[AArch64] Fix PR34625 -mtune without -mcpu should not set -target-cpu

When -mtune is used on AArch64 the -target-cpu is passed the value of the
cpu given to -mtune. As well as setting micro-architectural features of the
-mtune cpu, this will also add the architectural features such as support
for instructions. This can result in the backend using instructions that
are supported in the -mtune cpu but not supported in the target
architecture. For example use of the v8.1-a LSE extensions with -march=v8.
    
This change removes the setting of -target-cpu for -mtune, the -mcpu must
be used to set -target-cpu. This has the effect of removing all non-hard
coded benefits of mtune but it does produce correct output when -mtune cpu
with a later architecture than v8 is used.
    
Fixes PR34625
    
Differential Revision: https://reviews.llvm.org/D39179

llvm-svn: 316424

1 files changed, 10 insertions, 6 deletions

diff --git a/clang/lib/Driver/ToolChains/Arch/AArch64.cpp b/clang/lib/Driver/ToolChains/Arch/AArch64.cpp
index 36186f0ac32..ad04aedd098 100644
--- a/clang/lib/Driver/ToolChains/Arch/AArch64.cpp
+++ b/clang/lib/Driver/ToolChains/Arch/AArch64.cpp
@@ -20,14 +20,12 @@ using namespace clang;
 using namespace llvm::opt;
 
 /// getAArch64TargetCPU - Get the (LLVM) name of the AArch64 cpu we are
-/// targeting. Set \p A to the Arg corresponding to the -mcpu or -mtune
-/// arguments if they are provided, or to nullptr otherwise.
+/// targeting. Set \p A to the Arg corresponding to the -mcpu argument if it is
+/// provided, or to nullptr otherwise.
 std::string aarch64::getAArch64TargetCPU(const ArgList &Args, Arg *&A) {
   std::string CPU;
-  // If we have -mtune or -mcpu, use that.
-  if ((A = Args.getLastArg(clang::driver::options::OPT_mtune_EQ))) {
-    CPU = StringRef(A->getValue()).lower();
-  } else if ((A = Args.getLastArg(options::OPT_mcpu_EQ))) {
+  // If we have -mcpu, use that.
+  if ((A = Args.getLastArg(options::OPT_mcpu_EQ))) {
     StringRef Mcpu = A->getValue();
     CPU = Mcpu.split("+").first.lower();
   }
@@ -122,6 +120,12 @@ getAArch64MicroArchFeaturesFromMtune(const Driver &D, StringRef Mtune,
                                      const ArgList &Args,
                                      std::vector<StringRef> &Features) {
   std::string MtuneLowerCase = Mtune.lower();
+  // Check CPU name is valid
+  std::vector<StringRef> MtuneFeatures;
+  StringRef Tune;
+  if (!DecodeAArch64Mcpu(D, MtuneLowerCase, Tune, MtuneFeatures))
+    return false;
+
   // Handle CPU name is 'native'.
   if (MtuneLowerCase == "native")
     MtuneLowerCase = llvm::sys::getHostCPUName();