[PATCH] Add default parameter to MCSymbol accessors to control IsUsed

[Rafael Espíndola via llvm-commits]

With your patch we produce

000000000000  00010000000a R_X86_64_32       0000000000000000 zed + 0
000000000004  00010000000a R_X86_64_32       0000000000000000 zed + 0

gas produces

000000000000  00040000000a R_X86_64_32       0000000000000000 bar + 0
000000000004  00050000000a R_X86_64_32       0000000000000000 zed + 0

So we go from producing an error to producing an output that is
different from gas, which is bad thing.

The part about not setting IsUsed unconditionally is still nice, so if
you don't mind finishing up a NFC patch that just refines what is a
use that would be great.

Cheers,
Rafael



On 25 August 2015 at 09:26, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
> Taking a look.
>
> On 21 August 2015 at 17:34, Vedant Kumar <vsk at apple.com> wrote:
>> Ping
>>
>> We were discussing "invalid reassignment to non-absolute variable" and whether we still need it around.
>>
>> thanks
>> vedant
>>
>>> On Aug 13, 2015, at 9:31 AM, Vedant Kumar <vsk at apple.com> wrote:
>>>
>>> Hi Rafael,
>>>
>>> Sorry for the delay on this.
>>>
>>>> I think that dates from the early days of MC. My understanding is that
>>>> it was added just as a safety so that we have to reason about code
>>>> like
>>>>
>>>> foo = bar
>>>> .long foo
>>>> foo = zed
>>>> .long foo
>>>>
>>>> Should that produce two relocations with bar, two with foo or one each?
>>>
>>> Here is what GAS does:
>>>
>>> SYMBOL TABLE:
>>> 00000000 l    d  .text        00000000 .text
>>> 00000000 l    d  .data        00000000 .data
>>> 00000000 l    d  .bss 00000000 .bss
>>> 00000000         *UND*        00000000 bar
>>> 00000000         *UND*        00000000 zed
>>>
>>> RELOCATION RECORDS FOR [.text]:
>>> OFFSET   TYPE              VALUE
>>> 00000000 R_386_32          bar
>>> 00000004 R_386_32          zed
>>>
>>> Here is a different test that initially triggered this issue:
>>>
>>>> foo = bar
>>>> .long foo
>>>> foo = bar
>>>> .long foo
>>>
>>>
>>> And here is GAS's output:
>>>
>>> RELOCATION RECORDS FOR [.text]:
>>> OFFSET   TYPE              VALUE
>>> 00000000 R_386_32          bar
>>> 00000004 R_386_32          bar
>>>
>>> With the patch I suggested, both of these examples just work with llvm-mc. Here's what the relocation table looks like on Darwin for comparison:
>>>
>>> Relocation information (__TEXT,__text) 2 entries
>>> address  pcrel length extern type    scattered symbolnum/value
>>> 00000004 0     2      1      0       0         1
>>> 00000000 0     2      1      0       0         1
>>>
>>> This looks sane to me. I've updated the patch to get rid of the "invalid reassignment to non-absolute variable" error and fixed the test cases. Could you review it for any issues?
>>>
>>> thanks
>>> vedant
>>>
>>> <MCSymbol-patch1-NFC.patch><MCSymbol-patch2.patch>
>>