
-    fatal("missing or unsupported -arch " + archName);
+    error("unsupported architecture " + getArchitectureName(config->arch));
+    config->arch = AK_unknown;
+    return createUnknownTargetInfo();
   }
 }
 
@@ -664,7 +729,8 @@
 
   // TODO: add logic here as we support more archs. E.g. i386 should default
   // to PIE from 10.7
-  assert(config->arch == AK_x86_64 || config->arch == AK_x86_64h);
+  assert(config->arch == AK_unknown || config->arch == AK_x86_64 ||
+         config->arch == AK_x86_64h);
 
   PlatformKind kind = config->platform.kind;
   if (kind == PlatformKind::macOS &&
@@ -749,7 +815,6 @@
 
   config = make<Configuration>();
   symtab = make<SymbolTable>();
-  target = createTargetInfo(args);
 
   config->entry = symtab->addUndefined(args.getLastArgValue(OPT_e, "_main"),
                                        /*file=*/nullptr,
@@ -827,7 +892,9 @@
     return !errorCount();
   }
 
-  initLLVM(); // must be run before any call to addFile()
+  // These must be run before any call to addFile().
+  target = createTargetInfo(args);
+  initLLVM();
 
   for (const auto &arg : args) {
     const auto &opt = arg->getOption();
diff --git a/lld/MachO/Target.h b/lld/MachO/Target.h
--- a/lld/MachO/Target.h
+++ b/lld/MachO/Target.h
@@ -111,6 +111,7 @@
 
 TargetInfo *createX86_64TargetInfo();
 TargetInfo *createARM64TargetInfo();
+TargetInfo *createUnknownTargetInfo();
 
 extern TargetInfo *target;
 
diff --git a/lld/MachO/Target.cpp b/lld/MachO/Target.cpp
--- a/lld/MachO/Target.cpp
+++ b/lld/MachO/Target.cpp
@@ -43,4 +43,45 @@
   return valid;
 }
 
+// Dummy target that we use when we cannot figure out the right target. We
+// obviously cannot emit any valid binaries with this target; it's just
+// initialized in order that we can emit additional error messages before
+// exiting.
+class UnknownTarget : public TargetInfo {
+public:
+  UnknownTarget();
+
+  uint64_t
+  getEmbeddedAddend(llvm::MemoryBufferRef, const llvm::MachO::section_64 &,
+                    const llvm::MachO::relocation_info) const override {
+    return 0;
+  }
+
+  void relocateOne(uint8_t *loc, const Reloc &, uint64_t va,
+                   uint64_t pc) const override {}
+
+  void writeStub(uint8_t *buf, const Symbol &) const override {}
+  void writeStubHelperHeader(uint8_t *buf) const override {}
+  void writeStubHelperEntry(uint8_t *buf, const DylibSymbol &,
+                            uint64_t entryAddr) const override {}
+
+  void relaxGotLoad(uint8_t *loc, uint8_t type) const override {}
+
+  const RelocAttrs &getRelocAttrs(uint8_t type) const override {
+    llvm_unreachable("unknown arch has no relocations");
+  }
+
+  uint64_t getPageSize() const override { return 1024; }
+};
+
+UnknownTarget::UnknownTarget() {
+  cpuType = 0;
+  cpuSubtype = 0;
+}
+
+TargetInfo *macho::createUnknownTargetInfo() {
+  static UnknownTarget t;
+  return &t;
+}
+
 TargetInfo *macho::target = nullptr;
diff --git a/lld/test/MachO/arch.s b/lld/test/MachO/arch.s
--- a/lld/test/MachO/arch.s
+++ b/lld/test/MachO/arch.s
@@ -1,8 +1,15 @@
 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-darwin %s -o %t.o
 # RUN: %lld -o /dev/null %t.o
-# RUN: not %lld -arch i386 -o /dev/null %t.o 2>&1 | FileCheck %s
-# CHECK: error: missing or unsupported -arch i386
+# RUN: not %lld -arch i386 -o /dev/null %t.o 2>&1 | FileCheck %s --check-prefix=I386
+# RUN: not %lld -arch foo -o /dev/null %t.o 2>&1 | FileCheck %s --check-prefix=FOO
+# RUN: rm -f %t.a
+# RUN: llvm-ar rcs %t.a %t.o
+# RUN: not %lld -o /dev/null %t.a 2>&1 | FileCheck %s --check-prefix=ARCHIVE
+
+# I386: error: unsupported architecture i386
+# FOO: error: invalid -arch foo
+# ARCHIVE: error: unable to infer architecture from input files, please use -arch to specify
 
