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

Maciej W. Rozycki
2022-03-16T18:26:54Z

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