Re: [PATCH] gdb: mips: Fix the handling of complex type of function return value

Youling Tang
2022-03-17T07:02:17Z

Hi, Maciej

On 03/17/2022 02:26 AM, Maciej W. Rozycki wrote:
> On Wed, 16 Mar 2022, Andrew Burgess wrote:
>
>>>>> $ objdump -d outputs/gdb.base/varargs/varargs
>>>>> 00000001200012e8 <find_max_float_real>:
>>>>> ...
>>>>> 1200013b8: c7c10000 lwc1 $f1,0(s8)
>>>>> 1200013bc: c7c00004 lwc1 $f0,4(s8)
>>>>> 1200013c0: 46000886 mov.s $f2,$f1
>>>>> 1200013c4: 46000046 mov.s $f1,$f0
>>>>> 1200013c8: 46001006 mov.s $f0,$f2
>>>>> 1200013cc: 46000886 mov.s $f2,$f1
>>>>> 1200013d0: 03c0e825 move sp,s8
>>>>> 1200013d4: dfbe0038 ld s8,56(sp)
>>>>> 1200013d8: 67bd0080 daddiu sp,sp,128
>>>>> 1200013dc: 03e00008 jr ra
>>>>> 1200013e0: 00000000 nop
>>>>>
>>>>> From the above disassembly, we can see that when the return value of the
>>>>> function is a complex type and len <= 2 * MIPS64_REGSIZE, the return value
>>>>> will be passed through $f0 and $f2, so fix the corresponding processing
>>>>> in mips_n32n64_return_value().
>>>>>
>>>>> $ make check RUNTESTFLAGS='GDB=../gdb gdb.base/varargs.exp --outdir=test'
>>>>>
>>>>> Before applying the patch:
>>>>> FAIL: gdb.base/varargs.exp: print find_max_float_real(4, fc1, fc2, fc3, fc4)
>>>>> FAIL: gdb.base/varargs.exp: print find_max_double_real(4, dc1, dc2, dc3, dc4)
>>>>>
>>>>> # of expected passes 9
>>>>> # of unexpected failures 2
>>>>>
>>>>> After applying the patch:
>>>>> # of expected passes 11
>>>>>
>>>>> Signed-off-by: Youling Tang <tangyouling@loongson.cn>
>>>>> ---
>>>>> gdb/mips-tdep.c | 3 ++-
>>>>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/gdb/mips-tdep.c b/gdb/mips-tdep.c
>>>>> index 7e37578..cddb8f8 100644
>>>>> --- a/gdb/mips-tdep.c
>>>>> +++ b/gdb/mips-tdep.c
>>>>> @@ -5224,9 +5224,10 @@ mips_n32n64_return_value (struct gdbarch *gdbarch, struct value *function,
>>>>>
>>>>> if (TYPE_LENGTH (type) > 2 * MIPS64_REGSIZE)
>>>>> return RETURN_VALUE_STRUCT_CONVENTION;
>>>>> - else if (type->code () == TYPE_CODE_FLT
>>>>> + else if ((type->code () == TYPE_CODE_FLT
>>>>> && TYPE_LENGTH (type) == 16
>>>>> && tdep->mips_fpu_type != MIPS_FPU_NONE)
>>>> Hi,
>>>>
>>>> Just minor note, those 2 lines above should be indented 2 more space I
>>>> think (so the && operator continues to vertically align with "type->code
>>>> ()").
>>>>
>>>>> + || (type->code () == TYPE_CODE_COMPLEX))
>>>> I do not think the extra set of parens are requires (but they do no harm
>>>> either).
>>> Ok, I will modify this code style in the next version.
>> This change looks good to me with the style adjusments that Lancelot
>> pointed out.
> This has to be double-checked, because as I recall we have an ABI bug in
> GCC in this area. Which is also the reason why the relevant test cases
> have not been fixed in 15+ years now (I've been aware of this issue).
>
> OTOH, if things have been like this for so long, then I suppose they need
> to stay as they are. In any case I think this does have to be thoroughly
> understood and documented.
Thanks for your pointing out.If GCC's processing does not follow the ABI
call parameter specification, then this will be a GCC bug.GDB will remain
as is, without relevant modifications.

Thanks,
Youling.
>
> Maciej