 .text
 .global _main
diff --git a/lld/test/MachO/arm64-reloc-got-load.s b/lld/test/MachO/arm64-reloc-got-load.s
--- a/lld/test/MachO/arm64-reloc-got-load.s
+++ b/lld/test/MachO/arm64-reloc-got-load.s
@@ -4,11 +4,11 @@
 # RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/main.s -o %t/main.o
 # RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/foobar.s -o %t/foobar.o
 
-# RUN: %lld -lSystem -arch arm64 -o %t/static %t/main.o %t/foobar.o
+# RUN: %lld -lSystem -o %t/static %t/main.o %t/foobar.o
 # RUN: llvm-objdump --macho -d --no-show-raw-insn --syms %t/static | FileCheck %s --check-prefix=STATIC
 
-# RUN: %lld -lSystem -arch arm64 -dylib -o %t/libfoo.dylib %t/foobar.o
-# RUN: %lld -lSystem -arch arm64 -o %t/main %t/main.o %t/libfoo.dylib
+# RUN: %lld -lSystem -dylib -o %t/libfoo.dylib %t/foobar.o
+# RUN: %lld -lSystem -o %t/main %t/main.o %t/libfoo.dylib
 # RUN: llvm-objdump --macho -d --no-show-raw-insn --section-headers %t/main | FileCheck %s --check-prefix=DYLIB
 
 # STATIC-LABEL: _main:
diff --git a/lld/test/MachO/arm64-reloc-pointer-to-got.s b/lld/test/MachO/arm64-reloc-pointer-to-got.s
--- a/lld/test/MachO/arm64-reloc-pointer-to-got.s
+++ b/lld/test/MachO/arm64-reloc-pointer-to-got.s
@@ -1,7 +1,7 @@
 # REQUIRES: aarch64
 
 # RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %s -o %t.o
-# RUN: %lld -lSystem -arch arm64 -o %t %t.o
+# RUN: %lld -lSystem -o %t %t.o
 # RUN: llvm-objdump --macho -d --full-contents --section-headers %t | FileCheck %s
 
 ## FIXME: Even though we have reserved a GOT slot for _foo due to
diff --git a/lld/test/MachO/arm64-relocs.s b/lld/test/MachO/arm64-relocs.s
--- a/lld/test/MachO/arm64-relocs.s
+++ b/lld/test/MachO/arm64-relocs.s
@@ -1,6 +1,6 @@
 # REQUIRES: aarch64, shell
 # RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %s -o %t.o
-# RUN: %lld -dylib -arch arm64 -lSystem -o %t %t.o
+# RUN: %lld -dylib -lSystem -o %t %t.o
 # RUN: (llvm-objdump --syms %t; llvm-objdump --macho -d --section=__const %t) | FileCheck %s
 
 # CHECK-LABEL: SYMBOL TABLE:
diff --git a/lld/test/MachO/codemodel.ll b/lld/test/MachO/codemodel.ll
--- a/lld/test/MachO/codemodel.ll
+++ b/lld/test/MachO/codemodel.ll
@@ -1,7 +1,7 @@
 ; REQUIRES: x86
 ; RUN: llvm-as %s -o %t.o
-; RUN: %lld -lSystem %t.o -o %ts -mllvm -code-model=small
-; RUN: %lld -lSystem %t.o -o %tl -mllvm -code-model=large
+; RUN: %lld -arch x86_64 -lSystem %t.o -o %ts -mllvm -code-model=small
+; RUN: %lld -arch x86_64 -lSystem %t.o -o %tl -mllvm -code-model=large
 ; RUN: llvm-objdump -d %ts | FileCheck %s --check-prefix=CHECK-SMALL
 ; RUN: llvm-objdump -d %tl | FileCheck %s --check-prefix=CHECK-LARGE
 
diff --git a/lld/test/MachO/cpu-string.ll b/lld/test/MachO/cpu-string.ll
--- a/lld/test/MachO/cpu-string.ll
+++ b/lld/test/MachO/cpu-string.ll
@@ -1,11 +1,11 @@
 ; REQUIRES: x86
 ; RUN: llvm-as %s -o %t.o
 
