Re: patch v2: [RFC 2/2] ftrace/MIPS: Add support for C version of recordmcount

[wu zhangjin <wuzhangjin@gmail.com>]

On Tue, Oct 26, 2010 at 9:36 PM, Maciej W. Rozycki <macro@linux-mips.org> wrote:
> On Mon, 25 Oct 2010, John Reiser wrote:
>
>> Here's a second try [discard the first] for handling MIPS64 in 
>> recordmcount.[ch].
>
>  Thanks for your contribution -- I appreciate it as undoubtedly do the
> others -- no need to feel frustrated. :)
>
>  Please note that the unusual relocation format comes from the fact the
> MIPS n64 ABI supplement was developed before the ELF64 gABI was finalised
> -- no surprise here given MIPS processors were the first 64-bit around.
> Then the gABI took a more generic approach to handle compound relocations.
> Even `readelf' you cited only got it right as recently as in 2005 (BFD was
> OK from the beginning though AFAIR, so code generated as well as e.g.
> `objdump' reports were right).
>
>  Your change looks good to me overall (I haven't tried building or running
> it -- I take your word you've got it right).  I have a small nit though --
> see below.
>
>> +typedef unsigned char myElf64_byte;
>> +typedef struct {
>> +     Elf64_Addr    r_offset;               /* Address */
>> +     struct {
>> +             Elf64_Word r_sym;
>> +             myElf64_byte r_ssym;  /* Special sym: gp-relative, etc. */
>> +             myElf64_byte r_type3;
>> +             myElf64_byte r_type2;
>> +             myElf64_byte r_type;
>> +     } r_info;
>> +     Elf64_Sxword  r_addend;               /* Addend */
>> +} MIPS64_Rela;
>> +
>> +static uint64_t MIPS64_r_sym(Elf64_Rel const *rp)
>> +{
>> +     return w(((MIPS64_Rela const *)rp)->r_info.r_sym);
>> +}
>> +
>> +static void MIPS64_r_info(Elf64_Rel *const rp, unsigned sym, unsigned type)
>> +{
>> +     MIPS64_Rela *const m64rp = (MIPS64_Rela *)rp;
>> +     m64rp->r_info.r_sym = w(sym);
>> +     m64rp->r_info.r_ssym = 0;
>> +     m64rp->r_info.r_type3 = 0;
>> +     m64rp->r_info.r_type2 = 0;
>> +     m64rp->r_info.r_type = type;
>> +}
>
>  Please try and avoid type punning, i.e. use a union to switch between the
> gABI format of r_info and the n64 MIPS psABI variation.  You'll avoid the
> risk of GCC doing weird stuff wrt aliasing.

Hi, Maciej

will this help?

typedef struct {
        Elf64_Addr    r_offset;               /* Address */
        union {
                struct {
                        Elf64_Word r_sym;
                        myElf64_byte r_ssym;  /* Special sym:
gp-relative, etc. */
                        myElf64_byte r_type3;
                        myElf64_byte r_type2;
                        myElf64_byte r_type;
                } r_info;
                Elf64_Xword gABI_r_info;
        };
        Elf64_Sxword  r_addend;               /* Addend */
} MIPS64_Rela;

Regards,
Wu Zhangjin