[PATCH 2/2] riscv: Let compiler handle unaligned access when fixing R_RISCV_RELATIVE

[Adhemerval Zanella]

> On 23 Jun 2022, at 12:47, Kito Cheng <kito.cheng@sifive.com> wrote:
> 
> Explicitly say that might be unaligned, so that the compiler will generate
> appropriate code sequence for this relocation fix-up.
> 
> Although RISC-V Linux will enable the unaligned memory access handler by
> default, that is quite expensive in general, letting the compiler solve this
> will be much cheaper - just break down that into several store byte
> instructions.
> 
> ARM and MIPS has similar issue:
> 
> ARM: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51456
> MIPS: https://gcc.gnu.org/legacy-ml/gcc-help/2005-07/msg00325.html
> ---
> sysdeps/riscv/dl-machine.h | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/sysdeps/riscv/dl-machine.h b/sysdeps/riscv/dl-machine.h
> index 4bb858adaa..c83513e5b8 100644
> --- a/sysdeps/riscv/dl-machine.h
> +++ b/sysdeps/riscv/dl-machine.h
> @@ -157,7 +157,14 @@ __attribute__ ((always_inline))
> elf_machine_rela_relative (ElfW(Addr) l_addr, const ElfW(Rela) *reloc,
> 			  void *const reloc_addr)
> {
> -  *(ElfW(Addr) *) reloc_addr = l_addr + reloc->r_addend;
> +  /* R_RISCV_RELATIVE might located in debug info section which might not
> +     aligned to XLEN bytes. */
> +  struct unaligned
> +    {
> +      ElfW(Addr) x;
> +    } __attribute__ ((packed, may_alias));
> +  /* Support relocations on unaligned offsets.  */
> +  ((struct unaligned *) reloc_addr)->x = l_addr + reloc->r_addend;
> }

Although the same strategy on arm dl-machine.h, I think the most usual way
is to use a a memcpy since it avoid the type cast (which strictly is a UB):

  /* R_RISCV_RELATIVE might located in debug info section which might not
     aligned to XLEN bytes.  Also support relocations on unaligned offsets.  */
  ElfW(Addr) value = l_addr + relic->r_addend;
  memcpy (reloc_addr, &value, sizeof value);
  

> 
> /* Perform a relocation described by R_INFO at the location pointed to
> -- 
> 2.34.0
>