-; RUN: %lld %t.o -o %t.dylib -dylib
+; RUN: %lld -arch x86_64 %t.o -o %t.dylib -dylib
 ; RUN: llvm-objdump -d --section="__text" --no-leading-addr --no-show-raw-insn %t.dylib | FileCheck %s
 ; CHECK: nop{{$}}
 
-; RUN: %lld -mcpu znver1 %t.o -o %t.znver1.dylib -dylib
+; RUN: %lld -arch x86_64 -mcpu znver1 %t.o -o %t.znver1.dylib -dylib
 ; RUN: llvm-objdump -d --section="__text" --no-leading-addr --no-show-raw-insn %t.znver1.dylib | FileCheck %s --check-prefix=ZNVER1
 
 ; ZNVER1: nopw
diff --git a/lld/test/MachO/fat-arch.s b/lld/test/MachO/fat-arch.s
--- a/lld/test/MachO/fat-arch.s
+++ b/lld/test/MachO/fat-arch.s
@@ -2,10 +2,10 @@
 # RUN: llvm-mc -filetype=obj -triple=i386-apple-darwin %s -o %t.i386.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t.x86_64.o
 # RUN: llvm-lipo %t.i386.o %t.x86_64.o -create -o %t.fat.o
-# RUN: %lld -o /dev/null %t.fat.o
+# RUN: %lld -arch x86_64 -o /dev/null %t.fat.o
 
 # RUN: llvm-lipo %t.i386.o -create -o %t.noarch.o
-# RUN: not %lld -o /dev/null %t.noarch.o 2>&1 | \
+# RUN: not %lld -arch x86_64 -o /dev/null %t.noarch.o 2>&1 | \
 # RUN:    FileCheck %s -DFILE=%t.noarch.o
 # CHECK: error: unable to find matching architecture in [[FILE]]
 
diff --git a/lld/test/MachO/header.s b/lld/test/MachO/header.s
--- a/lld/test/MachO/header.s
+++ b/lld/test/MachO/header.s
@@ -2,10 +2,10 @@
 # RUN: rm -rf %t && mkdir -p %t
 # RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t/x86_64-test.o
 # RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %s -o %t/arm64-test.o
-# RUN: %lld -arch x86_64 -platform_version macos 10.5.0 11.0 -o %t/x86-64-executable %t/x86_64-test.o
-# RUN: %lld -arch arm64 -o %t/arm64-executable %t/arm64-test.o
-# RUN: %lld -arch x86_64 -dylib -o %t/x86-64-dylib %t/x86_64-test.o
-# RUN: %lld -arch arm64  -dylib -o %t/arm64-dylib %t/arm64-test.o
+# RUN: %lld -platform_version macos 10.5.0 11.0 -o %t/x86-64-executable %t/x86_64-test.o
+# RUN: %lld -o %t/arm64-executable %t/arm64-test.o
+# RUN: %lld -dylib -o %t/x86-64-dylib %t/x86_64-test.o
+# RUN: %lld -dylib -o %t/arm64-dylib %t/arm64-test.o
 
 # RUN: llvm-objdump --macho --all-headers %t/x86-64-executable | FileCheck %s -DCAPS=LIB64
 # RUN: llvm-objdump --macho --all-headers %t/arm64-executable | FileCheck %s -DCAPS=0x00
diff --git a/lld/test/MachO/invalid/invalid-executable.s b/lld/test/MachO/invalid/invalid-executable.s
--- a/lld/test/MachO/invalid/invalid-executable.s
+++ b/lld/test/MachO/invalid/invalid-executable.s
@@ -1,7 +1,7 @@
 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-darwin %s -o %t.o
-# RUN: %lld -o %t %t.o
-# RUN: not %lld -o /dev/null %t 2>&1 | FileCheck %s -DFILE=%t
+# RUN: %lld -lSystem -o %t %t.o
+# RUN: not %lld -lSystem -o /dev/null %t 2>&1 | FileCheck %s -DFILE=%t
 # CHECK: error: [[FILE]]: unhandled file type
 
