[llvm] r293936 - Change how we handle section symbols on ELF.

[Mikael Holmén via llvm-commits]

Hi Rafael,

Did you have any opinion about this?

The small reproducer further down in the mail crashes on xcore.

Regards,
Mikael


 > llc -O0 -march=xcore foo.ll -o -
 >
 > on
 >
 > @bar = internal global i32 zeroinitializer
 >
 > define void @".dp.bss"() {
 >   ret void
 > }


On 02/06/2017 09:37 AM, Mikael Holmén wrote:
>
>
> On 02/03/2017 03:50 PM, Rafael Avila de Espindola wrote:
>> Mikael Holmén <mikael.holmen at ericsson.com> writes:
>>
>>> Hi Rafael,
>>>
>>> I have a question about this change.
>> ...
>>>> This patch includes a few hacks to avoid changing our behaviour when
>>>> handling conflicts between section symbols and other symbols. I
>>>> reported pr31850 to track that.
>>>
>>> For my out-of-tree backend, we output all code in a section called
>>> exactly "code". With your patch we now run into the below problem if we
>>> then also have a function called "code".
>>>
>>> For foo.ll:
>>>
>>> define void @code() {
>>>    ret void
>>> }
>>>
>>> If I do
>>>
>>> build/bin/llc -O0 -mtriple phoenix foo.ll -o -
>>>
>>> we get
>>>
>>>          .section code
>>> __FILE_START:
>>>          .global code
>>> llc: ../lib/MC/MCAsmStreamer.cpp:440: virtual void (anonymous
>>> namespace)::MCAsmStreamer::EmitLabel(llvm::MCSymbol *): Assertion
>>> `Symbol->isUndefined() && "Cannot define a symbol twice!"' failed.
>>>
>>> So now we seem to get a clash between the section and function names,
>>> and the assertion fails.
>>>
>>> I suppose this was not intended or are we doing something stupid in our
>>> backend?
>>
>> It was not the intention to change it. What is line 440 for you?
>
> Ah, sorry.
>
> It's the assert on the first line in EmitLabel below that blows:
>
> void MCAsmStreamer::EmitLabel(MCSymbol *Symbol) {
>   assert(Symbol->isUndefined() && "Cannot define a symbol twice!");
>   MCStreamer::EmitLabel(Symbol);
>
>   Symbol->print(OS, MAI);
>   OS << MAI->getLabelSuffix();
>
>   EmitEOL();
> }
>
> We have setup our "code" section with this in our *TargetObjectFile.cpp:
>
>   // .text
>   TextSection = Ctx.getELFSection("code", ELF::SHT_LOUSER + 0,
>                                   ELF::SHF_ALLOC | ELF::SHF_EXECINSTR);
>
>> Upstream it is just an empty line.
>>
>> I tried to reproduce the problem with
>>
>> define void @".text"() {
>>   ret void
>> }
>>
>> but that doesn't crash and the function symbol takes precedence over the
>> text one.
>
> If I do
>
> llc -O0 -march=xcore foo.ll -o -
>
> on
>
> @bar = internal global i32 zeroinitializer
>
> define void @".dp.bss"() {
>   ret void
> }
>
> then I get the same crash for xcore.
>
> Regards,
> Mikael
>
>>
>> Cheers,
>> Rafael
>>
>