RISC-V: Implement TLS Descriptors.

This implements TLS Descriptors (TLSDESC) as specified in [1].

The 4-instruction sequence is implemented as a single RTX insn for
simplicity, but this can be revisited later if instruction scheduling or
more flexible RA is desired.

The default remains to be the traditional TLS model, but can be configured
with --with_tls={trad,desc}. The choice can be revisited once toolchain
and libc support ships.

[1]: riscv-non-isa/riscv-elf-psabi-doc#373.

gcc/Changelog:
	* config/riscv/riscv.opt: Add -mtls-dialect to configure TLS flavor.
	* config.gcc: Add --with_tls configuration option to change the
	default TLS flavor.
	* config/riscv/riscv.h: Add TARGET_TLSDESC determined from
	-mtls-dialect and with_tls defaults.
	* config/riscv/riscv-opts.h: Define enum riscv_tls_type for the
	two TLS flavors.
	* config/riscv/riscv-protos.h: Define SYMBOL_TLSDESC symbol type.
	* config/riscv/riscv.md: Add instruction sequence for TLSDESC.
	* config/riscv/riscv.cc (riscv_symbol_insns): Add instruction
	sequence length data for TLSDESC.
	(riscv_legitimize_tls_address): Add lowering of TLSDESC.
	* doc/install.texi: Document --with-tls for RISC-V.
	* doc/invoke.texi: Document -mtls-dialect for RISC-V.

gcc/config.gcc

@@ -2492,6 +2492,7 @@ riscv*-*-linux*)
 	# Force .init_array support.  The configure script cannot always
 	# automatically detect that GAS supports it, yet we require it.
 	gcc_cv_initfini_array=yes
+	with_tls=${with_tls:-trad}
 	;;
 riscv*-*-elf* | riscv*-*-rtems*)
 	tm_file="elfos.h newlib-stdint.h ${tm_file} riscv/elf.h"
@@ -2534,6 +2535,7 @@ riscv*-*-freebsd*)
 	# Force .init_array support.  The configure script cannot always
 	# automatically detect that GAS supports it, yet we require it.
 	gcc_cv_initfini_array=yes
+	with_tls=${with_tls:-trad}
 	;;
 
 loongarch*-*-linux*)
@@ -4660,7 +4662,7 @@ case "${target}" in
 		;;
 
 	riscv*-*-*)
-		supported_defaults="abi arch tune riscv_attribute isa_spec"
+		supported_defaults="abi arch tune riscv_attribute isa_spec tls"
 
 		case "${target}" in
 		riscv-* | riscv32*) xlen=32 ;;
@@ -4788,6 +4790,17 @@ case "${target}" in
 				;;
 			esac
 		fi
+		# Handle --with-tls.
+		case "$with_tls" in
+		"" \
+		| trad | desc)
+			# OK
+			;;
+		*)
+			echo "Unknown TLS method used in --with-tls=$with_tls" 1>&2
+			exit 1
+			;;
+		esac
 
 		# Handle --with-multilib-list.
 		if test "x${with_multilib_list}" != xdefault; then

gcc/config/riscv/riscv-opts.h

@@ -137,4 +137,10 @@ enum riscv_stringop_strategy_enum {
 #define TARGET_MAX_LMUL                                                        \
   (int) (riscv_autovec_lmul == RVV_DYNAMIC ? RVV_M8 : riscv_autovec_lmul)
 
+/* TLS types.  */
+enum riscv_tls_type {
+  TLS_TRADITIONAL,
+  TLS_DESCRIPTORS
+};
+
 #endif /* ! GCC_RISCV_OPTS_H */

gcc/config/riscv/riscv-protos.h

@@ -33,9 +33,10 @@ enum riscv_symbol_type {
   SYMBOL_TLS,
   SYMBOL_TLS_LE,
   SYMBOL_TLS_IE,
-  SYMBOL_TLS_GD
+  SYMBOL_TLS_GD,
+  SYMBOL_TLSDESC,
 };
-#define NUM_SYMBOL_TYPES (SYMBOL_TLS_GD + 1)
+#define NUM_SYMBOL_TYPES (SYMBOL_TLSDESC + 1)
 
 /* Classifies an address.
 

