[v5] 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]: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/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.
	* testsuite/gcc.target/riscv/tls_1.x: Add TLSDESC GD test case.
	* testsuite/gcc.target/riscv/tlsdesc.c: Same as above.
---
No regression in gcc tests for rv32gcv and rv64gcv, tested alongside
the binutils and glibc implementation. Tested with --with-tls=desc.

v2: Add with_tls configuration option, and a few readability improvements.
    Added Changelog.
v3: Add documentation per Kito's suggestion.
    Fix minor issues pointed out by Kito and Jeff.
    Thanks Kito Cheng and Jeff Law for review.
v4: Add TLSDESC GD assembly test.
    Rebase on top of trunk.
v5: Trivial rebase on top of trunk.

I have recently addressed relaxation concerns on binutils and RVV
register save/restore on glibc, so I'm sending out a trivial rebase
with the hope that the full set can be merged soon.

 gcc/config.gcc                           | 15 ++++++++++++++-
 gcc/config/riscv/riscv-opts.h            |  6 ++++++
 gcc/config/riscv/riscv-protos.h          |  5 +++--
 gcc/config/riscv/riscv.cc                | 24 ++++++++++++++++++++----
 gcc/config/riscv/riscv.h                 |  9 +++++++--
 gcc/config/riscv/riscv.md                | 20 +++++++++++++++++++-
 gcc/config/riscv/riscv.opt               | 14 ++++++++++++++
 gcc/doc/install.texi                     |  3 +++
 gcc/doc/invoke.texi                      | 13 ++++++++++++-
 gcc/testsuite/gcc.target/riscv/tls_1.x   |  5 +++++
 gcc/testsuite/gcc.target/riscv/tlsdesc.c | 12 ++++++++++++
 11 files changed, 115 insertions(+), 11 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/riscv/tls_1.x
 create mode 100644 gcc/testsuite/gcc.target/riscv/tlsdesc.c

Hi Tatsuyuki:

Thanks for your hard work and keep updating, the patch set is LGTM, I
plan to commit this next week if no further comments :)

Hi MaskRay:

Thanks for your review on the patchset! just put you into the cc list
in case you have few more comments :)


