[LLVMdev] [lld] TBSS wrong size

[Dave Bozier]

May not be a bug. It may have been placed in .tdata instead. have a
look at the symbol table to see where these variables are.

On Wed, Jun 3, 2015 at 3:13 PM, Shankar Easwaram <shankarke at gmail.com> wrote:
> Ah this is a nasty bug. You might want to checkout if some of the TLS variables are allocated elsewhere? It cannot just disappear from the output.
>
>
>> On Jun 3, 2015, at 08:16, Adhemerval Zanella <adhemerval.zanella at linaro.org> wrote:
>>
>> Hi,
>>
>> Yes, ldd is generating wrong tbss size.  It is just considering one tbss section
>> and not calculating all sections from all objects.  The following example on
>> x86_64 shows the issue:
>>
>> --- t0.c ---
>>
>> #include <stdio.h>
>>
>> extern __thread int t0;
>> extern __thread int t1;
>> extern __thread int t2;
>> extern __thread int t3;
>>
>> __thread int t4;
>> __thread int t5;
>> __thread int t6;
>> __thread int t7;
>>
>> int main ()
>> {
>>  t4 = 1;
>>  t5 = 2;
>>  t6 = 3;
>>  t7 = 4;
>>
>>  printf ("%i %i %i %i\n", t0, t1, t2, t3);
>>  printf ("%i %i %i %i\n", t4, t5, t6, t7);
>>
>>  return 0;
>> }
>>
>> --- t1.c ---
>>
>> __thread int t0;
>> __thread int t1;
>> __thread int t2;
>> __thread int t3;
>>
>> -------------
>>
>> If you build with lld you will see:
>>
>> $ ./t-lld
>> 1 1 1 1
>> 1 2 3 4
>>
>> Because t{4,5,6,7} space is not taking in consideration.  In fact if you
>> check the resulting tbss for the test:
>>
>>  [15] .tbss             NOBITS           0000000000401000  00001000
>>       0000000000000010  0000000000000000 WAT       0     0     4
>>
>> Its size is just 0x10 (4 int), where it should be 0x20 (8 ints).
>>
>>
>>> On 02-06-2015 21:14, Shankar Easwaram wrote:
>>> Are you saying it generates wrong section size?  Tbss is very special and I checked the behavior on X86_64 to model it. It does not account for virtual address increase as libc allocates it.
>>>
>>>
>>>> On Jun 2, 2015, at 17:20, Adhemerval Zanella <adhemerval.zanella at linaro.org> wrote:
>>>>
>>>> Hi,
>>>>
>>>> I am tracking some TLS issues with lld and found that it is
>>>> generating wrong tbss size for case where multiple modules
>>>> have non initialized threads variables.  For instance:
>>>>
>>>> -- t0.c --
>>>>
>>>> __thread int x0;
>>>> __thread int x1;
>>>> __thread int x2;
>>>>
>>>> extern __thread int e0;
>>>> extern __thread int e1;
>>>> extern __thread int e2;
>>>> extern __thread int e3;
>>>>
>>>> int foo0 ()
>>>> {
>>>> return x0;
>>>> }
>>>>
>>>> int main ()
>>>> {
>>>> return x0;
>>>> }
>>>>
>>>> -- t1.c --
>>>>
>>>> __thread int e0;
>>>> __thread int e1;
>>>> __thread int e2;
>>>> __thread int e3;
>>>>
>>>> ---
>>>>
>>>> lld is generating (for aarch64):
>>>>
>>>> [14] .tbss             NOBITS           0000000000401000  00001000
>>>>      0000000000000010  0000000000000000 WAT       0     0     4
>>>>
>>>> Where is just taking in consideration the largest tbss segment, not all
>>>> from all objects.  ld generates a correct output:
>>>>
>>>> [17] .tbss             NOBITS           0000000000410dec  00000dec
>>>>      000000000000001c  0000000000000000 WAT       0     0     4
>>>>
>>>> My initial idea is that 'lib/ReaderWriter/ELF/SegmentChunks.cpp' /
>>>> Segment<ELFT>::assignVirtualAddress is setting wrong slice values, however
>>>> playing with this I could not find a correct logic to handle the TBSS.
>>>>
>>>> Any ideas where lld is possible messing the TBSS segments sizes?
>
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