[ClangImporter] Let clang pick default target CPU. (#80451)

Currently, ClangImporter has some logic to pick the default arm64
target CPU (when -target-cpu isn't passed) based on the target OS
and arch variant.

This was originally done long ago in 3cf3f42, and later
maintained through 49a6c8e, because it was necessary when
clang (targeting darwin) relied on -arch to set this sort of default.

Clang has migrated to full -target triples for a while now, and has
sophisticated logic for picking default target CPUs and features based
on the target triple.

Currently, we override that by passing our -mcpu explicitly.
Instead, allow clang to pick its defaults, and only pass -mcpu
when asked via -target-cpu.

This is visible in the test, with arm64 macOS now defaulting to M1.
The same applies to simulators, per Triple::isTargetMachineMac.

rdar://148377686

Author: ahmedbougacha

lib/ClangImporter/ClangImporter.cpp

@@ -821,39 +821,15 @@ importer::addCommonInvocationArguments(
     switch (triple.getArch()) {
     case llvm::Triple::x86:
     case llvm::Triple::x86_64:
-      // `-mcpu` is deprecated and an alias for `-mtune`. We need to pass
-      // `-march` and `-mtune` to behave identically to the `apple-a\d+` cases
-      // below.
+      // For x86, `-mcpu` is deprecated and an alias of `-mtune`. We need to
+      // pass `-march` and `-mtune` to behave like `-mcpu` on other targets.
       invocationArgStrs.push_back("-march=" + importerOpts.TargetCPU);
       invocationArgStrs.push_back("-mtune=" + importerOpts.TargetCPU);
       break;
     default:
       invocationArgStrs.push_back("-mcpu=" + importerOpts.TargetCPU);
       break;
     }
-  } else if (triple.isOSDarwin()) {
-    // Special case CPU based on known deployments:
-    //   - arm64 deploys to apple-a7
-    //   - arm64 on macOS
-    //   - arm64 for iOS/tvOS/watchOS simulators
-    //   - arm64e deploys to apple-a12
-    // and arm64e (everywhere) and arm64e macOS defaults to the "apple-a12" CPU
-    // for Darwin, but Clang only detects this if we use -arch.
-    if (triple.getArchName() == "arm64e")
-      invocationArgStrs.push_back("-mcpu=apple-a12");
-    else if (triple.isAArch64() && triple.isMacOSX())
-      invocationArgStrs.push_back("-mcpu=apple-a12");
-    else if (triple.isAArch64() && triple.isSimulatorEnvironment() &&
-             (triple.isiOS() || triple.isWatchOS()))
-      invocationArgStrs.push_back("-mcpu=apple-a12");
-    else if (triple.isAArch64() && triple.isSimulatorEnvironment() &&
-             triple.isXROS())
-      invocationArgStrs.push_back("-mcpu=apple-a12");
-    else if (triple.getArch() == llvm::Triple::aarch64 ||
-             triple.getArch() == llvm::Triple::aarch64_32 ||
-             triple.getArch() == llvm::Triple::aarch64_be) {
-      invocationArgStrs.push_back("-mcpu=apple-a7");
-    }
   } else if (triple.getArch() == llvm::Triple::systemz) {
     invocationArgStrs.push_back("-march=z13");
   }

test/ClangImporter/invocation-mcpu.swift

@@ -1,7 +1,12 @@
+// RUN: %swift %s -typecheck -parse-stdlib -dump-clang-diagnostics -target arm64e-apple-macos11.0 2>&1 | %FileCheck -check-prefix=CHECK-APPLE-M1 %s
+// RUN: %swift %s -typecheck -parse-stdlib -dump-clang-diagnostics -target arm64-apple-ios13.0-simulator 2>&1 | %FileCheck -check-prefix=CHECK-APPLE-M1 %s
+// CHECK-APPLE-M1: '-target-cpu' 'apple-m1' '-target-feature'
+
 // RUN: %swift %s -typecheck -parse-stdlib -dump-clang-diagnostics -target arm64e-apple-ios13.0 2>&1 | %FileCheck -check-prefix=CHECK-APPLE-A12 %s
-// RUN: %swift %s -typecheck -parse-stdlib -dump-clang-diagnostics -target arm64e-apple-macos11.0 2>&1 | %FileCheck -check-prefix=CHECK-APPLE-A12 %s
-// RUN: %swift %s -typecheck -parse-stdlib -dump-clang-diagnostics -target arm64e-apple-ios13.0-simulator 2>&1 | %FileCheck -check-prefix=CHECK-APPLE-A12 %s
-// CHECK-APPLE-A12: '-mcpu=apple-a12'
+// CHECK-APPLE-A12: '-target-cpu' 'apple-a12' '-target-feature'
 
 // RUN: %swift %s -typecheck -parse-stdlib -dump-clang-diagnostics -target arm64-apple-ios13.0 2>&1 | %FileCheck -check-prefix=CHECK-APPLE-A7 %s
-// CHECK-APPLE-A7: '-mcpu=apple-a7'
+// CHECK-APPLE-A7: '-target-cpu' 'apple-a7' '-target-feature'
+
+// RUN: %swift %s -typecheck -parse-stdlib -dump-clang-diagnostics -target arm64-apple-ios13.0 -target-cpu apple-a16 2>&1 | %FileCheck -check-prefix=CHECK-APPLE-A16 %s
+// CHECK-APPLE-A16: '-target-cpu' 'apple-a16' '-target-feature'