On Wed, Nov 18, 2020 at 12:51:02PM +0100, Tobias Burnus wrote:
> As noted by Matthias when bootstrapping with AMD GCN support [PR97827]:
> Assembler source code generated by GCC might no longer assembly with
> LLVM's 'mc' since LLVM 11.
>
> The reason is that GCC generates on purpose first the section with
> the flags, e.g. (via mergeable_constant_section)
> .section .rodata.cst8,"aM",@progbits,8
> and then for subsequent uses, it does not repeat the flags:
> .section .rodata.cst8
>
> GNU assembler warns (and with as >=2.35 gives an error) if the flags
> do not match, but not if the attributes/flags are left in the other
> same-named sections (as above) – just if they are specified and different.
>
> LLVM since February (in git) and released with LLVM 11 (12 Oct 2020)
> does a similar check – but without the no-error-if-no-flag exception:
> strtod.s:4472:2: error: changed section flags for .rodata.cst8, expected:
> 0x12
> strtod.s:4472:2: error: changed section entsize for .rodata.cst8, expected:
> 8
>
>
> The solution done by the attached patch is to emit the full flags also
> for SECTION_MERGE.
>
> Side note: For AMD GCN, we rely on LLVM as "GNU as" does not handle
> this target, yet; still, also in general, it makes sense to be
> compatible with llvm-mc.
>
> OK?
I think we shouldn't do this except when targetting the (buggy) llvm
assembler.
Specifying section flags just on first .section directive and not others
is correct, there is no point repeating that and GNU as (but I think many
other assemblers) has been supporting it that way forever.
The only time one needs to specify the section flags again is for comdat
sections because then the section name is not unique, one needs section name
and comdat pair...
Jakub
