Apple: Improve comments for -arch linker argument

Author: madsmtm

compiler/rustc_target/src/spec/base/apple/mod.rs

@@ -116,21 +116,40 @@ fn pre_link_args(os: &'static str, arch: Arch, abi: TargetAbi) -> LinkArgs {
     };
     let sdk_version = min_version.clone();
 
-    let mut args = TargetOptions::link_args(
-        LinkerFlavor::Darwin(Cc::No, Lld::No),
-        &["-arch", arch.target_name(), "-platform_version"],
-    );
+    let mut args = LinkArgs::new();
+    // From the man page for ld64 (`man ld`):
+    // > The linker accepts universal (multiple-architecture) input files,
+    // > but always creates a "thin" (single-architecture), standard Mach-O
+    // > output file. The architecture for the output file is specified using
+    // > the -arch option.
+    //
+    // The linker has heuristics to determine the desired architecture, but to
+    // be safe, and to avoid a warning, we set the architecture explicitly.
+    //
+    // Supported architecture names can be found in the source:
+    // https://github.com/apple-oss-distributions/ld64/blob/ld64-951.9/src/abstraction/MachOFileAbstraction.hpp#L578-L648
+    let ld_arch = match arch {
+        Armv7k => "armv7k",
+        Armv7s => "armv7s",
+        Arm64 => "arm64",
+        Arm64e => "arm64e",
+        Arm64_32 => "arm64_32",
+        // ld64 doesn't understand i686, so fall back to i386 instead
+        //
+        // Same story when linking with cc, since that ends up invoking ld64.
+        I386 | I686 => "i386",
+        X86_64 => "x86_64",
+        X86_64h => "x86_64h",
+    };
+    add_link_args(&mut args, LinkerFlavor::Darwin(Cc::No, Lld::No), &["-arch", ld_arch]);
+
     add_link_args_iter(
         &mut args,
         LinkerFlavor::Darwin(Cc::No, Lld::No),
-        [platform_name, min_version, sdk_version].into_iter(),
+        ["-platform_version".into(), platform_name, min_version, sdk_version].into_iter(),
     );
     if abi != TargetAbi::MacCatalyst {
-        add_link_args(
-            &mut args,
-            LinkerFlavor::Darwin(Cc::Yes, Lld::No),
-            &["-arch", arch.target_name()],
-        );
+        add_link_args(&mut args, LinkerFlavor::Darwin(Cc::Yes, Lld::No), &["-arch", ld_arch]);
     } else {
         add_link_args_iter(
             &mut args,

compiler/rustc_target/src/spec/targets/i686_apple_darwin.rs

@@ -2,8 +2,7 @@ use crate::spec::base::apple::{macos_llvm_target, opts, Arch, TargetAbi};
 use crate::spec::{Cc, FramePointer, LinkerFlavor, Lld, Target, TargetOptions};
 
 pub(crate) fn target() -> Target {
-    // ld64 only understands i386 and not i686
-    let arch = Arch::I386;
+    let arch = Arch::I686;
     let mut base = opts("macos", arch, TargetAbi::Normal);
     base.max_atomic_width = Some(64);
     base.add_pre_link_args(LinkerFlavor::Darwin(Cc::Yes, Lld::No), &["-m32"]);
@@ -13,9 +12,7 @@ pub(crate) fn target() -> Target {
         // Clang automatically chooses a more specific target based on
         // MACOSX_DEPLOYMENT_TARGET. To enable cross-language LTO to work
         // correctly, we do too.
-        //
-        // While ld64 doesn't understand i686, LLVM does.
-        llvm_target: macos_llvm_target(Arch::I686).into(),
+        llvm_target: macos_llvm_target(arch).into(),
         metadata: crate::spec::TargetMetadata {
             description: Some("32-bit macOS (10.12+, Sierra+)".into()),
             tier: Some(3),