Xcode: Explicitly specify default native architecture on macOS

When `CMAKE_OSX_ARCHITECTURES` is not specified, we add the Xcode
setting `ONLY_ACTIVE_ARCH = YES` with the intention of targeting the
native architecture of the host.  However, the default `ARCHS` value
chosen by "Xcode 12 Universal Apps" includes multiple architectures.
Add an explicit `ARCHS` setting with value `$(NATIVE_ARCH_ACTUAL)`
to tell Xcode to use the host's native architecture only.

Fixes: #20893

1 files changed, 11 insertions, 1 deletions

diff --git a/Source/cmGlobalXCodeGenerator.cxx b/Source/cmGlobalXCodeGenerator.cxx
index e006be2..65828ce 100644
--- a/Source/cmGlobalXCodeGenerator.cxx
+++ b/Source/cmGlobalXCodeGenerator.cxx
@@ -12,6 +12,7 @@
 
 #include <cm/memory>
 #include <cmext/algorithm>
+#include <cmext/string_view>
 
 #include "cmsys/RegularExpression.hxx"
 
@@ -3118,6 +3119,14 @@ bool cmGlobalXCodeGenerator::CreateXCodeObjects(
   if (archs.empty()) {
     // Tell Xcode to use NATIVE_ARCH instead of ARCHS.
     buildSettings->AddAttribute("ONLY_ACTIVE_ARCH", this->CreateString("YES"));
+    // When targeting macOS, use only the host architecture.
+    if (this->SystemName == "Darwin"_s &&
+        (!sysroot || !*sysroot ||
+         cmSystemTools::LowerCase(sysroot).find("macos") !=
+           std::string::npos)) {
+      buildSettings->AddAttribute("ARCHS",
+                                  this->CreateString("$(NATIVE_ARCH_ACTUAL)"));
+    }
   } else {
     // Tell Xcode to use ARCHS (ONLY_ACTIVE_ARCH defaults to NO).
     buildSettings->AddAttribute("ARCHS", this->CreateString(archs));
@@ -3217,7 +3226,8 @@ void cmGlobalXCodeGenerator::ComputeArchitectures(cmMakefile* mf)
   }
 
   if (this->Architectures.empty()) {
-    // With no ARCHS we use ONLY_ACTIVE_ARCH.
+    // With no ARCHS we use ONLY_ACTIVE_ARCH and possibly a
+    // platform-specific default ARCHS placeholder value.
     // Look up the arch that Xcode chooses in this case.
     if (const char* arch = mf->GetDefinition("CMAKE_XCODE_ARCHS")) {
       this->ObjectDirArchDefault = arch;