On Fri, Mar 29, 2024 at 1:53 PM Tatsuyuki Ishi <ishitatsuyuki@gmail.com> wrote:
>
> 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]: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/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.
>         * testsuite/gcc.target/riscv/tls_1.x: Add TLSDESC GD test case.
>         * testsuite/gcc.target/riscv/tlsdesc.c: Same as above.
> ---
> No regression in gcc tests for rv32gcv and rv64gcv, tested alongside
> the binutils and glibc implementation. Tested with --with-tls=desc.
>
> v2: Add with_tls configuration option, and a few readability improvements.
>     Added Changelog.
> v3: Add documentation per Kito's suggestion.
>     Fix minor issues pointed out by Kito and Jeff.
>     Thanks Kito Cheng and Jeff Law for review.
> v4: Add TLSDESC GD assembly test.
>     Rebase on top of trunk.
> v5: Trivial rebase on top of trunk.
>
> I have recently addressed relaxation concerns on binutils and RVV
> register save/restore on glibc, so I'm sending out a trivial rebase
> with the hope that the full set can be merged soon.
>
>  gcc/config.gcc                           | 15 ++++++++++++++-
>  gcc/config/riscv/riscv-opts.h            |  6 ++++++
>  gcc/config/riscv/riscv-protos.h          |  5 +++--
>  gcc/config/riscv/riscv.cc                | 24 ++++++++++++++++++++----
>  gcc/config/riscv/riscv.h                 |  9 +++++++--
>  gcc/config/riscv/riscv.md                | 20 +++++++++++++++++++-
>  gcc/config/riscv/riscv.opt               | 14 ++++++++++++++
>  gcc/doc/install.texi                     |  3 +++
>  gcc/doc/invoke.texi                      | 13 ++++++++++++-
>  gcc/testsuite/gcc.target/riscv/tls_1.x   |  5 +++++
>  gcc/testsuite/gcc.target/riscv/tlsdesc.c | 12 ++++++++++++
>  11 files changed, 115 insertions(+), 11 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.target/riscv/tls_1.x
>  create mode 100644 gcc/testsuite/gcc.target/riscv/tlsdesc.c
>
> diff --git a/gcc/config.gcc b/gcc/config.gcc
> index 17873ac2103..1a5870672d2 100644
> --- a/gcc/config.gcc
> +++ b/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*)
> @@ -4671,7 +4673,7 @@ 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 ;;
> @@ -4801,6 +4803,17 @@ 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
> diff --git a/gcc/config/riscv/riscv-opts.h b/gcc/config/riscv/riscv-opts.h
> index 392b9169240..1b2dd5757a8 100644
> --- a/gcc/config/riscv/riscv-opts.h
> +++ b/gcc/config/riscv/riscv-opts.h
> @@ -154,4 +154,10 @@ enum rvv_vector_bits_enum {
>  #define TARGET_MAX_LMUL                                                        \
>    (int) (rvv_max_lmul == RVV_DYNAMIC ? RVV_M8 : rvv_max_lmul)
>
> +/* TLS types.  */
> +enum riscv_tls_type {
> +  TLS_TRADITIONAL,
> +  TLS_DESCRIPTORS
> +};
> +
>  #endif /* ! GCC_RISCV_OPTS_H */
> diff --git a/gcc/config/riscv/riscv-protos.h b/gcc/config/riscv/riscv-protos.h
> index b8735593805..066a5a5a7cc 100644
> --- a/gcc/config/riscv/riscv-protos.h
> +++ b/gcc/config/riscv/riscv-protos.h
> @@ -34,9 +34,10 @@ 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.
>
> diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
> index fe9976bfffe..deac3e4e9e3 100644
> --- a/gcc/config/riscv/riscv.cc
> +++ b/gcc/config/riscv/riscv.cc
> @@ -1072,6 +1072,7 @@ 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.  */
>      case SYMBOL_FORCE_TO_MEM: return 3; /* AUIPC + LD + the reference.  */
