Re: [PING^2] [PATCH] PR gold/18609

[Cary Coutant <ccoutant at gmail dot com>]

> +template<int size, int valsize>
> +class x86_64_overflow_check
> +{
> +public:
> +  typedef typename elfcpp::Elf_types<size>::Elf_Addr Address;
> +
> +  static inline bool
> +  has_overflow_signed(Address value)
> +  {
> +    // limit = 1 << (valsize - 1) without shift count exceeding size of type
> +    Address limit = static_cast<Address>(1) << ((valsize - 1) >> 1);
> +    limit <<= ((valsize - 1) >> 1);
> +    limit <<= ((valsize - 1) - 2 * ((valsize - 1) >> 1));
> +    return value + limit > (limit << 1) - 1;
> +  }
> +
> +  static inline bool
> +  has_overflow_unsigned(Address value)
> +  {
> +    Address limit = static_cast<Address>(1) << ((valsize - 1) >> 1);
> +    limit <<= ((valsize - 1) >> 1);
> +    limit <<= ((valsize - 1) - 2 * ((valsize - 1) >> 1));
> +    return value > (limit << 1) - 1;
> +  }
> +};

Use the functions from the Bits class in reloc.h.

> -       // If the relocation symbol isn't IFUNC,
> -       // and is local, then we will convert
> +       typename elfcpp::Elf_types<size>::Elf_Addr value
> +           = reloc.get_r_offset() + reloc.get_r_addend() - lsym.get_st_value();
> +
> +       // If the relocation symbol isn't IFUNC, and is local,
> +       // and branch and its target are in the same output section,
> +       // and distance isn't overflow, then we will convert
>         // mov foo@GOTPCREL(%rip), %reg
>         // to lea foo(%rip), %reg.
>         // in Relocate::relocate.
>         if (r_type == elfcpp::R_X86_64_GOTPCREL
>             && reloc.get_r_offset() >= 2
> -           && !is_ifunc)
> +           && !is_ifunc
> +           && strcmp(object->section_name(lsym.get_st_shndx()).c_str(),
> +                     ".text") == 0
> +           && !x86_64_overflow_check<size, 32>::has_overflow_signed(value))

We shouldn't be looking at the section name at all. That doesn't
really implement what the comment says, anyway.

The overflow check is only necessary for size == 64, right? You can
use Bits<32>::has_overflow(value).

> @@ -2905,14 +2937,21 @@ Target_x86_64<size>::Scan::global(Symbol_table* symtab,
>        {
>         // The symbol requires a GOT entry.
>         Output_data_got<64, false>* got = target->got_section(symtab, layout);
> +       Sized_symbol<size>* ssym = static_cast<Sized_symbol<size>*>(gsym);
> +       typename elfcpp::Elf_types<size>::Elf_Addr value
> +           = reloc.get_r_offset() + reloc.get_r_addend() - ssym->value();
>
>         // If we convert this from
>         // mov foo@GOTPCREL(%rip), %reg
>         // to lea foo(%rip), %reg.
> -       // in Relocate::relocate, then there is nothing to do here.
> +       // in Relocate::relocate and
> +       // branch and its target are in the same output section and
> +       // distance isn't overflow, then there is nothing to do here.
>         if (r_type == elfcpp::R_X86_64_GOTPCREL
>             && reloc.get_r_offset() >= 2
> -           && Target_x86_64<size>::can_convert_mov_to_lea(gsym))
> +           && Target_x86_64<size>::can_convert_mov_to_lea(gsym)
> +           && gsym->output_section() == output_section
> +           && !x86_64_overflow_check<size, 32>::has_overflow_signed(value))

Likewise.

> -      if (rela.get_r_offset() >= 2
> +      typename elfcpp::Elf_types<size>::Elf_Addr value;
> +      value = psymval->value(object, addend) - address;
> +      if (!x86_64_overflow_check<size, 32>::has_overflow_signed(value)
> +         && rela.get_r_offset() >= 2

Likewise.

-cary