On Thu, Jul 5, 2018 at 4:12 PM Tom de Vries <[email protected]> wrote:
>
> On 07/05/2018 01:39 PM, Richard Biener wrote:
> > On Thu, Jul 5, 2018 at 1:25 PM Tom de Vries <[email protected]> wrote:
> >>
> >> [ was: Re: [testsuite/guality, committed] Prevent optimization of local in
> >> vla-1.c ]
> >>
> >> On Wed, Jul 04, 2018 at 02:32:27PM +0200, Tom de Vries wrote:
> >>> On 07/03/2018 11:05 AM, Tom de Vries wrote:
> >>>> On 07/02/2018 10:16 AM, Jakub Jelinek wrote:
> >>>>> On Mon, Jul 02, 2018 at 09:44:04AM +0200, Richard Biener wrote:
> >>>>>> Given the array has size i + 1 it's upper bound should be 'i' and 'i'
> >>>>>> should be available via DW_OP_[GNU_]entry_value.
> >>>>>>
> >>>>>> I see it is
> >>>>>>
> >>>>>> <175> DW_AT_upper_bound : 10 byte block: 75 1 8 20 24 8 20 26 31
> >>>>>> 1c (DW_OP_breg5 (rdi): 1; DW_OP_const1u: 32; DW_OP_shl;
> >>>>>> DW_OP_const1u: 32; DW_OP_shra; DW_OP_lit1; DW_OP_minus)
> >>>>>>
> >>>>>> and %rdi is 1. Not sure why gdb fails to print it's length. Yes, the
> >>>>>> storage itself doesn't have a location but the
> >>>>>> type specifies the size.
> >>>>>>
> >>>>>> (gdb) ptype a
> >>>>>> type = char [variable length]
> >>>>>> (gdb) p sizeof(a)
> >>>>>> $3 = 0
> >>>>>>
> >>>>>> this looks like a gdb bug to me?
> >>>>>>
> >>>>
> >>>> With gdb patch:
> >>>> ...
> >>>> diff --git a/gdb/findvar.c b/gdb/findvar.c
> >>>> index 8ad5e25cb2..ebaff923a1 100644
> >>>> --- a/gdb/findvar.c
> >>>> +++ b/gdb/findvar.c
> >>>> @@ -789,6 +789,8 @@ default_read_var_value
> >>>> break;
> >>>>
> >>>> case LOC_OPTIMIZED_OUT:
> >>>> + if (is_dynamic_type (type))
> >>>> + type = resolve_dynamic_type (type, NULL,
> >>>> + /* Unused address. */ 0);
> >>>> return allocate_optimized_out_value (type);
> >>>>
> >>>> default:
> >>>> ...
> >>>>
> >>>> I get:
> >>>> ...
> >>>> $ ./gdb -batch -ex "b f1" -ex "r" -ex "p sizeof (a)" vla-1.exe
> >>>> Breakpoint 1 at 0x4004a8: file vla-1.c, line 17.
> >>>>
> >>>> Breakpoint 1, f1 (i=i@entry=5) at vla-1.c:17
> >>>> 17 return a[0];
> >>>> $1 = 6
> >>>> ...
> >>>>
> >>>
> >>> Well, for -O1 and -O2.
> >>>
> >>> For O3, I get instead:
> >>> ...
> >>> $ ./gdb vla-1.exe -q -batch -ex "b f1" -ex "run" -ex "p sizeof (a)"
> >>> Breakpoint 1 at 0x4004b0: f1. (2 locations)
> >>>
> >>> Breakpoint 1, f1 (i=5) at vla-1.c:17
> >>> 17 return a[0];
> >>> $1 = 0
> >>> ...
> >>>
> >>
> >> Hi,
> >>
> >> When compiling guality/vla-1.c with -O3 -g, vla 'a[i + 1]' in f1 is
> >> optimized
> >> away, but f1 still contains a debug expression describing the upper bound
> >> of the
> >> vla (D.1914):
> >> ...
> >> __attribute__((noinline))
> >> f1 (intD.6 iD.1900)
> >> {
> >> <bb 2>
> >> saved_stack.1_2 = __builtin_stack_save ();
> >> # DEBUG BEGIN_STMT
> >> # DEBUG D#3 => i_1(D) + 1
> >> # DEBUG D#2 => (long intD.8) D#3
> >> # DEBUG D#1 => D#2 + -1
> >> # DEBUG D.1914 => (sizetype) D#1
> >> ...
> >>
> >> Then f1 is cloned to a version f1.constprop with no parameters, eliminating
> >> parameter i, and 'DEBUG D#3 => i_1(D) + 1' turns into 'D#3 => NULL'.
> >> Consequently, 'print sizeof (a)' yields '0' in gdb.
> >
> > So does gdb correctly recognize there isn't any size available or do we
> > somehow
> > generate invalid debug info, not recognizing that D#3 => NULL means
> > "optimized out" and thus all dependent expressions are "optimized out" as
> > well?
> >
> > That is, shouldn't gdb do
> >
> > (gdb) print sizeof (a)
> > <optimized out>
> >
> > ?
>
> The type for the vla gcc is emitting is an DW_TAG_array_type with
> DW_TAG_subrange_type without DW_AT_upper_bound or DW_AT_count, which
> makes the upper bound value 'unknown'. So I'd say the debug info is valid.
OK, that sounds reasonable. I wonder if languages like Ada have a way
to declare an array type with unknown upper bound but known lower bound.
For
typedef int arr[];
arr *x;
we generate just
<1><2d>: Abbrev Number: 2 (DW_TAG_typedef)
<2e> DW_AT_name : arr
<32> DW_AT_decl_file : 1
<33> DW_AT_decl_line : 1
<34> DW_AT_decl_column : 13
<35> DW_AT_type : <0x39>
<1><39>: Abbrev Number: 3 (DW_TAG_array_type)
<3a> DW_AT_type : <0x44>
<3e> DW_AT_sibling : <0x44>
<2><42>: Abbrev Number: 4 (DW_TAG_subrange_type)
<2><43>: Abbrev Number: 0
which does
(gdb) ptype arr
type = int []
(gdb) ptype x
type = int (*)[]
(gdb) p sizeof (arr)
$1 = 0
so I wonder whether the patch makes it print <optimized out>
instead? I think both 0 and <optimized out> are less than ideal
and maybe <variable size> would be better. In the type case
above it's certainly not "optimized out".
> Using this gdb patch:
> ...
> diff --git a/gdb/eval.c b/gdb/eval.c
> index 9db6e7c69d..ea6f782c5b 100644
> --- a/gdb/eval.c
> +++ b/gdb/eval.c
> @@ -3145,6 +3145,8 @@ evaluate_subexp_for_sizeof (...)
> {
> val = evaluate_subexp (NULL_TYPE, exp, pos, EVAL_NORMAL);
> type = value_type (val);
> + if (TYPE_LENGTH (type) == 0)
> + return allocate_optimized_out_value (size_type);
> }
> else
> (*pos) += 4;
> ...
>
> I get:
> ...
> $ ./gdb vla-1.exe -batch -ex "b f1" -ex run -ex "p sizeof (a)"
> Breakpoint 1 at 0x4004b0: f1. (2 locations)
>
> Breakpoint 1, f1 (i=5) at vla-1.c:17
> 17 return a[0];
> $1 = <optimized out>
> ...
>
> Thanks,
> - Tom
