[llvm] r259641 - [mips] Add SHF_MIPS_GPREL flag to the MIPS .sbss and .sdata sections

[Daniel Sanders via llvm-commits]

> -----Original Message-----
> From: Simon Atanasyan [mailto:simon at atanasyan.com]
> Sent: 06 February 2016 16:55
> To: Rafael Espíndola; Daniel Sanders
> Cc: llvm-commits
> Subject: Re: [llvm] r259641 - [mips] Add SHF_MIPS_GPREL flag to the MIPS
> .sbss and .sdata sections
> 
> On Fri, Feb 5, 2016 at 4:44 PM, Rafael Espíndola
> <rafael.espindola at gmail.com> wrote:
> > On 5 February 2016 at 07:19, Simon Atanasyan <simon at atanasyan.com>
> wrote:
> >> Unfortunately now it is impossible. llvm-mc does not support some
> >> necessary flags etc.
> >
> > Which flags? Is it being worked on? It is *very* scary to be able to
> > produce a .o to which there is no .s equivalent.
> 
> I was not correct - llvm-mc does not support MIPS specific .sdata and
> .sbss directives to create the corresponding sections. Also it does
> not know about default flags and section types need to be assigned to
> these sections so ".section .sdata" is accepted but produces .sdata
> section with no type and no flags.
> 
> The attached patch partially fixes this problem (.sdata and .sbss
> directives still unsupported) and converts original test case to .s
> form. I am not sure about correctness of the selected approach.
> Daniel, what do you think about that?
> 
> Regards,
> Simon Atanasyan

I definitely agree that we should be able use llvm-mc to test this. As you've said, the reason we can't right now is that .sdata/.sbss aren't implemented. To add to that, the reason they aren't implemented yet is because they aren't needed for our first Integrated Assembler milestone. This milestone will have been reached when we can recurse LLVM, and assemble the linux kernel. We're pretty close on both but there's a couple issues remaining.

Your patch looks correct to me w.r.t MIPS but I've a concern for other targets. I don't think ELFAsmParser::ParseSectionArguments() should be adding SHF_MIPS_* values without first checking whether the target is MIPS. Similarly, I don't think we should be adding special default flags on targets where .sdata/.sbss aren't special.