Re: [PATCH v3] RISC-V: Implement TLS Descriptors.

[Tatsuyuki Ishi <ishitatsuyuki@gmail.com>]

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> On Nov 21, 2023, at 15:59, Fangrui Song <maskray@google.com> wrote:
> 
> On Mon, Nov 20, 2023 at 6:20 AM Tatsuyuki Ishi <ishitatsuyuki@gmail.com <mailto: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.
> 
> Nit: One dash for --mtls-dialect.

Ack. Thanks.

>> ---
>> No regression in gcc tests for rv64gc, 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.
>> 
>> I've considered gating this behind a GAS feature test, but it seems
>> nontrivial especially for restricting the variants available at runtime.
>> Since TLS descriptors is not selected by default, I've decided to leave it
>> ungated.
>> 
>> In other news, I have made some progress on binutils side, and I'll try to
>> update the GAS / ld patch set with relaxation included, by the end of this
>> month.
> 
> Thanks for the update.  I understand the complexity adding a runtime
> test when the feature also requires binutils and rtld support.
> I hope that we add a test checking assembly under
> gcc/testsuite/gcc.target/riscv/tls , otherwise as a non-default test,
> when this breaks, it may be difficult to figure it out.
> (glibc/elf/tst-* will need a runtime test, but GCC needs to have its own.)

Checking assembly sounds reasonable.  I’ll look into it after finishing the binutils stuff.

>> 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       | 21 ++++++++++++++++++++-
>> gcc/config/riscv/riscv.opt      | 14 ++++++++++++++
>> gcc/doc/install.texi            |  3 +++
>> gcc/doc/invoke.texi             | 13 ++++++++++++-
>> 9 files changed, 99 insertions(+), 11 deletions(-)
>> 
>> diff --git a/gcc/config.gcc b/gcc/config.gcc
>> index 415e0e1ebc5..2c1a7179b02 100644
>> --- a/gcc/config.gcc
>> +++ b/gcc/config.gcc
>> @@ -2434,6 +2434,7 @@ 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 ${tm_file} riscv/elf.h"
>> @@ -2476,6 +2477,7 @@ riscv*-*-freebsd*)
>>        # Force .init_array support.  The configure script cannot always
>>        # automatically detect that GAS supports it, yet we require it.
>>        gcc_cv_initfini_array=yes
>> +       with_tls=${with_tls:-trad}
>>        ;;
>> 
>> loongarch*-*-linux*)
>> @@ -4566,7 +4568,7 @@ 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 ;;
>> @@ -4694,6 +4696,17 @@ 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 378a17699cd..db03f35430a 100644
>> --- a/gcc/config/riscv/riscv-opts.h
>> +++ b/gcc/config/riscv/riscv-opts.h
>> @@ -319,4 +319,10 @@ enum riscv_entity
>> #define TARGET_VECTOR_VLS                                                      \
>>   (TARGET_VECTOR && riscv_autovec_preference == RVV_SCALABLE)
>> 
>> +/* TLS types.  */
>> +enum riscv_tls_type {
>> +  TLS_TRADITIONAL,
>> +  TLS_DESCRIPTORS
>> +};
>> +
> 
> I wonder whether `enum class` can be used instead and we can get rid
> of the `TLS_` prefix.
> gcc/config/ already has some uses.

Sounds reasonable. If others have no objections I’ll put it in a revision.

>> #endif /* ! GCC_RISCV_OPTS_H */
>> diff --git a/gcc/config/riscv/riscv-protos.h b/gcc/config/riscv/riscv-protos.h
>> index 472c00dc439..9b7471f7591 100644
>> --- a/gcc/config/riscv/riscv-protos.h
>> +++ b/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.
>> 
>> diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
>> index 49062bef9fc..c158e224aaa 100644
>> --- a/gcc/config/riscv/riscv.cc
>> +++ b/gcc/config/riscv/riscv.cc
>> @@ -799,6 +799,7 @@ 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 ();
>>     }
>> @@ -1734,7 +1735,7 @@ 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
>> @@ -1750,9 +1751,24 @@ 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 e18a0081297..faea78f5f4c 100644
>> --- a/gcc/config/riscv/riscv.h
>> +++ b/gcc/config/riscv/riscv.h
>> @@ -59,6 +59,7 @@ 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 \
>> @@ -68,8 +69,9 @@ 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
>> @@ -1122,4 +1124,7 @@ extern void riscv_remove_unneeded_save_restore_calls (void);
>> #define OPTIMIZE_MODE_SWITCHING(ENTITY) (TARGET_VECTOR)
>> #define NUM_MODES_FOR_MODE_SWITCHING {VXRM_MODE_NONE, riscv_vector::FRM_NONE}
>> 
>> +/* 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 b456fa6abb3..7c66c64b893 100644
>> --- a/gcc/config/riscv/riscv.md
>> +++ b/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
>> 
>> @@ -121,6 +121,7 @@
>>    (T1_REGNUM                  6)
>>    (S0_REGNUM                  8)
>>    (S1_REGNUM                  9)
>> +   (A0_REGNUM                  10)
>>    (S2_REGNUM                  18)
>>    (S3_REGNUM                  19)
>>    (S4_REGNUM                  20)
>> @@ -1869,6 +1870,24 @@
>>   [(set_attr "got" "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 6304efebfd5..9ba690f8497 100644
>> --- a/gcc/config/riscv/riscv.opt
>> +++ b/gcc/config/riscv/riscv.opt
>> @@ -311,3 +311,17 @@ Enum(riscv_autovec_lmul) String(m8) Value(RVV_M8)
>> -param=riscv-autovec-lmul=
>> Target RejectNegative Joined Enum(riscv_autovec_lmul) Var(riscv_autovec_lmul) Init(RVV_M1)
>> -param=riscv-autovec-lmul=<string>     Set the RVV LMUL of auto-vectorization in the RISC-V port.
>> +
>> +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 e099cd0b568..6ee533c0a40 100644
>> --- a/gcc/doc/install.texi
>> +++ b/gcc/doc/install.texi
>> @@ -1174,6 +1174,9 @@ 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 f2c1067ab7d..77b82c46311 100644
>> --- a/gcc/doc/invoke.texi
>> +++ b/gcc/doc/invoke.texi
>> @@ -1234,7 +1234,8 @@ See RS/6000 and PowerPC Options.
>> -mstack-protector-guard=@var{guard}  -mstack-protector-guard-reg=@var{reg}
>> -mstack-protector-guard-offset=@var{offset}
>> -mcsr-check -mno-csr-check
>> --minline-atomics  -mno-inline-atomics}
>> +-minline-atomics  -mno-inline-atomics
>> +-mtls-dialect=desc  -mtls-dialect=trad}
>> 
>> @emph{RL78 Options}
>> @gccoptlist{-msim  -mmul=none  -mmul=g13  -mmul=g14  -mallregs
>> @@ -29403,6 +29404,16 @@ 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
>> --
>> 2.42.1
>> 
> 
> 
> -- 
> 宋方睿