 .text
diff --git a/lld/test/MachO/invalid/invalid-fat-offset.s b/lld/test/MachO/invalid/invalid-fat-offset.s
--- a/lld/test/MachO/invalid/invalid-fat-offset.s
+++ b/lld/test/MachO/invalid/invalid-fat-offset.s
@@ -1,6 +1,6 @@
 # REQUIRES: x86
 # RUN: yaml2obj %s -o %t.o
-# RUN: not %lld -o /dev/null %t.o 2>&1 | \
+# RUN: not %lld -arch x86_64 -o /dev/null %t.o 2>&1 | \
 # RUN:    FileCheck %s -DFILE=%t.o
 # CHECK: error: [[FILE]]: slice extends beyond end of file
 
diff --git a/lld/test/MachO/invalid/invalid-relocation-pcrel.yaml b/lld/test/MachO/invalid/invalid-relocation-pcrel.yaml
--- a/lld/test/MachO/invalid/invalid-relocation-pcrel.yaml
+++ b/lld/test/MachO/invalid/invalid-relocation-pcrel.yaml
@@ -2,7 +2,7 @@
 # RUN: rm -rf %t; mkdir %t
 # RUN: yaml2obj %s -o %t/test.o
 # RUN: llvm-ar rcs %t/test.a %t/test.o
-# RUN: not %lld -o /dev/null %t/test.a 2>&1 | FileCheck %s
+# RUN: not %lld -arch x86_64 -o /dev/null %t/test.a 2>&1 | FileCheck %s
 #
 # CHECK: error: UNSIGNED relocation must not be PC-relative at offset 1 of __TEXT,__text in {{.*}}test.a(test.o)
 
diff --git a/lld/test/MachO/lc-linker-option.ll b/lld/test/MachO/lc-linker-option.ll
--- a/lld/test/MachO/lc-linker-option.ll
+++ b/lld/test/MachO/lc-linker-option.ll
@@ -3,7 +3,7 @@
 # RUN: split-file %s %t
 
 # RUN: llvm-as %t/framework.ll -o %t/framework.o
-# RUN: %lld %t/framework.o -o %t/frame
+# RUN: %lld -arch x86_64 %t/framework.o -o %t/frame
 # RUN: llvm-objdump --macho --all-headers %t/frame | FileCheck --check-prefix=FRAME %s \
 # RUN:  --implicit-check-not LC_LOAD_DYLIB
 # FRAME:          cmd LC_LOAD_DYLIB
@@ -11,12 +11,12 @@
 # FRAME-NEXT:    name /System/Library/Frameworks/CoreFoundation.framework/CoreFoundation
 
 # RUN: llvm-as %t/l.ll -o %t/l.o
-# RUN: %lld %t/l.o -o %t/l
+# RUN: %lld -arch x86_64 %t/l.o -o %t/l
 # RUN: llvm-objdump --macho --all-headers %t/l | FileCheck --check-prefix=LIB %s \
 # RUN:  --implicit-check-not LC_LOAD_DYLIB
 
 ## Check that we don't create duplicate LC_LOAD_DYLIBs.
-# RUN: %lld -lSystem %t/l.o -o %t/l
+# RUN: %lld -arch x86_64 -lSystem %t/l.o -o %t/l
 # RUN: llvm-objdump --macho --all-headers %t/l | FileCheck --check-prefix=LIB %s \
 # RUN:  --implicit-check-not LC_LOAD_DYLIB
 # LIB:          cmd LC_LOAD_DYLIB
@@ -24,7 +24,7 @@
 # LIB-NEXT:    name /usr/lib/libSystem.B.dylib
 
 # RUN: llvm-as %t/invalid.ll -o %t/invalid.o
-# RUN: not %lld %t/invalid.o -o /dev/null 2>&1 | FileCheck --check-prefix=INVALID %s
+# RUN: not %lld -arch x86_64 %t/invalid.o -o /dev/null 2>&1 | FileCheck --check-prefix=INVALID %s
 # INVALID: error: -why_load is not allowed in LC_LINKER_OPTION
 
 #--- framework.ll
diff --git a/lld/test/MachO/linkonce.ll b/lld/test/MachO/linkonce.ll
--- a/lld/test/MachO/linkonce.ll
+++ b/lld/test/MachO/linkonce.ll
@@ -2,9 +2,9 @@
 ; RUN: rm -rf %t; split-file %s %t
 ; RUN: opt -module-summary %t/first.ll -o %t/first.o
 ; RUN: opt -module-summary %t/second.ll -o %t/second.o
-; RUN: %lld -dylib -lSystem %t/first.o %t/second.o -o %t/12
+; RUN: %lld -arch x86_64 -dylib -lSystem %t/first.o %t/second.o -o %t/12
 ; RUN: llvm-objdump --syms %t/12 | FileCheck %s --check-prefix=FIRST
-; RUN: %lld -dylib -lSystem %t/second.o %t/first.o -o %t/21
+; RUN: %lld -arch x86_64 -dylib -lSystem %t/second.o %t/first.o -o %t/21
 ; RUN: llvm-objdump --syms %t/21 | FileCheck %s --check-prefix=SECOND
 
