diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -126,11 +126,9 @@
 }
 
 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");
+  StringRef archName = args.getLastArgValue(OPT_arch);
+  if (archName.empty())
+    fatal("must specify -arch");
   config->arch = MachO::getArchitectureFromName(archName);
   switch (MachO::getCPUTypeFromArchitecture(config->arch).first) {
   case MachO::CPU_TYPE_X86_64:
@@ -563,8 +561,10 @@
 static PlatformInfo getPlatformVersion(const opt::ArgList &args) {
   const opt::Arg *arg = args.getLastArg(OPT_platform_version);
   PlatformInfo platform;
-  if (!arg)
+  if (!arg) {
+    error("must specify -platform_version");
     return platform;
+  }
 
   StringRef platformStr = arg->getValue(0);
   StringRef minVersionStr = arg->getValue(1);
diff --git a/lld/MachO/InputFiles.cpp b/lld/MachO/InputFiles.cpp
--- a/lld/MachO/InputFiles.cpp
+++ b/lld/MachO/InputFiles.cpp
@@ -511,6 +511,7 @@
           getArchitectureName(config->arch));
     return;
   }
+  // TODO: check platform too
 
   if (const load_command *cmd = findCommand(hdr, LC_LINKER_OPTION)) {
     auto *c = reinterpret_cast<const linker_option_command *>(cmd);
diff --git a/lld/test/MachO/lit.local.cfg b/lld/test/MachO/lit.local.cfg
--- a/lld/test/MachO/lit.local.cfg
+++ b/lld/test/MachO/lit.local.cfg
@@ -2,7 +2,13 @@
 
 import os
 
-lld = ('ld64.lld -syslibroot ' +
+# We specify the most commonly-used arch and platform version in our tests here
+# Tests which need different settings can just append to this, as only the last
+# value will be used.
+#
+# Note however that this does not apply to `-syslibroot`: each instance of that
+# flag will append to the set of library roots.
+lld = ('ld64.lld -arch x86_64 -platform_version macos 10.0 11.0 -syslibroot ' +
     os.path.join(config.test_source_root, "MachO", "Inputs", "MacOSX.sdk"))
 config.substitutions.append(('%lld', lld + ' -fatal_warnings'))
 config.substitutions.append(('%no_fatal_warnings_lld', lld))
diff --git a/lld/test/MachO/syslibroot.test b/lld/test/MachO/syslibroot.test
--- a/lld/test/MachO/syslibroot.test
+++ b/lld/test/MachO/syslibroot.test
@@ -1,56 +1,68 @@
 # Ensure that a nonexistent path is ignored with a syslibroot
 
-RUN: ld64.lld -v -dylib -o /dev/null -syslibroot /var/empty | FileCheck %s -check-prefix CHECK-NONEXISTENT-SYSLIBROOT
+RUN: ld64.lld -arch x86_64 -platform_version macos 10 11 -v -dylib -o /dev/null \
+RUN:   -syslibroot /var/empty | FileCheck %s -check-prefix CHECK-NONEXISTENT-SYSLIBROOT
 
 CHECK-NONEXISTENT-SYSLIBROOT: Library search paths:
 CHECK-NONEXISTENT-SYSLIBROOT-NEXT: Framework search paths:
 
 RUN: mkdir -p %t/usr/lib
-RUN: ld64.lld -v -dylib -o /dev/null -syslibroot %t 2>&1 | FileCheck %s -check-prefix CHECK-SYSLIBROOT -DROOT=%t
+RUN: ld64.lld -arch x86_64 -platform_version macos 10 11 -v -dylib -o /dev/null \
+RUN:   -syslibroot %t 2>&1 | FileCheck %s -check-prefix CHECK-SYSLIBROOT -DROOT=%t
 
 CHECK-SYSLIBROOT-NOT: directory not found{{.*}}usr/local/lib
 CHECK-SYSLIBROOT: Library search paths:
 CHECK-SYSLIBROOT-NEXT: [[ROOT]]/usr/lib
 
 RUN: mkdir -p %t/Library/libxml2-development
-RUN: ld64.lld -v -dylib -o /dev/null -syslibroot %t -L /Library/libxml2-development | FileCheck %s -check-prefix CHECK-ABSOLUTE-PATH-REROOTED -DROOT=%t
+RUN: ld64.lld -arch x86_64 -platform_version macos 10 11 -v -dylib -o /dev/null \
+RUN:   -syslibroot %t -L /Library/libxml2-development | FileCheck %s -check-prefix CHECK-ABSOLUTE-PATH-REROOTED -DROOT=%t
 
 CHECK-ABSOLUTE-PATH-REROOTED: Library search paths:
 CHECK-ABSOLUTE-PATH-REROOTED: [[ROOT]]/Library/libxml2-development
 CHECK-ABSOLUTE-PATH-REROOTED: [[ROOT]]/usr/lib
 
