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Re: [PATCH v3 2/2] Implement pahole-like 'ptype /o' option
Thanks for looking into this.
On Tuesday, December 12 2017, Pedro Alves wrote:
> On 12/12/2017 06:19 AM, Sergio Durigan Junior wrote:
>
>> BTW, a little status update.
>>
>> Apparently the patch can't handle bitfields very well. I've found a few
>> cases where the bitfield handling gets confused, printing wrong
>> offsets/sizes/holes. Bitfields can be extremely complex to deal with
>> when it comes to offsets...
>>
>> I spent hours trying to improve the patch, managed to make some
>> progress, but there are still corner cases to fix. For example, the
>> patch doesn't deal well with this case:
>>
>> struct aa {
>> /* 0 | 1 */ char aa;
>> /* 1: 1 | 1 */ unsigned char a : 7;
>> /* 1:15 | 4 */ int b : 10;
>> } /* total size: 4 bytes */
>>
>> In this case, the bitfield "b" would be combined with the previous
>> bitfield "a", like pahole reports:
>>
>> struct aa {
>> char aa; /* 0 1 */
>> unsigned char a:7; /* 1: 1 1 */
>>
>> /* Bitfield combined with previous fields */
>>
>> int b:10; /* 0: 7 4 */
>> }
>>
>> Confusing... I'm not sure why pahole reports b's offset to be 0.
>
> 0 seems right to me. The bitfield's type is int, with size 4,
> and it lives at byte offset 0:
>
> <2><53>: Abbrev Number: 5 (DW_TAG_member)
> <54> DW_AT_name : (indirect string, offset: 0x4ae): b
> <58> DW_AT_decl_file : 1
> <59> DW_AT_decl_line : 7
> <5a> DW_AT_type : <0x71>
> <5e> DW_AT_byte_size : 4
> <5f> DW_AT_bit_size : 10
> <60> DW_AT_bit_offset : 7
> <61> DW_AT_data_member_location: 0 <<<
>
> Replacing the 0 with 1, like:
>
> int b:10; /* 1: 7 4 */
>
> would look incorrect to me, because that'd make '(char*)&aa + 1 + 4'
> (address of containing object + byte offset + byte size) overshoot the
> size of the containing object.
OK. The GDB patch is printing "1:15" because the offset is calculated
as:
(bitpos + offset_bitpos) / TARGET_CHAR_BIT
In this particular case, offset_bitpos is zero because we're not inside
an inner struct, so we don't care about it. bitpos is TYPE_FIELD_BITPOS
(type, field_idx), which is 15 in this case, because sizeof (aa.aa) +
bitsof (aa.a) = 15. When we divide it by TARGET_CHAR_BIT, we get 1. It
seems using TYPE_FIELD_BITPOS is not reliable when dealing with
bitfields.
So there is something I'm not accounting here.
> What is that number after ":" in bitfields supposed to mean in
> pahole's output (and I assume that that's what you're trying
> to emulate)? We're missing documentation for that.
Yeah. I tried to find documentation on the output, but couldn't.
Yesterday I was reading pahole's code. From what I understand, it is
the number of bits left in the object.
> It seems like it's supposed to mean the number of bits left in the
> containing anonymous object (i.e., in the 4 bytes of the declared
> int)? Then "0:7" seems right, given:
>
> sizeof (int) * 8 - bitsof(aa.a) - bitsof(aa.a) - bits(aa.b)
> => sizeof (int) * 8 - 7 - 10
> => 7
I guess you meant:
sizeof (int) * 8 - bitsof (aa.a) - bitsof(aa.b)
Right?
But yeah, it makes sense when you consider that the first bitfield got
"promoted" from "unsigned char" to "int". I haven't figured out a way
to keep track of these type changes in order to calculate the right
offsets, remaining space and holes.
> It took me a while to get to this conclusion (and writing a lot
> of text that I ended up deleting... :-P), because I originally
> assumed that this was meant to be the field's bit offset.
>
>>
>> Also, the patch doesn't understand cases like this:
>>
>> struct tyu
>> {
>> unsigned int a1 : 1;
>> unsigned int a2 : 3;
>> unsigned int a9 : 23;
>> /* PROBLEM HAPPENS HERE */
>> unsigned char a0 : 2;
>> uint64_t a3;
>> unsigned int a4 : 5;
>> uint64_t a5 : 3;
>> };
>>
>> In this case, we're switching types in the middle of the bitfield. The
>> bitfield "unsigned char a0" would be combined with the bitfields above
>> it, but the patch doesn't know that:
>>
>> struct tyu {
>> /* 0:31 | 4 */ unsigned int a1 : 1;
>> /* 0:28 | 4 */ unsigned int a2 : 3;
>> /* 0: 5 | 4 */ unsigned int a9 : 23;
>> /* 3: 6 | 1 */ unsigned char a0 : 2;
>> /* XXX 3-bit hole */
>> /* XXX 4-byte hole */
>> /* 8 | 8 */ uint64_t a3;
>> /* 16:27 | 4 */ unsigned int a4 : 5;
>> /* 16:56 | 8 */ uint64_t a5 : 3;
>> } /* total size: 24 bytes */
>>
>> Whereas pahole reports:
>>
>> struct tyu {
>> unsigned int a1:1; /* 0:31 4 */
>> unsigned int a2:3; /* 0:28 4 */
>> unsigned int a9:23; /* 0: 5 4 */
>>
>> /* XXX 253 bits hole, try to pack */
>> /* Bitfield combined with next fields */
>>
>> unsigned char a0:2; /* 3: 3 1 */
>>
>> /* XXX 6 bits hole, try to pack */
>> /* XXX 4 bytes hole, try to pack */
>>
>> uint64_t a3; /* 8 8 */
>> unsigned int a4:5; /* 16:27 4 */
>> uint64_t a5:3; /* 16:56 8 */
>> };
>>
>> Hm, TBH pahole itself seems a bit confused here, saying there is a
>> 253-bit hole...
>
> A 253-bit hole does look odd.
Yes. I haven't checked all the offsets too; there may be
inconsistencies.
>>
>>
>> Anyway, long story short, this is much more complex that I thought it
>> would be (TM). I am extremely tired right now and can't continue, but I
>> intend to resume work tomorrow morning. But I'd like to leave a few
>> options on the table:
>>
>> 1) Remove the patch from the 8.1 wishlist, which will unblock the
>> branching.
>>
>> 2) Remove the bitfield handling from the patch, leaving it
>> feature-incomplete, but working.
>>
>> 3) Push the patch with these known limitations, document them, mark them
>> as KFAIL in the testcase, and open a bug to fix them (I personally
>> wouldn't like this).
>>
>> 4) Wait more time until these issues are resolved.
>>
>
> Joel said that we'd re-evaluate Wednesday, so there's still some time.
OK, I'll keep trying here.
Thanks,
--
Sergio
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