libiberty: Fix up > 64K section handling in simple_object_elf_copy_lto_debug_section...

libiberty: Fix up > 64K section handling in simple_object_elf_copy_lto_debug_section [PR116614]

cat abc.C
  #define A(n) struct T##n {} t##n;
  #define B(n) A(n##0) A(n##1) A(n##2) A(n##3) A(n##4) A(n##5) A(n##6) A(n##7) A(n##8) A(n##9)
  #define C(n) B(n##0) B(n##1) B(n##2) B(n##3) B(n##4) B(n##5) B(n##6) B(n##7) B(n##8) B(n##9)
  #define D(n) C(n##0) C(n##1) C(n##2) C(n##3) C(n##4) C(n##5) C(n##6) C(n##7) C(n##8) C(n##9)
  #define E(n) D(n##0) D(n##1) D(n##2) D(n##3) D(n##4) D(n##5) D(n##6) D(n##7) D(n##8) D(n##9)
  E(1) E(2) E(3)
  int main () { return 0; }
./xg++ -B ./ -o abc{.o,.C} -flto -flto-partition=1to1 -O2 -g -fdebug-types-section -c
./xgcc -B ./ -o abc{,.o} -flto -flto-partition=1to1 -O2
(not included in testsuite as it takes a while to compile) FAILs with
lto-wrapper: fatal error: Too many copied sections: Operation not supported
compilation terminated.
/usr/bin/ld: error: lto-wrapper failed
collect2: error: ld returned 1 exit status

The following patch fixes that.  Most of the 64K+ section support for
reading and writing was already there years ago (and especially reading used
quite often already) and a further bug fixed in it in the PR104617 fix.

Yet, the fix isn't solely about removing the
  if (new_i - 1 >= SHN_LORESERVE)
    {
      *err = ENOTSUP;
      return "Too many copied sections";
    }
5 lines, the missing part was that the function only handled reading of
the .symtab_shndx section but not copying/updating of it.
If the result has less than 64K-epsilon sections, that actually wasn't
needed, but e.g. with -fdebug-types-section one can exceed that pretty
easily (reported to us on WebKitGtk build on ppc64le).
Updating the section is slightly more complicated, because it basically
needs to be done in lock step with updating the .symtab section, if one
doesn't need to use SHN_XINDEX in there, the section should (or should be
updated to) contain SHN_UNDEF entry, otherwise needs to have whatever would
be overwise stored but couldn't fit.  But repeating due to that all the
symtab decisions what to discard and how to rewrite it would be ugly.

So, the patch instead emits the .symtab_shndx section (or sections) last
and prepares the content during the .symtab processing and in a second
pass when going just through .symtab_shndx sections just uses the saved
content.

2024-09-07  Jakub Jelinek  <jakub@redhat.com>

	PR lto/116614
	* simple-object-elf.c (SHN_COMMON): Align comment with neighbouring
	comments.
	(SHN_HIRESERVE): Use uppercase hex digits instead of lowercase for
	consistency.
	(simple_object_elf_find_sections): Formatting fixes.
	(simple_object_elf_fetch_attributes): Likewise.
	(simple_object_elf_attributes_merge): Likewise.
	(simple_object_elf_start_write): Likewise.
	(simple_object_elf_write_ehdr): Likewise.
	(simple_object_elf_write_shdr): Likewise.
	(simple_object_elf_write_to_file): Likewise.
	(simple_object_elf_copy_lto_debug_section): Likewise.  Don't fail for
	new_i - 1 >= SHN_LORESERVE, instead arrange in that case to copy
	over .symtab_shndx sections, though emit those last and compute their
	section content when processing associated .symtab sections.  Handle
	simple_object_internal_read failure even in the .symtab_shndx reading
	case.