>      default: gcc_unreachable ();
> @@ -2220,7 +2221,7 @@ 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
> @@ -2236,9 +2237,24 @@ 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:
> diff --git a/gcc/config/riscv/riscv.h b/gcc/config/riscv/riscv.h
> index da089a03e9d..10ad4aedd70 100644
> --- a/gcc/config/riscv/riscv.h
> +++ b/gcc/config/riscv/riscv.h
> @@ -64,6 +64,7 @@ extern const char *riscv_arch_help (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 \
> @@ -73,8 +74,9 @@ extern const char *riscv_arch_help (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
> @@ -1228,4 +1230,7 @@ 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 */
> diff --git a/gcc/config/riscv/riscv.md b/gcc/config/riscv/riscv.md
> index 0346cc3859d..c2b4323c53a 100644
> --- a/gcc/config/riscv/riscv.md
> +++ b/gcc/config/riscv/riscv.md
> @@ -48,7 +48,7 @@
>    UNSPEC_TLS_LE
>    UNSPEC_TLS_IE
>    UNSPEC_TLS_GD
> -
> +  UNSPEC_TLSDESC
>    ;; High part of PC-relative address.
>    UNSPEC_AUIPC
>
> @@ -2089,6 +2089,24 @@
>     (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
> diff --git a/gcc/config/riscv/riscv.opt b/gcc/config/riscv/riscv.opt
> index 7d625af02d5..8da0764eb4b 100644
> --- a/gcc/config/riscv/riscv.opt
> +++ b/gcc/config/riscv/riscv.opt
> @@ -606,3 +606,17 @@ Enum(rvv_vector_bits) String(zvl) Value(RVV_VECTOR_BITS_ZVL)
>  mrvv-vector-bits=
>  Target RejectNegative Joined Enum(rvv_vector_bits) Var(rvv_vector_bits) Init(RVV_VECTOR_BITS_SCALABLE)
>  -mrvv-vector-bits=<string>     Set the kind of bits for an RVV vector register.
> +
> +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.
> diff --git a/gcc/doc/install.texi b/gcc/doc/install.texi
> index 269fe7ec870..120bb045f4d 100644
> --- a/gcc/doc/install.texi
> +++ b/gcc/doc/install.texi
> @@ -1187,6 +1187,9 @@ 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
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index d09074e13de..e4112bbe61e 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -1262,7 +1262,8 @@ 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
> @@ -30975,6 +30976,16 @@ 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
> diff --git a/gcc/testsuite/gcc.target/riscv/tls_1.x b/gcc/testsuite/gcc.target/riscv/tls_1.x
> new file mode 100644
> index 00000000000..cf334ef55ed
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/riscv/tls_1.x
> @@ -0,0 +1,5 @@
> +extern __thread unsigned gd;
> +
> +unsigned get() {
> +       return gd;
> +}
> diff --git a/gcc/testsuite/gcc.target/riscv/tlsdesc.c b/gcc/testsuite/gcc.target/riscv/tlsdesc.c
> new file mode 100644
> index 00000000000..f9ac9fe0d6d
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/riscv/tlsdesc.c
> @@ -0,0 +1,12 @@
> +/* { dg-require-effective-target tls_native } */
> +/* { dg-options "-O2 -fpic -mtls-dialect=desc --save-temps" } */
> +/* { dg-require-effective-target fpic } */
> +
> +#include "tls_1.x"
> +
> +/* { dg-final { scan-assembler-times "auipc\t\[a-x0-9\]+,%tlsdesc_hi" 1 } } */
> +/* { dg-final { scan-assembler-times "lw\t\[a-x0-9\]+,%tlsdesc_load_lo" 1 { target { rv32 } } } } */
> +/* { dg-final { scan-assembler-times "ld\t\[a-x0-9\]+,%tlsdesc_load_lo" 1 { target { rv64 } } } }*/
> +/* { dg-final { scan-assembler-times "addi\ta0,\[a-x0-9\]+,%tlsdesc_add_lo" 1 } } */
> +/* { dg-final { scan-assembler-times "jalr\tt0,\[a-x0-9\]+,%tlsdesc_call" 1 } } */
> +/* { dg-final { cleanup-saved-temps } } */
> --
> 2.44.0
>

