Empty section flags

[H.J. Lu]

On Tue, Mar 3, 2020 at 1:20 PM Fangrui Song <i@maskray.me> wrote:
>
> On Sun, Feb 9, 2020 at 9:21 PM Alan Modra <amodra@gmail.com> wrote:
> >
> > On Thu, Feb 06, 2020 at 05:25:33PM +0000, bd1976 llvm wrote:
> > > Hi Alan, thanks for the input here. I wonder if it wouldn't be more
> > > consistent to error in all cases - even in the case of different group
> > > signatures. The only exception would need to be for the special section
> > > names (.text, .debug_str, etc...) that the assembler has special knowledge
> > > of (as you explained).
> >
> > Yes, let's see how that goes.
> > https://sourceware.org/ml/binutils/2020-02/msg00129.html
> >
> > > I wonder why creating multiple sections with the
> > > same name for section directives with different group signatures was
> > > implemented - why not just require the use of a distinct section name for
> > > these?
> >
> > I think plain ".text" for a group's text section is fine.  Distict
> > names would just be yet another thing to track for a group.
> >
> > > Or, now that GNU has the ",unique,N" assembly extension (
> > > https://sourceware.org/ml/binutils/2020-02/msg00028.html) that could be
> > > used if the section name is fixed - it would then be explicit in the source
> > > code that another section with the same name will be created.
> >
> > Perhaps, but we aren't designing a new toolchain.  Backwards
> > compatibility can't be discarded without compelling reasons.
> >
> > --
> > Alan Modra
> > Australia Development Lab, IBM
>
> For empty flags, should there be an error as well?
>
>   .section .foo,"ax",@progbits; .byte 1
>   .section .foo,"",@progbits; .byte 2  # no diagnostic
>   .section .foo,"a",@progbits; .byte 3  # Error: changed section
> attributes for .foo
>
> Context: https://github.com/ClangBuiltLinux/linux/issues/913
>
> I lean toward an error for consistency, and I will try making the LLVM
> MC side rule stick.

[hjl@gnu-cfl-2 tmp]$ cat x.s
.section .foo,"",@progbits; .byte 2
[hjl@gnu-cfl-2 tmp]$ gcc -c x.s
[hjl@gnu-cfl-2 tmp]$ readelf -SW x.o | grep foo
  [ 4] .foo              PROGBITS        0000000000000000 000040
000001 00      0   0  1
[hjl@gnu-cfl-2 tmp]$

Unless it is disallowed by gABI/psABI, assembler should allow it.
Sometimes, I found a need to create odd object files, like zero-sized
relocation section,  for linker test.  Assembler should have more
flexibilities within gABI/psABI.

-- 
H.J.