gcc/config/riscv/riscv.cc

@@ -895,6 +895,7 @@ static int riscv_symbol_insns (enum riscv_symbol_type type)
     case SYMBOL_ABSOLUTE: return 2; /* LUI + the reference.  */
     case SYMBOL_PCREL: return 2; /* AUIPC + the reference.  */
     case SYMBOL_TLS_LE: return 3; /* LUI + ADD TP + the reference.  */
+    case SYMBOL_TLSDESC: return 6; /* 4-instruction call + ADD TP + the reference.  */
     case SYMBOL_GOT_DISP: return 3; /* AUIPC + LD GOT + the reference.  */
     default: gcc_unreachable ();
     }
@@ -1999,7 +2000,7 @@ riscv_call_tls_get_addr (rtx sym, rtx result)
 static rtx
 riscv_legitimize_tls_address (rtx loc)
 {
-  rtx dest, tp, tmp;
+  rtx dest, tp, tmp, a0;
   enum tls_model model = SYMBOL_REF_TLS_MODEL (loc);
 
 #if 0
@@ -2015,9 +2016,24 @@ riscv_legitimize_tls_address (rtx loc)
       /* Rely on section anchors for the optimization that LDM TLS
 	 provides.  The anchor's address is loaded with GD TLS. */
     case TLS_MODEL_GLOBAL_DYNAMIC:
-      tmp = gen_rtx_REG (Pmode, GP_RETURN);
-      dest = gen_reg_rtx (Pmode);
-      emit_libcall_block (riscv_call_tls_get_addr (loc, tmp), dest, tmp, loc);
+      if (TARGET_TLSDESC)
+	{
+	  static unsigned seqno;
+	  tp = gen_rtx_REG (Pmode, THREAD_POINTER_REGNUM);
+	  a0 = gen_rtx_REG (Pmode, GP_ARG_FIRST);
+	  dest = gen_reg_rtx (Pmode);
+
+	  emit_insn (gen_tlsdesc (Pmode, loc, GEN_INT (seqno)));
+	  emit_insn (gen_add3_insn (dest, a0, tp));
+	  seqno++;
+	}
+      else
+	{
+	  tmp = gen_rtx_REG (Pmode, GP_RETURN);
+	  dest = gen_reg_rtx (Pmode);
+	  emit_libcall_block (riscv_call_tls_get_addr (loc, tmp), dest, tmp,
+			      loc);
+	}
       break;
 
     case TLS_MODEL_INITIAL_EXEC:

gcc/config/riscv/riscv.h

@@ -62,6 +62,7 @@ extern const char *riscv_multi_lib_check (int argc, const char **argv);
    --with-abi is ignored if -mabi is specified.
    --with-tune is ignored if -mtune or -mcpu is specified.
    --with-isa-spec is ignored if -misa-spec is specified.
+   --with-tls is ignored if -mtls-dialect is specified.
 
    But using default -march/-mtune value if -mcpu don't have valid option.  */
 #define OPTION_DEFAULT_SPECS \
@@ -71,8 +72,9 @@ extern const char *riscv_multi_lib_check (int argc, const char **argv);
   {"arch", "%{!march=*:"						\
 	   "  %{!mcpu=*:-march=%(VALUE)}"				\
 	   "  %{mcpu=*:%:riscv_expand_arch_from_cpu(%* %(VALUE))}}" },	\
-  {"abi", "%{!mabi=*:-mabi=%(VALUE)}" }, \
-  {"isa_spec", "%{!misa-spec=*:-misa-spec=%(VALUE)}" }, \
+  {"abi", "%{!mabi=*:-mabi=%(VALUE)}" },				\
+  {"isa_spec", "%{!misa-spec=*:-misa-spec=%(VALUE)}" },			\
+  {"tls", "%{!mtls-dialect=*:-mtls-dialect=%(VALUE)}"},         	\
 
 #ifdef IN_LIBGCC2
 #undef TARGET_64BIT
@@ -1206,4 +1208,7 @@ extern void riscv_remove_unneeded_save_restore_calls (void);
 #define HAVE_POST_MODIFY_DISP TARGET_XTHEADMEMIDX
 #define HAVE_PRE_MODIFY_DISP  TARGET_XTHEADMEMIDX
 
