Re: [PATCH] RISC-V: Add --[no-]relax-gp to ld

[Fangrui Song <i@maskray.me>]

On Wed, Jul 13, 2022 at 1:26 AM Fangrui Song <maskray@google.com> wrote:
>
> From: Fangrui Song <i@maskray.me>
>
> --relax enables all relaxations. --no-relax-gp disables GP relaxation to
> allow measuring its effect.
>
> Link: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/issues/298
>
> bfd/
>     * elfnn-riscv.c (struct riscv_elf_link_hash_table): Add params.
>     (riscv_elfNN_set_options): New.
>     (riscv_info_to_howto_rela): Check relax_gp.
>     (_bfd_riscv_relax_section): Likewise.
>     * elfxx-riscv.h (struct riscv_elf_params): New.
>     (riscv_elf32_set_options): New.
>     (riscv_elf64_set_options): New.
> ld/
>     * emultempl/riscvelf.em: Add option parsing.
>     * testsuite/ld-riscv-elf/code-model-relax-medlow-01-norelaxgp.d: New.
>     * testsuite/ld-riscv-elf/pcgp-relax-01-norelaxgp.d: New.
>     * testsuite/ld-riscv-elf/pcgp-relax-02.d: Test --relax --relax-gp can be
>       used together.
> ---
>  bfd/elfnn-riscv.c                             | 22 ++++++++++---
>  bfd/elfxx-riscv.h                             | 11 +++++++
>  ld/emultempl/riscvelf.em                      | 31 +++++++++++++++++++
>  .../code-model-relax-medlow-01-norelaxgp.d    |  4 +++
>  ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp    |  2 ++
>  .../ld-riscv-elf/pcgp-relax-01-norelaxgp.d    | 18 +++++++++++
>  ld/testsuite/ld-riscv-elf/pcgp-relax-02.d     |  2 +-
>  7 files changed, 84 insertions(+), 6 deletions(-)
>  create mode 100644 ld/testsuite/ld-riscv-elf/code-model-relax-medlow-01-norelaxgp.d
>  create mode 100644 ld/testsuite/ld-riscv-elf/pcgp-relax-01-norelaxgp.d
>
> diff --git a/bfd/elfnn-riscv.c b/bfd/elfnn-riscv.c
> index 8f9f0d8a86a..527c3e74e97 100644
> --- a/bfd/elfnn-riscv.c
> +++ b/bfd/elfnn-riscv.c
> @@ -118,6 +118,9 @@ struct riscv_elf_link_hash_table
>  {
>    struct elf_link_hash_table elf;
>
> +  /* Various options and other info passed from the linker.  */
> +  struct riscv_elf_params *params;
> +
>    /* Short-cuts to get to dynamic linker sections.  */
>    asection *sdyntdata;
>
> @@ -157,6 +160,13 @@ struct riscv_elf_link_hash_table
>      && elf_hash_table_id (elf_hash_table (p)) == RISCV_ELF_DATA)       \
>     ? (struct riscv_elf_link_hash_table *) (p)->hash : NULL)
>
> +void
> +riscv_elfNN_set_options (struct bfd_link_info *link_info,
> +                        struct riscv_elf_params *params)
> +{
> +  riscv_elf_hash_table (link_info)->params = params;
> +}
> +
>  static bool
>  riscv_info_to_howto_rela (bfd *abfd,
>                           arelent *cache_ptr,
> @@ -4289,7 +4299,9 @@ _bfd_riscv_relax_lui (bfd *abfd,
>                       bool undefined_weak)
>  {
>    bfd_byte *contents = elf_section_data (sec)->this_hdr.contents;
> -  bfd_vma gp = riscv_global_pointer_value (link_info);
> +  bfd_vma gp = riscv_elf_hash_table (link_info)->params->relax_gp
> +                  ? riscv_global_pointer_value (link_info)
> +                  : 0;
>    int use_rvc = elf_elfheader (abfd)->e_flags & EF_RISCV_RVC;
