[PATCH] dwarf2out: Fix up field_byte_offset [PR101378]

Richard Biener rguenther@suse.de
Wed Nov 10 09:52:42 GMT 2021


On Wed, 10 Nov 2021, Jakub Jelinek wrote:

> Hi!
> 
> For PCC_BITFIELD_TYPE_MATTERS field_byte_offset has quite large code
> to deal with it since many years ago (see it e.g. in GCC 3.2, although it
> used to be on HOST_WIDE_INTs, then on double_ints, now on offset_ints).
> But that code apparently isn't able to cope with members with empty class
> types with [[no_unique_address]] attribute, because the empty classes have
> non-zero type size but zero decl size and so one can end up from the
> computation with negative offset or offset 1 byte smaller than it should be.
> For !PCC_BITFIELD_TYPE_MATTERS, we just use
>     tree_result = byte_position (decl);
> which seems exactly right even for the empty classes or anything which is
> not a bitfield (and for which we don't add DW_AT_bit_offset attribute).
> So, instead of trying to handle those no_unique_address members in the
> current already very complicated code, this limits it to bitfields.
> 
> stor-layout.c PCC_BITFIELD_TYPE_MATTERS handling also affects only
> bitfields, twice it checks DECL_BIT_FIELD and once DECL_BIT_FIELD_TYPE.
> 
> The only thing I'm unsure about is whether the test should be
> DECL_BIT_FIELD or DECL_BIT_FIELD_TYPE should be tested.  I thought it
> doesn't matter, but it seems stor-layout.c in some cases clears
> DECL_BIT_FIELD if their TYPE_MODE can express the type exactly, and
> dwarf2out.c (gen_field_die) uses
>   if (DECL_BIT_FIELD_TYPE (decl))
> to decide if DW_AT_bit_offset etc. attributes should be added.
> So maybe I should go with && DECL_BIT_FIELD_TYPE (decl) instead.

You need DECL_BIT_FIELD_TYPE if you want to know whether it is
a bitfield.  DECL_BIT_FIELD can only be used to test whether the
size is not a multiple of BITS_PER_UNIT.

So the question is whether the code makes a difference if
the bitfield is int a : 8; int b : 8; int c : 16; for example.
If so then DECL_BIT_FIELD_TYPE is needed, otherwise what you
test probably doesn't matter.

> On
> struct S { int e; int a : 1, b : 7, c : 8, d : 16; } s;
> struct T { int a : 1, b : 7; long long c : 8; int d : 16; } t;
> it doesn't make a difference though on x86_64, ppc64le nor ppc64...
> 
> I think Ada has bitfields of aggregate types, so CCing Eric, though
> I'd hope it doesn't have bitfields where type size is smaller than
> field decl size like C++ has.
> 
> Bootstrapped/regtested on x86_64-linux, i686-linux, powerpc64le-linux
> and powerpc64-linux and Pedro has tested it on GDB testsuite.
> 
> I can bootstrap/regtest the
> +      && DECL_BIT_FIELD_TYPE (decl)
> version too.
> 
> 2021-11-10  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR debug/101378
> 	* dwarf2out.c (field_byte_offset): Do the PCC_BITFIELD_TYPE_MATTERS
> 	handling only for DECL_BIT_FIELD decls.
> 
> 	* g++.dg/debug/dwarf2/pr101378.C: New test.
> 
> --- gcc/dwarf2out.c.jj	2021-11-05 10:19:46.339457342 +0100
> +++ gcc/dwarf2out.c	2021-11-09 15:01:51.425437717 +0100
> @@ -19646,6 +19646,7 @@ field_byte_offset (const_tree decl, stru
>       properly dynamic byte offsets only when PCC bitfield type doesn't
>       matter.  */
>    if (PCC_BITFIELD_TYPE_MATTERS
> +      && DECL_BIT_FIELD (decl)
>        && TREE_CODE (DECL_FIELD_OFFSET (decl)) == INTEGER_CST)

What's more interesting is the INTEGER_CST restriction - I'm sure
that Ada allows bitfields to follow variable position other fields.
Even C does:

void foo (int n)
{
  struct S { int a[n]; int b : 5;  int c : 3; } s;
}

runs into the code above and ends up not honoring 
PCC_BITFIELD_TYPE_MATTERS ...

Richard.

>      {
>        offset_int object_offset_in_bits;
> --- gcc/testsuite/g++.dg/debug/dwarf2/pr101378.C.jj	2021-11-09 15:17:39.504975396 +0100
> +++ gcc/testsuite/g++.dg/debug/dwarf2/pr101378.C	2021-11-09 15:17:28.067137556 +0100
> @@ -0,0 +1,13 @@
> +// PR debug/101378
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-gdwarf-5 -dA" }
> +// { dg-final { scan-assembler-times "0\[^0-9x\\r\\n\]* DW_AT_data_member_location" 1 } }
> +// { dg-final { scan-assembler-times "1\[^0-9x\\r\\n\]* DW_AT_data_member_location" 1 } }
> +// { dg-final { scan-assembler-times "2\[^0-9x\\r\\n\]* DW_AT_data_member_location" 1 } }
> +// { dg-final { scan-assembler-not "-1\[^0-9x\\r\\n\]* DW_AT_data_member_location" } }
> +
> +struct E {};
> +struct S
> +{
> +  [[no_unique_address]] E e, f, g;
> +} s;
> 
> 	Jakub
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Ivo Totev; HRB 36809 (AG Nuernberg)


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