+/* Check TLS Descriptors mechanism is selected.  */
+#define TARGET_TLSDESC (riscv_tls_dialect == TLS_DESCRIPTORS)
+
 #endif /* ! GCC_RISCV_H */

gcc/config/riscv/riscv.md

@@ -47,7 +47,7 @@
   UNSPEC_TLS_LE
   UNSPEC_TLS_IE
   UNSPEC_TLS_GD
-
+  UNSPEC_TLSDESC
   ;; High part of PC-relative address.
   UNSPEC_AUIPC
 
@@ -1975,6 +1975,24 @@
    (set_attr "type" "load")
    (set_attr "mode" "<MODE>")])
 
+(define_insn "@tlsdesc<mode>"
+  [(set (reg:P A0_REGNUM)
+	    (unspec:P
+			[(match_operand:P 0 "symbolic_operand" "")
+			 (match_operand:P 1 "const_int_operand")]
+			UNSPEC_TLSDESC))
+   (clobber (reg:P T0_REGNUM))]
+  "TARGET_TLSDESC"
+  {
+    return ".LT%1: auipc\ta0,%%tlsdesc_hi(%0)\;"
+           "<load>\tt0,%%tlsdesc_load_lo(.LT%1)(a0)\;"
+           "addi\ta0,a0,%%tlsdesc_add_lo(.LT%1)\;"
+           "jalr\tt0,t0,%%tlsdesc_call(.LT%1)";
+  }
+  [(set_attr "type" "multi")
+   (set_attr "length" "16")
+   (set_attr "mode" "<MODE>")])
+
 (define_insn "auipc<mode>"
   [(set (match_operand:P           0 "register_operand" "=r")
 	(unspec:P

gcc/config/riscv/riscv.opt

@@ -551,3 +551,17 @@ Enum(riscv_stringop_strategy) String(vector) Value(USE_VECTOR)
 mmemcpy-strategy=
 Target RejectNegative Joined Enum(riscv_stringop_strategy) Var(riscv_memcpy_strategy) Init(USE_AUTO)
 Specify memcpy expansion strategy.
+
+Enum
+Name(tls_type) Type(enum riscv_tls_type)
+The possible TLS dialects:
+
+EnumValue
+Enum(tls_type) String(trad) Value(TLS_TRADITIONAL)
+
+EnumValue
+Enum(tls_type) String(desc) Value(TLS_DESCRIPTORS)
+
+mtls-dialect=
+Target RejectNegative Joined Enum(tls_type) Var(riscv_tls_dialect) Init(TLS_TRADITIONAL) Save
+Specify TLS dialect.

gcc/doc/install.texi

@@ -1185,6 +1185,9 @@ Specify the default TLS dialect, for systems were there is a choice.
 For ARM targets, possible values for @var{dialect} are @code{gnu} or
 @code{gnu2}, which select between the original GNU dialect and the GNU TLS
 descriptor-based dialect.
+For RISC-V targets, possible values for @var{dialect} are @code{trad} or
+@code{desc}, which select between the traditional GNU dialect and the GNU TLS
+descriptor-based dialect.
 
 @item --enable-multiarch
 Specify whether to enable or disable multiarch support.  The default is

gcc/doc/invoke.texi

@@ -1249,7 +1249,8 @@ See RS/6000 and PowerPC Options.
 -minline-atomics  -mno-inline-atomics
 -minline-strlen  -mno-inline-strlen
 -minline-strcmp  -mno-inline-strcmp
--minline-strncmp  -mno-inline-strncmp}
+-minline-strncmp  -mno-inline-strncmp
+-mtls-dialect=desc  -mtls-dialect=trad}
 
 @emph{RL78 Options}
 @gccoptlist{-msim  -mmul=none  -mmul=g13  -mmul=g14  -mallregs
@@ -29870,6 +29871,16 @@ which register to use as base register for reading the canary,
 and from what offset from that base register. There is no default
 register or offset as this is entirely for use within the Linux
 kernel.
+
+@opindex mtls-dialect=desc
+@item -mtls-dialect=desc
+Use TLS descriptors as the thread-local storage mechanism for dynamic accesses
+of TLS variables.
+
+@opindex mtls-dialect=trad
+@item -mtls-dialect=trad
+Use traditional TLS as the thread-local storage mechanism for dynamic accesses
+of TLS variables.  This is the default.
 @end table
 
 @node RL78 Options