[ARM] Introduce -mtp=auto and make it the default

This adds a new value auto to the possible values of the existing -mtp= clang option which controls how the thread pointer is found. auto means the same as soft if the target architecture doesn't support a hardware thread pointer at all; otherwise it means the same as cp15.

This behavior is the default in gcc version 7.3.0 and later. The new auto option is therefore also the default in clang, so this change aligns clang with gcc.

Author: Zhenhang1213

clang/lib/Driver/ToolChains/Arch/ARM.cpp

@@ -239,7 +239,7 @@ arm::ReadTPMode arm::getReadTPMode(const Driver &D, const ArgList &Args,
       D.Diag(diag::err_drv_invalid_mtp) << A->getAsString(Args);
     return ReadTPMode::Invalid;
   }
-  return ReadTPMode::Soft;
+  return (isHardTPSupported(Triple) ? ReadTPMode::TPIDRURO : ReadTPMode::Soft);
 }
 
 void arm::setArchNameInTriple(const Driver &D, const ArgList &Args,
@@ -574,11 +574,13 @@ llvm::ARM::FPUKind arm::getARMTargetFeatures(const Driver &D,
       A->ignoreTargetSpecific();
   }
 
-  if (getReadTPMode(D, Args, Triple, ForAS) == ReadTPMode::TPIDRURW)
+  arm::ReadTPMode TPMode = getReadTPMode(D, Args, Triple, ForAS);
+
+  if (TPMode == ReadTPMode::TPIDRURW)
     Features.push_back("+read-tp-tpidrurw");
-  if (getReadTPMode(D, Args, Triple, ForAS) == ReadTPMode::TPIDRURO)
+  else if (TPMode == ReadTPMode::TPIDRURO)
     Features.push_back("+read-tp-tpidruro");
-  if (getReadTPMode(D, Args, Triple, ForAS) == ReadTPMode::TPIDRPRW)
+  else if (TPMode == ReadTPMode::TPIDRPRW)
     Features.push_back("+read-tp-tpidrprw");
 
   const Arg *ArchArg = Args.getLastArg(options::OPT_march_EQ);

clang/test/Driver/arm-thread-pointer.c

@@ -42,4 +42,4 @@
 
 // RUN: %clang --target=armv7-linux -### -S %s 2>&1 | \
 // RUN: FileCheck -check-prefix=ARMv7_THREAD_POINTER_NON %s
-// ARMv7_THREAD_POINTER_NON-NOT: "-target-feature" "+read-tp-tpidruro"
+// ARMv7_THREAD_POINTER_NON: "-target-feature" "+read-tp-tpidruro"