 ; FIRST:  w    O __TEXT,first  _foo
diff --git a/lld/test/MachO/lto-object-path.ll b/lld/test/MachO/lto-object-path.ll
--- a/lld/test/MachO/lto-object-path.ll
+++ b/lld/test/MachO/lto-object-path.ll
@@ -4,10 +4,10 @@
 ; RUN: rm -rf %t; mkdir %t
 ; RUN: llvm-as %s -o %t/test.o
 
-; RUN: %lld %t/test.o -o %t/test
+; RUN: %lld -arch x86_64 %t/test.o -o %t/test
 ; RUN: llvm-nm -pa %t/test | FileCheck %s --check-prefixes CHECK,NOOBJPATH
 
-; RUN: %lld %t/test.o -o %t/test -object_path_lto %t/lto-temps
+; RUN: %lld -arch x86_64 %t/test.o -o %t/test -object_path_lto %t/lto-temps
 ; RUN: llvm-nm -pa %t/test | FileCheck %s --check-prefixes CHECK,OBJPATH -DDIR=%t/lto-temps
 
 ; CHECK:          0000000000000000                - 00 0000    SO /tmp/test.cpp
diff --git a/lld/test/MachO/lto-save-temps.ll b/lld/test/MachO/lto-save-temps.ll
--- a/lld/test/MachO/lto-save-temps.ll
+++ b/lld/test/MachO/lto-save-temps.ll
@@ -6,14 +6,14 @@
 ; RUN: rm -rf %t; split-file %s %t
 ; RUN: llvm-as %t/foo.ll -o %t/foo.o
 ; RUN: llvm-as %t/test.ll -o %t/test.o
-; RUN: %lld -save-temps %t/foo.o %t/test.o -o %t/test
+; RUN: %lld -arch x86_64 -save-temps %t/foo.o %t/test.o -o %t/test
 ; RUN: llvm-objdump -d --no-show-raw-insn %t/test.lto.o | FileCheck %s --check-prefix=ALL
 ; RUN: llvm-objdump -d --no-show-raw-insn %t/test | FileCheck %s --check-prefix=ALL
 
 ; RUN: rm -rf %t; split-file %s %t
 ; RUN: opt -module-summary %t/foo.ll -o %t/foo.o
 ; RUN: opt -module-summary %t/test.ll -o %t/test.o
-; RUN: %lld -save-temps %t/foo.o %t/test.o -o %t/test
+; RUN: %lld -arch x86_64 -save-temps %t/foo.o %t/test.o -o %t/test
 ; RUN: llvm-objdump -d --no-show-raw-insn %t/test1.lto.o | FileCheck %s --check-prefix=FOO
 ; RUN: llvm-objdump -d --no-show-raw-insn %t/test2.lto.o | FileCheck %s --check-prefix=MAIN
 ; RUN: llvm-objdump -d --no-show-raw-insn %t/test | FileCheck %s --check-prefix=ALL
diff --git a/lld/test/MachO/mattrs.ll b/lld/test/MachO/mattrs.ll
--- a/lld/test/MachO/mattrs.ll
+++ b/lld/test/MachO/mattrs.ll
@@ -3,10 +3,10 @@
 
 ;; Verify that LTO behavior can be tweaked using -mattr.
 
-; RUN: %lld -mcpu haswell -mllvm -mattr=+fma %t.o -o %t.dylib -dylib
+; RUN: %lld -arch x86_64 -mcpu haswell -mllvm -mattr=+fma %t.o -o %t.dylib -dylib
 ; RUN: llvm-objdump -d --section="__text" --no-leading-addr --no-show-raw-insn %t.dylib | FileCheck %s --check-prefix=FMA
 
-; RUN: %lld -mcpu haswell -mllvm -mattr=-fma %t.o -o %t.dylib -dylib
+; RUN: %lld -arch x86_64 -mcpu haswell -mllvm -mattr=-fma %t.o -o %t.dylib -dylib
 ; RUN: llvm-objdump -d --section="__text" --no-leading-addr --no-show-raw-insn %t.dylib | FileCheck %s --check-prefix=NO-FMA
 
