[clang] 742fa8a - [ARM] Introduce -mtp=auto and make it the default (#128901)

[via cfe-commits]

Author: Austin
Date: 2025-03-03T20:20:43+08:00
New Revision: 742fa8ac67796198dde99e18cdadeaf9b96c2f88

URL: https://github.com/llvm/llvm-project/commit/742fa8ac67796198dde99e18cdadeaf9b96c2f88
DIFF: https://github.com/llvm/llvm-project/commit/742fa8ac67796198dde99e18cdadeaf9b96c2f88.diff

LOG: [ARM] Introduce -mtp=auto and make it the default (#128901)

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.

Added: 
    

Modified: 
    clang/docs/ReleaseNotes.rst
    clang/include/clang/Driver/Options.td
    clang/lib/Driver/ToolChains/Arch/ARM.cpp
    clang/test/Driver/arm-thread-pointer.c

Removed: 
    


################################################################################
diff  --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index d2fc1ce07dd79..b572a166146ac 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -126,6 +126,10 @@ 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
 -------------------------
 

diff  --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index c4bd030e32b5b..d0414aba35209 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -4729,7 +4729,7 @@ 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'.">;

diff  --git a/clang/lib/Driver/ToolChains/Arch/ARM.cpp b/clang/lib/Driver/ToolChains/Arch/ARM.cpp
index 3aee540d501be..51454de1b9dcc 100644
--- a/clang/lib/Driver/ToolChains/Arch/ARM.cpp
+++ b/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);

diff  --git a/clang/test/Driver/arm-thread-pointer.c b/clang/test/Driver/arm-thread-pointer.c
index 5521e1865b276..985c5046f6d26 100644
--- a/clang/test/Driver/arm-thread-pointer.c
+++ b/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"