[clang][ARM] relax -mtp=cp15 for non-thumb cases

Building -march=armv6k Linux kernels with -mtp=cp15 fails to
compile:

error: hardware TLS register is not supported for the arm
sub-architecture

@ARDB found docs for ARM1176JZF-S (ARMv6K) that reference hard thread
pointer.

Relax our ARMv6 check for cases where we're targeting ARM via -marm (vs
Thumb1 via -mthumb).  This more closely matches the KConfig requirements
for where we plan to use these (ie. ARMv6K, ARMv7 (arm or thumb2)).

As @peter.smith mentions:
  on armv5 we can write the instruction to read/write to CP15 C13 with
  the ThreadID opcode. However on no armv5 implementation will the CP15
  C13 have a Thread ID register. The GCC intent seems to be whether the
  instruction is encodable rather than check what the CPU supports.

Link: ClangBuiltLinux/linux#1502
Link: https://developer.arm.com/documentation/ddi0301/h/system-control-coprocessor/system-control-processor-registers/c13--thread-and-process-id-registers

Reviewed By: ardb, peter.smith

Differential Revision: https://reviews.llvm.org/D114116

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

@@ -147,6 +147,16 @@ bool arm::useAAPCSForMachO(const llvm::Triple &T) {
          T.getOS() == llvm::Triple::UnknownOS || isARMMProfile(T);
 }
 
+// We follow GCC and support when the backend has support for the MRC/MCR
+// instructions that are used to set the hard thread pointer ("CP15 C13
+// Thread id").
+bool arm::isHardTPSupported(const llvm::Triple &Triple) {
+  int Ver = getARMSubArchVersionNumber(Triple);
+  llvm::ARM::ArchKind AK = llvm::ARM::parseArch(Triple.getArchName());
+  return Triple.isARM() || AK == llvm::ARM::ArchKind::ARMV6T2 ||
+         (Ver >= 7 && AK != llvm::ARM::ArchKind::ARMV8MBaseline);
+}
+
 // Select mode for reading thread pointer (-mtp=soft/cp15).
 arm::ReadTPMode arm::getReadTPMode(const Driver &D, const ArgList &Args,
                                    const llvm::Triple &Triple) {
@@ -156,10 +166,7 @@ arm::ReadTPMode arm::getReadTPMode(const Driver &D, const ArgList &Args,
             .Case("cp15", ReadTPMode::Cp15)
             .Case("soft", ReadTPMode::Soft)
             .Default(ReadTPMode::Invalid);
-    if (ThreadPointer == ReadTPMode::Cp15 &&
-        getARMSubArchVersionNumber(Triple) < 7 &&
-        llvm::ARM::parseArch(Triple.getArchName()) !=
-            llvm::ARM::ArchKind::ARMV6T2) {
+    if (ThreadPointer == ReadTPMode::Cp15 && !isHardTPSupported(Triple)) {
       D.Diag(diag::err_target_unsupported_tp_hard) << Triple.getArchName();
       return ReadTPMode::Invalid;
     }
@@ -430,7 +437,6 @@ void arm::getARMTargetFeatures(const Driver &D, const llvm::Triple &Triple,
   bool KernelOrKext =
       Args.hasArg(options::OPT_mkernel, options::OPT_fapple_kext);
   arm::FloatABI ABI = arm::getARMFloatABI(D, Triple, Args);
-  arm::ReadTPMode ThreadPointer = arm::getReadTPMode(D, Args, Triple);
   llvm::Optional<std::pair<const Arg *, StringRef>> WaCPU, WaFPU, WaHDiv,
       WaArch;
 
@@ -482,7 +488,7 @@ void arm::getARMTargetFeatures(const Driver &D, const llvm::Triple &Triple,
     }
   }
 
-  if (ThreadPointer == arm::ReadTPMode::Cp15)
+  if (getReadTPMode(D, Args, Triple) == ReadTPMode::Cp15)
     Features.push_back("+read-tp-hard");
 
   const Arg *ArchArg = Args.getLastArg(options::OPT_march_EQ);

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

@@ -53,6 +53,7 @@ FloatABI getARMFloatABI(const Driver &D, const llvm::Triple &Triple,
                         const llvm::opt::ArgList &Args);
 void setFloatABIInTriple(const Driver &D, const llvm::opt::ArgList &Args,
                          llvm::Triple &triple);
+bool isHardTPSupported(const llvm::Triple &Triple);
 ReadTPMode getReadTPMode(const Driver &D, const llvm::opt::ArgList &Args,
                          const llvm::Triple &Triple);
 void setArchNameInTriple(const Driver &D, const llvm::opt::ArgList &Args,

clang/test/Driver/clang-translation.c

@@ -115,16 +115,22 @@
 // ARMv7_THREAD_POINTER-HARD: "-target-feature" "+read-tp-hard"
 
 // RUN: %clang -target armv6t2-linux -mtp=cp15 -### -S %s 2>&1 | \
-// RUN: FileCheck -check-prefix=ARMv6T2_THREAD_POINTER-HARD %s
-// ARMv6T2_THREAD_POINTER-HARD: "-target-feature" "+read-tp-hard"
-
+// RUN: FileCheck -check-prefix=ARM_THREAD_POINTER-HARD %s
+// RUN: %clang -target thumbv6t2-linux -mtp=cp15 -### -S %s 2>&1 | \
+// RUN: FileCheck -check-prefix=ARM_THREAD_POINTER-HARD %s
+// RUN: %clang -target armv6k-linux -mtp=cp15 -### -S %s 2>&1 | \
+// RUN: FileCheck -check-prefix=ARM_THREAD_POINTER-HARD %s
+// RUN: %clang -target armv6-linux -mtp=cp15 -### -S %s 2>&1 | \
+// RUN: FileCheck -check-prefix=ARM_THREAD_POINTER-HARD %s
 // RUN: %clang -target armv5t-linux -mtp=cp15 -### -S %s 2>&1 | \
-// RUN: FileCheck -check-prefix=ARMv5_THREAD_POINTER_UNSUPP %s
-// ARMv5_THREAD_POINTER_UNSUPP: hardware TLS register is not supported for the armv5 sub-architecture
-
-// RUN: %clang -target thumbv6-linux -mtp=cp15 -### -S %s 2>&1 | \
-// RUN: FileCheck -check-prefix=ARMv6_THREAD_POINTER_UNSUPP %s
-// ARMv6_THREAD_POINTER_UNSUPP: hardware TLS register is not supported for the armv6 sub-architecture
+// RUN: FileCheck -check-prefix=ARM_THREAD_POINTER-HARD %s
+// ARM_THREAD_POINTER-HARD: "-target-feature" "+read-tp-hard"
+
+// RUN: %clang -target armv6-linux -mthumb -mtp=cp15 -### -S %s 2>&1 | \
+// RUN: FileCheck -check-prefix=THUMBv6_THREAD_POINTER_UNSUPP %s
+// RUN: %clang -target thumbv6-linux -mthumb -mtp=cp15 -### -S %s 2>&1 | \
+// RUN: FileCheck -check-prefix=THUMBv6_THREAD_POINTER_UNSUPP %s
+// THUMBv6_THREAD_POINTER_UNSUPP: hardware TLS register is not supported for the thumbv6 sub-architecture
 
 // RUN: %clang -target armv7-linux -mtp=soft -### -S %s 2>&1 | \
 // RUN: FileCheck -check-prefix=ARMv7_THREAD_POINTER_SOFT %s