On 11/24/2017 02:53 PM, Jakub Jelinek wrote:
> Hi!
>
> On most targets, N_SLINE has addresses relative to the start of
> the function, which means -gstabs{,+} is completely broken with
> hot/cold partitioning (fails to assemble almost anything that
> has both partitions). This used to be bearable when it wasn't
> the default, but now that we hot/cold partition by default it means
> STABS is completely unusable.
>
> Because STABS should die soon, I'm not trying to propose any extension,
> just emit something that assemble and be tolerable for debugging purposes
> (after all, debugging optimized code with stabs isn't really a good idea
> because it doesn't handle block fragments anyway).
>
> What the patch does is that it treats hot/cold partitioned functions
> basically as two functions, say main and main.cold.1, in the hot partition
> everything should be normal, except that lexical scopes that only appear in
> cold partition are not emitted in the [lr]brac tree for the hot partition.
> Then the cold partition is yet another N_FUN, set of N_SLINE relative to
> the start of the cold partition, and finally another [lr]brac block tree.
> This one doesn't include any lexical scopes that are solely in the hot
> partition, but we can have scopes that are in both, those are duplicated,
> sometimes with merged vars from multiple scopes.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> Could anybody from the debugger folks test it a little bit (say gdb
> testsuite if it has any -O2 -gstabs/-O2 -gstabs+ tests)?
>
> 2017-11-24 Jakub Jelinek <ja...@redhat.com>
>
> PR debug/81307
> * dbxout.c (lastlineno): New variable.
> (dbx_debug_hooks): Use dbxout_switch_text_section as
> switch_text_section debug hook.
> (dbxout_function_end): Switch to current_function_section
> rather than function_section. If crtl->has_bb_partition,
> output just one N_FUN, depending on in_cold_section_p.
> (dbxout_source_line): Remember last lineno in lastlineno.
> (dbxout_switch_text_section): New function.
> (dbxout_function_decl): Adjust dbxout_block caller.
> (dbx_block_with_cold_children): New function.
> (dbxout_block): Return true if any LBRAC/RBRAC have been
> emitted. Use dbx_block_with_cold_children at depth == 0
> in second partition. Add PARENT_BLOCKNUM argument, pass
> it optionally adjusted to children. Output LBRAC/RBRAC
> around recursive call only if the block is in the current
> partition, if not and anything was output, emit empty
> range LBRAC/RBRAC.
> * final.c (final_scan_insn): Compute cold_function_name
> before calling switch_text_section debug hook. Call
> that hook even if dwarf2out_do_frame if not emitting
> dwarf debug info.
Alternately, just issue a warning that -gstabs isn't supported when
hot/cold partitioning is enabled. I'm just not sure it's worth the
headache to bother making this even limp along.
No objection if you want to go ahead with your patch, you've already
done the work, but fixing bugs in stabs support, ewwww.
Jeff