 ; FMA:      <_foo>:
diff --git a/lld/test/MachO/module-asm.ll b/lld/test/MachO/module-asm.ll
--- a/lld/test/MachO/module-asm.ll
+++ b/lld/test/MachO/module-asm.ll
@@ -1,6 +1,6 @@
 ; REQUIRES: x86
 ; RUN: llvm-as %s -o %t.o
-; RUN: %lld %t.o -o %t
+; RUN: %lld -arch x86_64 %t.o -o %t
 ; RUN: llvm-objdump -d %t | FileCheck %s
 
 ; CHECK:      <_foo>:
diff --git a/lld/test/MachO/objc-arc-contract.ll b/lld/test/MachO/objc-arc-contract.ll
--- a/lld/test/MachO/objc-arc-contract.ll
+++ b/lld/test/MachO/objc-arc-contract.ll
@@ -5,15 +5,15 @@
 ;; which doesn't know how to handle it.
 
 ; RUN: llvm-as %s -o %t.o
-; RUN: %lld -dylib -lSystem %t.o -o %t --lto-legacy-pass-manager
+; RUN: %lld -arch x86_64 -dylib -lSystem %t.o -o %t --lto-legacy-pass-manager
 ; RUN: llvm-objdump -d %t | FileCheck %s
-; RUN: %lld -dylib -lSystem %t.o -o %t --no-lto-legacy-pass-manager
+; RUN: %lld -arch x86_64 -dylib -lSystem %t.o -o %t --no-lto-legacy-pass-manager
 ; RUN: llvm-objdump -d %t | FileCheck %s
 
 ; RUN: opt -module-summary %s -o %t.o
-; RUN: %lld -dylib -lSystem %t.o -o %t --lto-legacy-pass-manager
+; RUN: %lld -arch x86_64 -dylib -lSystem %t.o -o %t --lto-legacy-pass-manager
 ; RUN: llvm-objdump -d %t | FileCheck %s
-; RUN: %lld -dylib -lSystem %t.o -o %t --no-lto-legacy-pass-manager
+; RUN: %lld -arch x86_64 -dylib -lSystem %t.o -o %t --no-lto-legacy-pass-manager
 ; RUN: llvm-objdump -d %t | FileCheck %s
 
 ; CHECK:      <_foo>:
diff --git a/lld/test/MachO/reloc-subtractor.s b/lld/test/MachO/reloc-subtractor.s
--- a/lld/test/MachO/reloc-subtractor.s
+++ b/lld/test/MachO/reloc-subtractor.s
@@ -4,7 +4,7 @@
 # RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %s -o %t/arm64.o
 # RUN: %lld -lSystem %t/x86_64.o -o %t/x86_64
 # RUN: llvm-objdump --syms --full-contents --rebase %t/x86_64 | FileCheck %s
-# RUN: %lld -arch arm64 -lSystem %t/arm64.o -o %t/arm64
+# RUN: %lld -lSystem %t/arm64.o -o %t/arm64
 # RUN: llvm-objdump --syms --full-contents --rebase %t/arm64 | FileCheck %s
 
 # CHECK-LABEL: SYMBOL TABLE:
diff --git a/lld/test/MachO/reproduce-thin-archives.s b/lld/test/MachO/reproduce-thin-archives.s
--- a/lld/test/MachO/reproduce-thin-archives.s
+++ b/lld/test/MachO/reproduce-thin-archives.s
@@ -6,10 +6,10 @@
 # RUN: cd %t.dir
 # RUN: llvm-ar rcsT foo.a foo.o
 
-# RUN: %lld foo.a -o /dev/null --reproduce repro.tar
+# RUN: %lld -arch x86_64 foo.a -o /dev/null --reproduce repro.tar
 # RUN: tar tf repro.tar | FileCheck -DPATH='repro/%:t.dir' %s
 
-# RUN: %lld -all_load foo.a -o /dev/null --reproduce repro2.tar
+# RUN: %lld -arch x86_64 -all_load foo.a -o /dev/null --reproduce repro2.tar
 # RUN: tar tf repro2.tar | FileCheck -DPATH='repro2/%:t.dir' %s
 
 # CHECK-DAG: [[PATH]]/foo.a