Re: [PATCH] RISC-V: Optimize relax of GP with max_alignment.

[Nelson Chu <nelson@rivosinc.com>]

On Fri, Sep 23, 2022 at 2:48 PM <lifang_xia@linux.alibaba.com> wrote:
>
> From: Lifang Xia <lifang_xia@linux.alibaba.com>
>
> The max_alignment defined out of [gp-2K, gp+2k), the max_alignment
> shouldn't affect the relax of gp.
> If the symbol is in [gp-2K, gp+2k), the max_alignment would be
> replaced with the max_alignment of the section in [gp-2k, gp+2k).

This idea should works!  But I need to pass the riscv-gnu-toolchain
regression at least, though the regression shouldn't be broken by the
change.
Btw, also cc Palmer and Greg that they may have interested in and have
some though on this.

Thanks
Nelson

> bfd/
>         * elfnn-riscv.c (_bfd_riscv_get_max_alignment_in_gp): New.
>         (_bfd_riscv_relax_lui): The max_alignment of sections is from
>         [gp-2K, gp+2K).
>         (_bfd_riscv_relax_pc): Likewise.
> ld/
>         * ld/testsuite/ld-riscv-elf/relax-max-align.*: New tests.
> ---
>  bfd/elfnn-riscv.c                           | 36 ++++++++++++++++
>  ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp  |  1 +
>  ld/testsuite/ld-riscv-elf/relax-max-align.d | 46 +++++++++++++++++++++
>  ld/testsuite/ld-riscv-elf/relax-max-align.s | 29 +++++++++++++
>  4 files changed, 112 insertions(+)
>  create mode 100644 ld/testsuite/ld-riscv-elf/relax-max-align.d
>  create mode 100644 ld/testsuite/ld-riscv-elf/relax-max-align.s
>
> diff --git a/bfd/elfnn-riscv.c b/bfd/elfnn-riscv.c
> index 3d2ddf4e651..76f888d805c 100644
> --- a/bfd/elfnn-riscv.c
> +++ b/bfd/elfnn-riscv.c
> @@ -4262,6 +4262,28 @@ _bfd_riscv_get_max_alignment (asection *sec)
>    return (bfd_vma) 1 << max_alignment_power;
>  }
>
> +/* Traverse all output sections in [gp-2K, gp+2K) and return the max alignment.  */
> +
> +static bfd_vma
> +_bfd_riscv_get_max_alignment_in_gp (asection *sec, bfd_vma gp)
> +{
> +  unsigned int max_alignment_power = 0;
> +  asection *o;
> +
> +  if (sec == NULL)
> +    return 0;
> +
> +  for (o = sec->output_section->owner->sections; o != NULL; o = o->next)
> +    {
> +      if (VALID_ITYPE_IMM (sec_addr(o) - gp)
> +         || VALID_ITYPE_IMM (sec_addr(o) + o->size - gp))
> +       if (o->alignment_power > max_alignment_power)
> +         max_alignment_power = o->alignment_power;
> +    }

probably can use only one if rather than two.

> +  return (bfd_vma) 1 << max_alignment_power;
> +}
> +
>  /* Relax non-PIC global variable references to GP-relative references.  */
>
>  static bool
> @@ -4293,6 +4315,13 @@ _bfd_riscv_relax_lui (bfd *abfd,
>        if (h->u.def.section->output_section == sym_sec->output_section
>           && sym_sec->output_section != bfd_abs_section_ptr)
>         max_alignment = (bfd_vma) 1 << sym_sec->output_section->alignment_power;
> +
> +      /* Otherwise, consider the alignment of sections in [gp-2K,gp+2K). */
> +      else if (!undefined_weak){
> +         bfd_vma new_max_alignment = _bfd_riscv_get_max_alignment_in_gp (sym_sec, gp);
> +         if (new_max_alignment)
> +           max_alignment = new_max_alignment;
> +      }
>      }

if (...)
  max_alignment = ...
else if (!undefined_weak)
{
  /* Otherwise, ...  */
  ...
}

>    /* Is the reference in range of x0 or gp?
> @@ -4559,6 +4588,13 @@ _bfd_riscv_relax_pc (bfd *abfd ATTRIBUTE_UNUSED,
>        if (h->u.def.section->output_section == sym_sec->output_section
>           && sym_sec->output_section != bfd_abs_section_ptr)
>         max_alignment = (bfd_vma) 1 << sym_sec->output_section->alignment_power;
> +
> +      /* Otherwise, consider the alignment of sections in [gp-2K,gp+2K). */
> +      else if (!undefined_weak){
> +         bfd_vma new_max_alignment = _bfd_riscv_get_max_alignment_in_gp (sym_sec, gp);
> +         if (new_max_alignment)
> +           max_alignment = new_max_alignment;
> +      }
>      }

Likewise, it is minor though.

>    /* Is the reference in range of x0 or gp?
> diff --git a/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp b/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp
> index df89e0ee68b..e53f16facfa 100644
> --- a/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp
> +++ b/ld/testsuite/ld-riscv-elf/ld-riscv-elf.exp
> @@ -169,6 +169,7 @@ if [istarget "riscv*-*-*"] {
>      run_dump_test "attr-merge-priv-spec-failed-05"
>      run_dump_test "attr-merge-priv-spec-failed-06"
>      run_dump_test "attr-phdr"
> +    run_dump_test "relax-max-align"
>      run_ld_link_tests [list \
>         [list "Weak reference 32" "-T weakref.ld -m[riscv_choose_ilp32_emul]" "" \
>             "-march=rv32i -mabi=ilp32" {weakref32.s} \
> diff --git a/ld/testsuite/ld-riscv-elf/relax-max-align.d b/ld/testsuite/ld-riscv-elf/relax-max-align.d
> new file mode 100644
> index 00000000000..4ba920da970
> --- /dev/null
> +++ b/ld/testsuite/ld-riscv-elf/relax-max-align.d
> @@ -0,0 +1,46 @@
> +#source: relax-max-align.s
> +#ld:
> +#objdump: -d
> +
> +.*:[   ]+file format .*
> +
> +
> +Disassembly of section .text:
> +
> +0+[0-9a-f]+ <_start>:
> +.*:[   ]+[0-9a-f]+[    ]+addi[         ]+.*<gdata>
> +.*:[   ]+[0-9a-f]+[    ]+jal[  ]+.*
> +.*:[   ]+[0-9a-f]+[    ]+j[    ]+.*
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +.*:[   ]+[0-9a-f]+[    ]+nop
> +
> +0+[0-9a-f]+ <func>:
> +.*:[   ]+[0-9a-f]+[    ]+ret
> +[      ]+...
> diff --git a/ld/testsuite/ld-riscv-elf/relax-max-align.s b/ld/testsuite/ld-riscv-elf/relax-max-align.s
> new file mode 100644
> index 00000000000..e8abe0f34a1
> --- /dev/null
> +++ b/ld/testsuite/ld-riscv-elf/relax-max-align.s
> @@ -0,0 +1,29 @@
> +
> +.global _start
> +_start:
> +       lui   a0, %hi(gdata)
> +       addi  a0, a0, %lo(gdata)
> +       call func
> +       j .
> +       .size _start, . - _start
> +
> +.global func
> +.align 7
> +func:
> +       ret
> +       .size func, . - func
> +
> +.data
> +padding:
> +       .long 0
> +       .long 0
> +       .long 0
> +       .long 0
> +       .size padding, . - padding
> +
> +.global gdata
> +.type gdata, object
> +gdata:
> +       .zero 4
> +       .size gdata, . - gdata
> +
> --
> 2.32.1 (Apple Git-133)
>