[Bug libgcc/99759] morestack.S should support .init_array.0 besides .ctors.65535

[i at maskray dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99759

--- Comment #3 from Fangrui Song <i at maskray dot me> ---
(In reply to Alan Modra from comment #2)
> > this means the ctors.65535 will come last.
> Nope, it will come first.  And since DT_INIT_ARRAY pointers are executed in
> the order they appear in the array, it will be one of the first to run. 
> .init_array and .ctors sorting is complicated.  ld.bfd will sort
> .init_array.0 (highest priority .init_array section) and .ctors.65535
> (highest priority .ctors section) together.
> 
> I assume this comment:
> > The input section description is quite close but does not sort .init_array.* and .ctors.* with the same priority together.
> is referring to lld.

`KEEP (*(SORT_BY_INIT_PRIORITY(.init_array.*)
SORT_BY_INIT_PRIORITY(.ctors.*)))`
The syntax is ambiguous. I can read it this way: place .init_array.* before
.ctors.* , but the behavior is (the ideal way):

.init_array     0x0000000000402ff9        0x7
                [!provide]                        PROVIDE (__init_array_start =
.)
 *(SORT_BY_INIT_PRIORITY(.init_array.*) SORT_BY_INIT_PRIORITY(.ctors.*))
 .init_array.5  0x0000000000402ff9        0x1 a.o
 .ctors.65529   0x0000000000402ffa        0x1 a.o
 .init_array.7  0x0000000000402ffb        0x1 a.o
 .ctors.65435   0x0000000000402ffc        0x1 a.o
 .init_array.100
                0x0000000000402ffd        0x1 a.o
 *(.init_array EXCLUDE_FILE(*crtend?.o *crtend.o *crtbegin?.o *crtbegin.o)
.ctors)
 .init_array    0x0000000000402ffe        0x1 a.o
 .ctors         0x0000000000402fff        0x1 a.o
                [!provide]                        PROVIDE (__init_array_end =
.)


It is unclear that contiguous SORT_BY_INIT_PRIORITY are sorted as a unit.

> > I don't see any issue here really in the end because GCC will produce init_array most of the time.
> So the issue really is that lld doesn't support mixing of .ctors.* and
> .init_array.*.

Yes.

> It might be nice for libgcc to use .init_array.0 here instead of
> .ctors.65536 whenever gcc will use .init_array in compiled code.

Yes. This is the only place I know where modern Linux distrubtions is still
using .ctors* in .o files.