Committed to trunk, thanks Tatsuyuki!

On Fri, Mar 29, 2024 at 2:32 PM Kito Cheng <kito.cheng@gmail.com> wrote:
>
> Hi Tatsuyuki:
>
> Thanks for your hard work and keep updating, the patch set is LGTM, I
> plan to commit this next week if no further comments :)
>
> Hi MaskRay:
>
> Thanks for your review on the patchset! just put you into the cc list
> in case you have few more comments :)
>
>
> On Fri, Mar 29, 2024 at 1:53 PM Tatsuyuki Ishi <ishitatsuyuki@gmail.com> wrote:
> >
> > 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]: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/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.
> >         * testsuite/gcc.target/riscv/tls_1.x: Add TLSDESC GD test case.
> >         * testsuite/gcc.target/riscv/tlsdesc.c: Same as above.
> > ---
> > No regression in gcc tests for rv32gcv and rv64gcv, tested alongside
> > the binutils and glibc implementation. Tested with --with-tls=desc.
> >
> > v2: Add with_tls configuration option, and a few readability improvements.
> >     Added Changelog.
> > v3: Add documentation per Kito's suggestion.
> >     Fix minor issues pointed out by Kito and Jeff.
> >     Thanks Kito Cheng and Jeff Law for review.
> > v4: Add TLSDESC GD assembly test.
> >     Rebase on top of trunk.
> > v5: Trivial rebase on top of trunk.
> >
> > I have recently addressed relaxation concerns on binutils and RVV
> > register save/restore on glibc, so I'm sending out a trivial rebase
> > with the hope that the full set can be merged soon.
> >
> >  gcc/config.gcc                           | 15 ++++++++++++++-
> >  gcc/config/riscv/riscv-opts.h            |  6 ++++++
> >  gcc/config/riscv/riscv-protos.h          |  5 +++--
> >  gcc/config/riscv/riscv.cc                | 24 ++++++++++++++++++++----
> >  gcc/config/riscv/riscv.h                 |  9 +++++++--
> >  gcc/config/riscv/riscv.md                | 20 +++++++++++++++++++-
> >  gcc/config/riscv/riscv.opt               | 14 ++++++++++++++
> >  gcc/doc/install.texi                     |  3 +++
> >  gcc/doc/invoke.texi                      | 13 ++++++++++++-
> >  gcc/testsuite/gcc.target/riscv/tls_1.x   |  5 +++++
> >  gcc/testsuite/gcc.target/riscv/tlsdesc.c | 12 ++++++++++++
> >  11 files changed, 115 insertions(+), 11 deletions(-)
> >  create mode 100644 gcc/testsuite/gcc.target/riscv/tls_1.x
> >  create mode 100644 gcc/testsuite/gcc.target/riscv/tlsdesc.c
> >
> > diff --git a/gcc/config.gcc b/gcc/config.gcc
> > index 17873ac2103..1a5870672d2 100644
> > --- a/gcc/config.gcc
> > +++ b/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*)
> > @@ -4671,7 +4673,7 @@ 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 ;;
> > @@ -4801,6 +4803,17 @@ 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
> > diff --git a/gcc/config/riscv/riscv-opts.h b/gcc/config/riscv/riscv-opts.h
> > index 392b9169240..1b2dd5757a8 100644
> > --- a/gcc/config/riscv/riscv-opts.h
> > +++ b/gcc/config/riscv/riscv-opts.h
> > @@ -154,4 +154,10 @@ enum rvv_vector_bits_enum {
> >  #define TARGET_MAX_LMUL                                                        \
> >    (int) (rvv_max_lmul == RVV_DYNAMIC ? RVV_M8 : rvv_max_lmul)
> >
> > +/* TLS types.  */
> > +enum riscv_tls_type {
> > +  TLS_TRADITIONAL,
> > +  TLS_DESCRIPTORS
> > +};
> > +
> >  #endif /* ! GCC_RISCV_OPTS_H */
> > diff --git a/gcc/config/riscv/riscv-protos.h b/gcc/config/riscv/riscv-protos.h
> > index b8735593805..066a5a5a7cc 100644
> > --- a/gcc/config/riscv/riscv-protos.h
> > +++ b/gcc/config/riscv/riscv-protos.h
> > @@ -34,9 +34,10 @@ 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.
> >
> > diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
> > index fe9976bfffe..deac3e4e9e3 100644
> > --- a/gcc/config/riscv/riscv.cc
> > +++ b/gcc/config/riscv/riscv.cc