>
>    BFD_ASSERT (rel->r_offset + 4 <= sec->size);
> @@ -4754,10 +4766,10 @@ _bfd_riscv_relax_section (bfd *abfd, asection *sec,
>                    || type == R_RISCV_TPREL_LO12_I
>                    || type == R_RISCV_TPREL_LO12_S)
>             relax_func = _bfd_riscv_relax_tls_le;
> -         else if (!bfd_link_pic (info)
> -                  && (type == R_RISCV_PCREL_HI20
> -                      || type == R_RISCV_PCREL_LO12_I
> -                      || type == R_RISCV_PCREL_LO12_S))
> +         else if ((type == R_RISCV_PCREL_HI20 || type == R_RISCV_PCREL_LO12_I
> +                   || type == R_RISCV_PCREL_LO12_S)
> +                  && !bfd_link_pic (info)
> +                  && riscv_elf_hash_table (info)->params->relax_gp)
>             relax_func = _bfd_riscv_relax_pc;
>           else
>             continue;
> diff --git a/bfd/elfxx-riscv.h b/bfd/elfxx-riscv.h
> index ea7126bdb4d..f9bd0ad882c 100644
> --- a/bfd/elfxx-riscv.h
> +++ b/bfd/elfxx-riscv.h
> @@ -27,6 +27,17 @@
>
>  #define RISCV_UNKNOWN_VERSION -1
>
> +struct riscv_elf_params
> +{
> +  /* Whether to relax code sequences to GP-relative addressing.  */
> +  bool relax_gp;
> +};
> +
> +extern void riscv_elf32_set_options (struct bfd_link_info *,
> +                                    struct riscv_elf_params *);
> +extern void riscv_elf64_set_options (struct bfd_link_info *,
> +                                    struct riscv_elf_params *);
> +
>  extern reloc_howto_type *
>  riscv_reloc_name_lookup (bfd *, const char *);
>
> diff --git a/ld/emultempl/riscvelf.em b/ld/emultempl/riscvelf.em
> index 645a807f239..37a0fd08e96 100644
> --- a/ld/emultempl/riscvelf.em
> +++ b/ld/emultempl/riscvelf.em
> @@ -25,6 +25,35 @@ fragment <<EOF
>  #include "elf/riscv.h"
>  #include "elfxx-riscv.h"
>
> +static struct riscv_elf_params params = { .relax_gp = 1 };
> +EOF
> +
> +# Define some shell vars to insert bits of code into the standard elf
> +# parse_args and list_options functions.  */
> +PARSE_AND_LIST_PROLOGUE=${PARSE_AND_LIST_PROLOGUE}'
> +enum risccv_opt
> +{
> +  OPTION_RELAX_GP = 321,
> +  OPTION_NO_RELAX_GP,
> +};
> +'
> +
> +PARSE_AND_LIST_LONGOPTS=${PARSE_AND_LIST_LONGOPTS}'
> +    { "relax-gp", no_argument, NULL, OPTION_RELAX_GP },
> +    { "no-relax-gp", no_argument, NULL, OPTION_NO_RELAX_GP },
> +'
> +
> +PARSE_AND_LIST_ARGS_CASES=${PARSE_AND_LIST_ARGS_CASES}'
> +    case OPTION_RELAX_GP:
> +      params.relax_gp = 1;
> +      break;
> +
> +    case OPTION_NO_RELAX_GP:
> +      params.relax_gp = 0;
> +      break;
> +'
> +
> +fragment <<EOF
>  static void
>  riscv_elf_before_allocation (void)
>  {
> @@ -96,6 +125,8 @@ riscv_create_output_section_statements (void)
>                " whilst linking %s binaries\n"), "RISC-V");
>        return;
>      }
> +
> +  riscv_elf${ELFSIZE}_set_options (&link_info, &params);
>  }
>
>  EOF
> diff --git a/ld/testsuite/ld-riscv-elf/code-model-relax-medlow-01-norelaxgp.d b/ld/testsuite/ld-riscv-elf/code-model-relax-medlow-01-norelaxgp.d
> new file mode 100644
> index 00000000000..8e40cc5f32d
