[Driver,AArch64] Add support for -mcpu=native.

[fhahn]

Summary

This patches adds support for passing -mcpu=native for AArch64. It will
get turned into the host CPU name, before we get the target features.

CPU = native is handled in a similar fashion in
getAArch64MicroArchFetauresFromMtune and getAArch64TargetCPU already.

Having a good test case for this is hard, as it depends on the host CPU
of the machine running the test. But we can check that native has been
replaced with something else.

When cross-compiling, we will get a CPU name from the host architecture
and get the clang compiler does not support '-mcpu=native' as error
message, which seems reasonable to me.

[peter.smith]

Looks ok to me. I've checked that gcc also supports -mcpu=native on an AArch64 machine. It also supports -march=native. While I don't think that this is necessary to support it for this patch is it worth considering as well?

[fhahn]

In D48931#1152325, @peter.smith wrote:

Looks ok to me. I've checked that gcc also supports -mcpu=native on an AArch64 machine. It also supports -march=native. While I don't think that this is necessary to support it for this patch is it worth considering as well?

Yep it should be possible to add support for -march=native by getting the host CPU and then getting the architecture version of the host CPU. I would prefer to do that in a separate patch, which I can put together tomorrow.

[peter.smith]

Looks good to me.

lib/Driver/ToolChains/Arch/AArch64.cpp

 
 // Check if the CPU name and feature modifiers in -mcpu are legal. If yes,
 // decode CPU and feature.
 static bool DecodeAArch64Mcpu(const Driver &D, StringRef Mcpu, StringRef &CPU,
                               std::vector<StringRef> &Features) {
   std::pair<StringRef, StringRef> Split = Mcpu.split("+");
   CPU = Split.first;
 
+  if (CPU == "native")
+    CPU = llvm::sys::getHostCPUName();
+
   if (CPU == "generic") {
     Features.push_back("+neon");
   } else {
     llvm::AArch64::ArchKind ArchKind = llvm::AArch64::parseCPUArch(CPU);
     if (!llvm::AArch64::getArchFeatures(ArchKind, Features))
       return false;
 
     unsigned Extension = llvm::AArch64::getDefaultExtensions(CPU, ArchKind);

test/Driver/aarch64-cpus.c

 
 // RUN: %clang -target arm64 -### -c %s 2>&1 | FileCheck -check-prefix=ARM64-GENERIC %s
 // RUN: %clang -target arm64 -mcpu=generic -### -c %s 2>&1 | FileCheck -check-prefix=ARM64-GENERIC %s
 // RUN: %clang -target arm64 -mlittle-endian -### -c %s 2>&1 | FileCheck -check-prefix=ARM64-GENERIC %s
 // RUN: %clang -target arm64 -mlittle-endian -mcpu=generic -### -c %s 2>&1 | FileCheck -check-prefix=ARM64-GENERIC %s
 
 // ARM64-GENERIC: "-cc1"{{.*}} "-triple" "arm64{{.*}}" "-target-cpu" "generic"
 
+// We cannot check much for -mcpu=native, but it should be replaced by either generic or a valid
+// Arm cpu string, depending on the host.
+// RUN: %clang -target arm64 -mcpu=native -### -c %s 2>&1 | FileCheck -check-prefix=ARM64-NATIVE %s
+// ARM64-NATIVE-NOT: "-cc1"{{.*}} "-triple" "arm64{{.*}}" "-target-cpu" "native"
+
 // RUN: %clang -target arm64-apple-darwin -arch arm64 -### -c %s 2>&1 | FileCheck -check-prefix=ARM64-DARWIN %s
 // ARM64-DARWIN: "-cc1"{{.*}} "-triple" "arm64{{.*}}" "-target-cpu" "cyclone"
 
 // RUN: %clang -target aarch64 -mcpu=cortex-a35 -### -c %s 2>&1 | FileCheck -check-prefix=CA35 %s
 // RUN: %clang -target aarch64 -mlittle-endian -mcpu=cortex-a35 -### -c %s 2>&1 | FileCheck -check-prefix=CA35 %s
 // RUN: %clang -target aarch64_be -mlittle-endian -mcpu=cortex-a35 -### -c %s 2>&1 | FileCheck -check-prefix=CA35 %s
 // RUN: %clang -target aarch64 -mtune=cortex-a35 -### -c %s 2>&1 | FileCheck -check-prefix=CA35-TUNE %s
 // RUN: %clang -target aarch64 -mlittle-endian -mtune=cortex-a35 -### -c %s 2>&1 | FileCheck -check-prefix=CA35-TUNE %s