> > @@ -1072,6 +1072,7 @@ 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.  */
> >      case SYMBOL_FORCE_TO_MEM: return 3; /* AUIPC + LD + the reference.  */
> >      default: gcc_unreachable ();
> > @@ -2220,7 +2221,7 @@ 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
> > @@ -2236,9 +2237,24 @@ 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:
> > diff --git a/gcc/config/riscv/riscv.h b/gcc/config/riscv/riscv.h
> > index da089a03e9d..10ad4aedd70 100644
> > --- a/gcc/config/riscv/riscv.h
> > +++ b/gcc/config/riscv/riscv.h
> > @@ -64,6 +64,7 @@ extern const char *riscv_arch_help (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 \
> > @@ -73,8 +74,9 @@ extern const char *riscv_arch_help (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
> > @@ -1228,4 +1230,7 @@ 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 */
> > diff --git a/gcc/config/riscv/riscv.md b/gcc/config/riscv/riscv.md
> > index 0346cc3859d..c2b4323c53a 100644
> > --- a/gcc/config/riscv/riscv.md
> > +++ b/gcc/config/riscv/riscv.md
> > @@ -48,7 +48,7 @@
> >    UNSPEC_TLS_LE
> >    UNSPEC_TLS_IE
> >    UNSPEC_TLS_GD
> > -
> > +  UNSPEC_TLSDESC
> >    ;; High part of PC-relative address.
> >    UNSPEC_AUIPC
> >
> > @@ -2089,6 +2089,24 @@
> >     (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
> > diff --git a/gcc/config/riscv/riscv.opt b/gcc/config/riscv/riscv.opt
> > index 7d625af02d5..8da0764eb4b 100644
> > --- a/gcc/config/riscv/riscv.opt
> > +++ b/gcc/config/riscv/riscv.opt
> > @@ -606,3 +606,17 @@ Enum(rvv_vector_bits) String(zvl) Value(RVV_VECTOR_BITS_ZVL)
> >  mrvv-vector-bits=
> >  Target RejectNegative Joined Enum(rvv_vector_bits) Var(rvv_vector_bits) Init(RVV_VECTOR_BITS_SCALABLE)
> >  -mrvv-vector-bits=<string>     Set the kind of bits for an RVV vector register.
> > +
> > +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.
> > diff --git a/gcc/doc/install.texi b/gcc/doc/install.texi
> > index 269fe7ec870..120bb045f4d 100644
> > --- a/gcc/doc/install.texi
> > +++ b/gcc/doc/install.texi
> > @@ -1187,6 +1187,9 @@ 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
> > diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> > index d09074e13de..e4112bbe61e 100644
> > --- a/gcc/doc/invoke.texi
> > +++ b/gcc/doc/invoke.texi
> > @@ -1262,7 +1262,8 @@ 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
> > @@ -30975,6 +30976,16 @@ 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
> > diff --git a/gcc/testsuite/gcc.target/riscv/tls_1.x b/gcc/testsuite/gcc.target/riscv/tls_1.x
> > new file mode 100644
> > index 00000000000..cf334ef55ed
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/riscv/tls_1.x
> > @@ -0,0 +1,5 @@
> > +extern __thread unsigned gd;
> > +
> > +unsigned get() {
> > +       return gd;
> > +}
> > diff --git a/gcc/testsuite/gcc.target/riscv/tlsdesc.c b/gcc/testsuite/gcc.target/riscv/tlsdesc.c
> > new file mode 100644
> > index 00000000000..f9ac9fe0d6d
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/riscv/tlsdesc.c
> > @@ -0,0 +1,12 @@
> > +/* { dg-require-effective-target tls_native } */
> > +/* { dg-options "-O2 -fpic -mtls-dialect=desc --save-temps" } */
> > +/* { dg-require-effective-target fpic } */
> > +
> > +#include "tls_1.x"
> > +
> > +/* { dg-final { scan-assembler-times "auipc\t\[a-x0-9\]+,%tlsdesc_hi" 1 } } */
> > +/* { dg-final { scan-assembler-times "lw\t\[a-x0-9\]+,%tlsdesc_load_lo" 1 { target { rv32 } } } } */
> > +/* { dg-final { scan-assembler-times "ld\t\[a-x0-9\]+,%tlsdesc_load_lo" 1 { target { rv64 } } } }*/
> > +/* { dg-final { scan-assembler-times "addi\ta0,\[a-x0-9\]+,%tlsdesc_add_lo" 1 } } */
> > +/* { dg-final { scan-assembler-times "jalr\tt0,\[a-x0-9\]+,%tlsdesc_call" 1 } } */
> > +/* { dg-final { cleanup-saved-temps } } */
> > --
> > 2.44.0
> >