Bug 99090 - gsplit-dwarf broken on riscv64-linux
Summary: gsplit-dwarf broken on riscv64-linux
Alias: None
Product: gcc
Classification: Unclassified
Component: debug (show other bugs)
Version: 10.0
: P3 normal
Target Milestone: ---
Assignee: Jakub Jelinek
Depends on: 91602
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Reported: 2021-02-13 21:20 UTC by Jim Wilson
Modified: 2021-03-03 08:58 UTC (History)
2 users (show)

See Also:
Target: riscv*-*-*
Known to work:
Known to fail:
Last reconfirmed: 2021-02-26 00:00:00

gcc11-pr99090.patch (1.29 KB, patch)
2021-02-26 18:24 UTC, Jakub Jelinek
Details | Diff

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Description Jim Wilson 2021-02-13 21:20:42 UTC
Enabling -gsplit-dwarf by default and trying a build hits an assert in dw2_asm_output_delta_uleb128 because HAVE_AS_LEB128 is not defined.

The problem appears to be in output_loc_list in dwarf2out.c which has in the dwarf_split_debug_info code
              /* FIXME: This will ICE ifndef HAVE_AS_LEB128.                    
                 For that case we probably need to emit DW_LLE_startx_endx,     
                 but we'd need 2 .debug_addr entries rather than just one.  */

riscv doesn't allow leb128 because of agressive linker relaxation, so we need the alternative solution here that works without HAVE_AS_LEB128.
Comment 1 Jakub Jelinek 2021-02-26 11:59:57 UTC
Is there any reason why can't .{u,s}leb128 be supported in .debug* sections when referring to labels in .debug* sections (where I don't see how aggressive linker relaxation should apply to those sections, those sections heavily rely on no linker relaxation)?
So essentially have partial as leb128 support instead of none at all?
Comment 2 Jim Wilson 2021-02-26 16:34:09 UTC
Yes we could have partial uleb128 support.  There is only a problem if at least one label is in the code section.

There is another proposed solution to add special relaxable relocations for uleb128 but the initial proposal had flaws, and no one has reviewed the second proposal yet.

Or we could change the -gsplit-dwarf support to work even when there is no uleb128 support.
Comment 3 Jakub Jelinek 2021-02-26 17:50:22 UTC
Making it work is desirable in any case.  But, the emitted debug sections will be in various spots less compact when .uleb128/.sleb128 don't support non-constant operands.
Though, I'm afraid the
              /* FIXME: This will ICE ifndef HAVE_AS_LEB128.
                 For that case we probably need to emit DW_LLE_startx_endx,
                 but we'd need 2 .debug_addr entries rather than just one.  */
              dw2_asm_output_delta_uleb128 (curr->end, curr->begin,
                                            "Location list length (%s)",
case is actually code addresses and not .debug section labels, sorry for confusion.
Comment 4 Jakub Jelinek 2021-02-26 18:24:01 UTC
Created attachment 50265 [details]

Untested fix.
Comment 5 Jim Wilson 2021-02-27 02:35:56 UTC
I tested it with a riscv-gnu-toolchain build and check.  The 4 -gsplit-dwarf testcases that fail without the patch work with the patch.

I also tried a build and check with -gsplit-dwarf enabled by default and discovered that there are a number of debug tests that fail simply because the output is a little different than what is expected.  But nothing else appeared to fail.
Comment 6 CVS Commits 2021-03-03 08:54:28 UTC
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:


commit r11-7472-gb5040344b9ca609e19ee59ba56cd4af9697a1692
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed Mar 3 09:53:58 2021 +0100

    dwarf2out: Fix -gsplit-dwarf on riscv or other non-.uleb128 targets [PR99090]
    As mentioned in the PR, riscv* only supports .uleb128 with constant
    arguments, doesn't support difference of two labels because of aggressive
    linker relaxations.  But I bet various other targets, especially those not
    using GNU assembler, might suffer from the same problem.
    As the FIXME comment in output_loc_list indicates, we ICE on
    -gsplit-dwarf on those targets whenever we need .debug_loclists, because
    we only emit DW_LLE_startx_length which requires working .uleb128 delta
    of 2 code section labels.  We can't use DW_LLE_base_addressx
    once followed by DW_LLE_offset_pair either because the latter suffers
    from the same issue - need .uleb128 difference of code section labels
    (and in that case not just for the second operand but also for the first).
    So, this patch implements what the comment said and emits DW_LLE_startx_endx
    instead, which wastes more space in .debug_addr, but will work.
    Bootstrapped/regtested on x86_64-linux and i686-linux and as written in the
    PR, Jim has tested it on riscv*linux.  Ok for trunk?
    BTW, for HAVE_AS_LEB128 -gdwarf-5 -gsplit-dwarf, maybe we should consider
    instead of always emitting DW_LLE_startx_length do all the optimizations
    that we do for HAVE_AS_LEB128 -gdwarf-5, or at least a subset of them.
    For !have_multiple_function_sections, we in that case emit just
    DW_LLE_offset_pair (that can certainly be a win for small TUs, we wouldn't
    need any .debug_addr entry in that case; on the other side, just using
    DW_LLE_offset_pair can be harmful for very large TUs especially if the
    loclist has many entries, emitting in that case a single DW_LLE_base_address
    or for -gsplit-dwarf DW_LLE_base_addressx followed by DW_LLE_offset_pair
    might be much smaller), and for have_multiple_function_sections figuring
    out if DW_LLE_base_address followed by DW_LLE_offset_pair entries
    or DW_LLE_start_length is bettter.  So perhaps a middle-ground for
    -gsplit-dwarf would be to always do the have_multiple_function_sections
    behavior, i.e. DW_LLE_base_addressx followed by DW_LLE_offset_pair vs.
    DW_LLE_startx_length decisions based on the ranges and their counts.
    And perhaps dwz could optimize afterwards, on linked binaries or shared
    libraries it knows all the offsets and could figure out optimal DW_LLE_*.
    2021-03-03  Jakub Jelinek  <jakub@redhat.com>
            PR debug/99090
            * dwarf2out.c (dw_loc_list_struct): Add end_entry member.
            (new_loc_list): Clear end_entry.
            (output_loc_list): Only use DW_LLE_startx_length for -gsplit-dwarf
            if HAVE_AS_LEB128, otherwise use DW_LLE_startx_endx.  Fix comment
            (index_location_lists): For dwarf_version >= 5 without HAVE_AS_LEB128,
            initialize also end_entry.
Comment 7 Jakub Jelinek 2021-03-03 08:58:01 UTC
Fixed on the trunk so far.