-RUN: ld64.lld -v -dylib -o /dev/null -syslibroot %t -L %t/Library/libxml2-development | FileCheck %s -check-prefix CHECK-PATH-WITHOUT-REROOT -DPATH=%t/Library/libxml2-development
+RUN: ld64.lld -arch x86_64 -platform_version macos 10 11 -v -dylib -o /dev/null \
+RUN:   -syslibroot %t -L %t/Library/libxml2-development | FileCheck %s -check-prefix CHECK-PATH-WITHOUT-REROOT -DPATH=%t/Library/libxml2-development
 CHECK-PATH-WITHOUT-REROOT:      Library search paths:
 CHECK-PATH-WITHOUT-REROOT-NEXT: [[PATH]]
 
 RUN: mkdir -p %t.2/usr/lib
-RUN: ld64.lld -v -dylib -o /dev/null -syslibroot %t -syslibroot %t.2 | FileCheck %s -check-prefix CHECK-SYSLIBROOT-MATRIX -DROOT=%t
+RUN: ld64.lld -arch x86_64 -platform_version macos 10 11 -v -dylib -o /dev/null \
+RUN:   -syslibroot %t -syslibroot %t.2 | FileCheck %s -check-prefix CHECK-SYSLIBROOT-MATRIX -DROOT=%t
 
 CHECK-SYSLIBROOT-MATRIX: Library search paths:
 CHECK-SYSLIBROOT-MATRIX: [[ROOT]]/usr/lib
 CHECK-SYSLIBROOT-MATRIX: [[ROOT]].2/usr/lib
 
-RUN: ld64.lld -v -dylib -o /dev/null -syslibroot %t -syslibroot %t.2 -syslibroot / | FileCheck %s -check-prefix CHECK-SYSLIBROOT-IGNORED -DROOT=%t
+RUN: ld64.lld -arch x86_64 -platform_version macos 10 11 -v -dylib -o /dev/null \
+RUN:   -syslibroot %t -syslibroot %t.2 -syslibroot / | \
+RUN:   FileCheck %s -check-prefix CHECK-SYSLIBROOT-IGNORED -DROOT=%t
 
 CHECK-SYSLIBROOT-IGNORED: Library search paths:
 CHECK-SYSLIBROOT-IGNORED-NOT: [[ROOT]]/usr/lib
 CHECK-SYSLIBROOT-IGNORED-NOT: [[ROOT]].2/usr/lib
 
 RUN: mkdir -p %t/System/Library/Frameworks
-RUN: ld64.lld -v -dylib -o /dev/null -syslibroot %t | FileCheck %s -check-prefix CHECK-SYSLIBROOT-FRAMEWORK -DROOT=%t
+RUN: ld64.lld -arch x86_64 -platform_version macos 10 11 -v -dylib -o /dev/null \
+RUN:   -syslibroot %t | FileCheck %s -check-prefix CHECK-SYSLIBROOT-FRAMEWORK -DROOT=%t
 
 CHECK-SYSLIBROOT-FRAMEWORK: Framework search paths:
 CHECK-SYSLIBROOT-FRAMEWORK: [[ROOT]]/System/Library/Frameworks
 
 RUN: mkdir -p %t/Library/Frameworks
 RUN: mkdir -p %t.2/Library/Frameworks
-RUN: ld64.lld -v -dylib -o /dev/null -syslibroot %t -syslibroot %t.2 -F /Library/Frameworks | FileCheck %s -check-prefix CHECK-SYSLIBROOT-FRAMEWORK-MATRIX -DROOT=%t
+RUN: ld64.lld -arch x86_64 -platform_version macos 10 11 -v -dylib -o /dev/null \
+RUN:   -syslibroot %t -syslibroot %t.2 -F /Library/Frameworks | \
+RUN:   FileCheck %s -check-prefix CHECK-SYSLIBROOT-FRAMEWORK-MATRIX -DROOT=%t
 
 CHECK-SYSLIBROOT-FRAMEWORK-MATRIX: Framework search paths:
 CHECK-SYSLIBROOT-FRAMEWORK-MATRIX: [[ROOT]]/Library/Frameworks
 CHECK-SYSLIBROOT-FRAMEWORK-MATRIX: [[ROOT]].2/Library/Frameworks
 
-RUN: ld64.lld -v -dylib -o /dev/null -syslibroot %t -syslibroot %t.2 -syslibroot / -F /Library/Frameworks | FileCheck %s -check-prefix CHECK-SYSLIBROOT-FRAMEWORK-IGNORED -DROOT=%t
+RUN: ld64.lld -arch x86_64 -platform_version macos 10 11 -v -dylib -o /dev/null \
+RUN:   -syslibroot %t -syslibroot %t.2 -syslibroot / -F /Library/Frameworks | \
+RUN:   FileCheck %s -check-prefix CHECK-SYSLIBROOT-FRAMEWORK-IGNORED -DROOT=%t
 
 CHECK-SYSLIBROOT-FRAMEWORK-IGNORED: Framework search paths:
 CHECK-SYSLIBROOT-FRAMEWORK-IGNORED-NOT: [[ROOT]]/Library/Frameworks