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

["Maciej W. Rozycki" <macro@orcam.me.uk>]

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.

  Maciej