Implement `sys::getHostCPUName()` for Darwin ARM

[beanz]

Summary

Currently there is no implementation of sys::getHostCPUName() for Darwin ARM targets. This patch makes it so that LLVM running on ARM makes reasonable guesses about the CPU features of the host CPU.

[efriedma]

llvm/lib/Support/Host.cpp

1236

What do 64-bit devices return here? (I assume the call returns CPU_TYPE_ARM for 32-bit code regardless of the CPU?)

1243

Can we skip the syscall on __aarch64__ targets, since you don't really check the result?

[beanz]

llvm/lib/Support/Host.cpp

1236

Modern Apple 64-bit devices can't execute 32bit code, so that really should never happen. If it does, returning a value that matches the binary's CPU type/subtype is probably reasonable.

I could break the arm 32/64-bit implementations up into separate ifdef blocks if that seems more sane to you. In that case we could return cyclone for arm 32 in the arm 64 implementation, but we would still likely want the sys call to determine if the cpu type is 64 or 32 bit.

1243

Sure... Until arm64e pointer authentication lands in upstream (see @rjmccall's RFC). At which point this changes. I tried to structure the code to take that into account.

[efriedma]

llvm/lib/Support/Host.cpp

1236

If you're not sure what cpu_type the kernel returns for a 32-bit userspace on top of a 64-bit kernel, you could check on x86. That said, I don't want to add codepaths you can't reasonably test.

Splitting the ifdef blocks probably makes sense.

1243

Okay.

[beanz]

Rebasing and updating to separate arm32 and arm64.

I beleive this is the simplest correct implementation based on the state of the in-tree Aarch64 support.

[efriedma]

LGTM

llvm/lib/Support/Host.cpp

 #include "Unix/Host.inc"
 #endif
 #ifdef _WIN32
 #include "Windows/Host.inc"
 #endif
 #ifdef _MSC_VER
 #include <intrin.h>
 #endif
-#if defined(__APPLE__) && (defined(__ppc__) || defined(__powerpc__))
+#if defined(__APPLE__) && (!defined(__x86_64__))
 #include <mach/host_info.h>
 #include <mach/mach.h>
 #include <mach/mach_host.h>
 #include <mach/machine.h>
 #endif
 
 #define DEBUG_TYPE "host-detection"
 
StringRef sys::getHostCPUName() {
   return detail::getHostCPUNameForARM(Content);
 }
 #elif defined(__linux__) && defined(__s390x__)
 StringRef sys::getHostCPUName() {
   std::unique_ptr<llvm::MemoryBuffer> P = getProcCpuinfoContent();
   StringRef Content = P ? P->getBuffer() : "";
   return detail::getHostCPUNameForS390x(Content);
 }
+#elif defined(__APPLE__) && defined(__aarch64__)
+StringRef sys::getHostCPUName() {
+  return "cyclone";
+}
+#elif defined(__APPLE__) && defined(__arm__)
+StringRef sys::getHostCPUName() {
+  host_basic_info_data_t hostInfo;
+  mach_msg_type_number_t infoCount;
+
+  infoCount = HOST_BASIC_INFO_COUNT;
+  mach_port_t hostPort = mach_host_self();
+  host_info(hostPort, HOST_BASIC_INFO, (host_info_t)&hostInfo,
+            &infoCount);
+  mach_port_deallocate(mach_task_self(), hostPort);
+
+  if (hostInfo.cpu_type != CPU_TYPE_ARM) {
+    assert(false && "CPUType not equal to ARM should not be possible on ARM");
+    return "generic";
+  }
+  switch (hostInfo.cpu_subtype) {
+    case CPU_SUBTYPE_ARM_V7S:
+      return "swift";
+    default:;
+    }
+  
+  return "generic";
+}
 #else
 StringRef sys::getHostCPUName() { return "generic"; }
 #endif
 
 #if defined(__linux__) && defined(__x86_64__)
 // On Linux, the number of physical cores can be computed from /proc/cpuinfo,
 // using the number of unique physical/core id pairs. The following
 // implementation reads the /proc/cpuinfo format on an x86_64 system.