[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 4.1.0 and later. The new
auto option is therefore also the default in clang, so this change
aligns clang with gcc.

Fixes #123864.

Author: Zhenhang1213

clang/docs/ReleaseNotes.rst

@@ -122,6 +122,10 @@ Deprecated Compiler Flags
 Modified Compiler Flags
 -----------------------
 
+- The ARM AArch32 ``-mtp`` option accepts and defaults to ``auto``, a value of ``auto`` uses the best available method of providing the frame pointer supported by the hardware. This matches
+  the behavior of ``-mtp`` in gcc. This changes the default behavior for ARM targets that provide the ``TPIDRURO`` register as this will be used instead of a call to the ``__aeabi_read_tp``.
+  Programs that use ``__aeabi_read_tp`` but do not use the ``TPIDRURO`` register must use ``-mtp=soft``. Fixes #123864
+
 Removed Compiler Flags
 -------------------------
 

clang/include/clang/Driver/Options.td

@@ -4711,7 +4711,7 @@ def mexecute_only : Flag<["-"], "mexecute-only">, Group<m_arm_Features_Group>,
 def mno_execute_only : Flag<["-"], "mno-execute-only">, Group<m_arm_Features_Group>,
   HelpText<"Allow generation of data access to code sections (ARM only)">;
 let Flags = [TargetSpecific] in {
-def mtp_mode_EQ : Joined<["-"], "mtp=">, Group<m_arm_Features_Group>, Values<"soft,cp15,tpidrurw,tpidruro,tpidrprw,el0,el1,el2,el3,tpidr_el0,tpidr_el1,tpidr_el2,tpidr_el3,tpidrro_el0">,
+def mtp_mode_EQ : Joined<["-"], "mtp=">, Group<m_arm_Features_Group>, Values<"soft,cp15,tpidrurw,tpidruro,tpidrprw,el0,el1,el2,el3,tpidr_el0,tpidr_el1,tpidr_el2,tpidr_el3,tpidrro_el0,auto">,
   HelpText<"Thread pointer access method. "
            "For AArch32: 'soft' uses a function call, or 'tpidrurw', 'tpidruro' or 'tpidrprw' use the three CP15 registers. 'cp15' is an alias for 'tpidruro'. "
            "For AArch64: 'tpidr_el0', 'tpidr_el1', 'tpidr_el2', 'tpidr_el3' or 'tpidrro_el0' use the five system registers. 'elN' is an alias for 'tpidr_elN'.">;

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

@@ -215,7 +215,8 @@ bool arm::isHardTPSupported(const llvm::Triple &Triple) {
 // Select mode for reading thread pointer (-mtp=soft/cp15).
 arm::ReadTPMode arm::getReadTPMode(const Driver &D, const ArgList &Args,
                                    const llvm::Triple &Triple, bool ForAS) {
-  if (Arg *A = Args.getLastArg(options::OPT_mtp_mode_EQ)) {
+  Arg *A = Args.getLastArg(options::OPT_mtp_mode_EQ);
+  if (A && A->getValue() != StringRef("auto")) {
     arm::ReadTPMode ThreadPointer =
         llvm::StringSwitch<arm::ReadTPMode>(A->getValue())
             .Case("cp15", ReadTPMode::TPIDRURO)
@@ -239,7 +240,7 @@ 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,
@@ -574,12 +575,14 @@ 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)
-    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");
+  else if (TPMode == ReadTPMode::TPIDRURO)
+    Features.push_back("+read-tp-tpidruro");
 
   const Arg *ArchArg = Args.getLastArg(options::OPT_march_EQ);
   const Arg *CPUArg = Args.getLastArg(options::OPT_mcpu_EQ);

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

@@ -41,6 +41,7 @@ enum class ReadTPMode {
   TPIDRURW,
   TPIDRURO,
   TPIDRPRW,
+  Auto,
 };
 
 enum class FloatABI {

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

@@ -42,4 +42,8 @@
 
 // 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"
+
+// RUN: %clang --target=armv7-linux -mtp=auto -### -S %s 2>&1 | \
+// RUN: FileCheck -check-prefix=ARMv7_THREAD_POINTER_Auto %s
+// ARMv7_THREAD_POINTER_Auto: "-target-feature" "+read-tp-tpidruro"