[Bug gas/21809] New: gas outputs data symbol markers "$d" in .debug_frame

[mark at klomp dot org]

https://sourceware.org/bugzilla/show_bug.cgi?id=21809

            Bug ID: 21809
           Summary: gas outputs data symbol markers "$d" in .debug_frame
           Product: binutils
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gas
          Assignee: unassigned at sourceware dot org
          Reporter: mark at klomp dot org
  Target Milestone: ---

On arm and aarch64:

$ echo "int main() { return 1; }" | gcc -g -xc -
$ eu-readelf -S ./a.out | grep debug_frame
[31] .debug_frame         PROGBITS     00000000 0007c0 000030  0        0   0 
4
$ eu-readelf -s ./a.out | grep '$d' | grep 31
   67: 00000010      0 NOTYPE  LOCAL  DEFAULT       31 $d

Note that theoretically it is allowed to have a data marker symbol at the start
of the section if the whole section contains data. But it is redundant and
optional. Also this symbol doesn't point to the start of the section but some
way in.

This is caused by the use of frag_align () in gas/dw2gencfi.c.
It is similar to an issue fixed with .debug_aranges:

commit d025d5e5b57fb59c56aa4d57b7fc138720a8e454
Author: Mark Wielaard <address@hidden>
Date:   Sat Jun 13 17:47:41 2015 -0400

    gas: Don't use frag_align but use plain padding to align .debug_aranges.

    out_debug_aranges uses frag_align to make sure the addresses start
    out aligned. Using frag_align will call frag_var[_init], which will
    end up calling TC_FRAG_INIT. On arm and aarch64 TC_FRAG_INIT will
    generate a $d mapping symbol for the .debug_aranges to show that at
    that point a sequence of data items starts.

    Such a symbol pointing into a non-allocated debug section will confuse
    eu-strip -g. And it seems inefficient and wrong in general to have
    additional mapping symbols for debug sections, which won't contain
    actual code in the first place.

    Just keep track of the aranges header size and use plain padding to
    align the addresses which avoids generating any mapping symbols on
    aarch64 and arm.

    Includes a testcase for aarch64 that PASS with this patch and shows
    the extra $d mapping symbol in .debug_aranges before.

    gas/ChangeLog

           * dwarf2dbg.c (out_header): Document EXPR->X_add_number value,
           out_debug_aranges depends on it.
           (out_debug_aranges): Track size of header to properly pad header
           for address alignment.

    gas/testsuite/ChangeLog

           * gas/aarch64/dwarf.d: New.
           * gas/aarch64/dwarf.s: New.

It is however not as to remove all uses of frag_align () and replace them with
simple padding in dw2gencfi.c.

It would best if aarch64_init_frag and arm_init_frag could be changed to not
emit marker symbols if the frag is associated with a data only section (like
SEC_DEBUGGING).

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