> --- /dev/null
> +++ b/ld/testsuite/ld-riscv-elf/code-model-relax-medlow-01-norelaxgp.d
> @@ -0,0 +1,4 @@
> +#source: code-model.s
> +#as: -march=rv64i -mabi=lp64 --defsym __medlow__=1
> +#ld: -Tcode-model-01.ld -melf64lriscv --no-relax-gp --relax
> +#error: .*relocation truncated to fit: R_RISCV_HI20 against `symbolL'
> diff --git a/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp b/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp
> index 272424b33e3..1711d12ba23 100644
> --- a/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp
> +++ b/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp
> @@ -122,6 +122,7 @@ if [istarget "riscv*-*-*"] {
>      run_dump_test "align-small-region"
>      run_dump_test "call-relax"
>      run_dump_test "pcgp-relax-01"
> +    run_dump_test "pcgp-relax-01-norelaxgp"
>      run_dump_test "pcgp-relax-02"
>      run_dump_test "c-lui"
>      run_dump_test "c-lui-2"
> @@ -141,6 +142,7 @@ if [istarget "riscv*-*-*"] {
>      run_dump_test "code-model-medany-weakref-01"
>      run_dump_test "code-model-medany-weakref-02"
>      run_dump_test "code-model-relax-medlow-01"
> +    run_dump_test "code-model-relax-medlow-01-norelaxgp"
>      run_dump_test "code-model-relax-medlow-02"
>      run_dump_test "code-model-relax-medlow-weakref-01"
>      run_dump_test "code-model-relax-medlow-weakref-02"
> diff --git a/ld/testsuite/ld-riscv-elf/pcgp-relax-01-norelaxgp.d b/ld/testsuite/ld-riscv-elf/pcgp-relax-01-norelaxgp.d
> new file mode 100644
> index 00000000000..d1344576ff3
> --- /dev/null
> +++ b/ld/testsuite/ld-riscv-elf/pcgp-relax-01-norelaxgp.d
> @@ -0,0 +1,18 @@
> +#source: pcgp-relax-01.s
> +#ld: --no-relax-gp --relax
> +#objdump: -d -Mno-aliases
> +
> +.*:[   ]+file format .*
> +
> +
> +Disassembly of section \.text:
> +
> +0+[0-9a-f]+ <_start>:
> +.*:[   ]+[0-9a-f]+[    ]+addi[         ]+a0,a0,[0-9]+
> +.*:[   ]+[0-9a-f]+[    ]+jal[          ]+ra,[0-9a-f]+ <_start>
> +.*:[   ]+[0-9a-f]+[    ]+auipc[        ]+a1,0x[0-9a-f]+
> +.*:[   ]+[0-9a-f]+[    ]+addi[         ]+a1,a1,[0-9]+ # [0-9a-f]+ <data_g>
> +.*:[   ]+[0-9a-f]+[    ]+lui[          ]+a2,0x[0-9a-f]+
> +.*:[   ]+[0-9a-f]+[    ]+addi[         ]+a2,a2,[0-9]+ # [0-9a-f]+ <data_g>
> +.*:[   ]+[0-9a-f]+[    ]+addi[         ]+a3,tp,0 # 0 <data_t>
> +.*:[   ]+[0-9a-f]+[    ]+auipc[        ]+a0,0x[0-9a-f]+
> diff --git a/ld/testsuite/ld-riscv-elf/pcgp-relax-02.d b/ld/testsuite/ld-riscv-elf/pcgp-relax-02.d
> index c6c73c54265..984d457aed9 100644
> --- a/ld/testsuite/ld-riscv-elf/pcgp-relax-02.d
> +++ b/ld/testsuite/ld-riscv-elf/pcgp-relax-02.d
> @@ -1,6 +1,6 @@
>  #source: pcgp-relax-02.s
>  #as:
> -#ld: --relax
> +#ld: --relax --relax-gp
>  #objdump: -d
>
>  .*:[   ]+file format .*
> --
> 2.37.0.144.g8ac04bfd2-goog
>

Ping. I maintain lld/ELF and we are considering a new opt-in option
--relax-gp to perform GP relaxation.
It will be opt-in since many users don't find the relaxation useful
and some feel GP relaxation will conflict with using GP for other
purposes (some ABI variants).
I wish that the two linkers agree on the same options.