So I was experimenting and doing sth like

diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 4a35e389e9..72d7962678 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -21328,7 +21328,11 @@ new_symbol (struct die_info *die, struct type *type, struct dwarf2_cu *cu,
       OBJSTAT (objfile, n_syms++);
 
       /* Cache this symbol's name and the name's demangled form (if any).  */
-      SYMBOL_SET_LANGUAGE (sym, cu->language, &objfile->objfile_obstack);
+      dwarf2_cu *lang_cu = cu;
+      attribute *spec = dwarf2_attr (die, DW_AT_abstract_origin, cu);
+      if (spec)
+       follow_die_ref (die, spec, &lang_cu);
+      SYMBOL_SET_LANGUAGE (sym, lang_cu->language, &objfile->objfile_obstack);
       linkagename = dwarf2_physname (name, die, cu);
       SYMBOL_SET_NAMES (sym, linkagename, strlen (linkagename), 0, objfile);
 
gets us further, now running into

#1  0x000000000055d2db in insert_symbol_hashed (dict=0x142a0e0, sym=0x142a020)
    at /space/rguenther/src/binutils-gdb/gdb/dictionary.c:690
690       gdb_assert (SYMBOL_LANGUAGE (sym) == DICT_LANGUAGE (dict)->la_language);
(top-gdb) l
685       unsigned int hash;
686       struct symbol **buckets = DICT_HASHED_BUCKETS (dict);
687
688       /* We don't want to insert a symbol into a dictionary of a different
689          language.  The two may not use the same hashing algorithm.  */
690       gdb_assert (SYMBOL_LANGUAGE (sym) == DICT_LANGUAGE (dict)->la_language);
691

for which there is a duplicate bugreport I think.  The issue here seems to be
that we are doing

#3  0x00000000004b6eaa in buildsym_compunit::finish_block_internal (
    this=0x13bc890, symbol=0x0, listhead=0x13bca10, old_blocks=0x0, 
    static_link=0x0, start=0x400bf0, end=0x4012cc, is_global=0, expandable=0)
    at /space/rguenther/src/binutils-gdb/gdb/buildsym.c:245
245                                     m_language, *listhead);
(top-gdb) l
240             }
241           else
242             {
243               BLOCK_DICT (block) =
244                 dict_create_hashed (&m_objfile->objfile_obstack,
245                                     m_language, *listhead);
246             }

with m_language == language_cplus but all symbols are language_c.

diff --git a/gdb/buildsym.c b/gdb/buildsym.c
index a9d2698584..ea0c9a537e 100644
--- a/gdb/buildsym.c
+++ b/gdb/buildsym.c
@@ -225,24 +225,27 @@ buildsym_compunit::finish_block_internal
           ? allocate_global_block (&m_objfile->objfile_obstack)
           : allocate_block (&m_objfile->objfile_obstack));
 
+  language lang = m_language;
+  if (*listhead && (*listhead)->nsyms != 0)
+    lang = (*listhead)->symbol[0]->ginfo.language;
   if (symbol)
     {
       BLOCK_DICT (block)
        = dict_create_linear (&m_objfile->objfile_obstack,
-                             m_language, *listhead);
+                             lang, *listhead);
     }
   else
     {
       if (expandable)
        {
-         BLOCK_DICT (block) = dict_create_hashed_expandable (m_language);
+         BLOCK_DICT (block) = dict_create_hashed_expandable (lang);
          dict_add_pending (BLOCK_DICT (block), *listhead);
        }
       else
        {
          BLOCK_DICT (block) =
            dict_create_hashed (&m_objfile->objfile_obstack,
-                               m_language, *listhead);
+                               lang, *listhead);
        }
     }

"cures" that, just getting us back to the first assert ;)

This time with

(top-gdb) p symbol->ginfo
$1 = {name = 0x13cdfa0 "main(int, char**)", value = {ivalue = 0, block = 0x0, 
    bytes = 0x0, address = 0x0, common_block = 0x0, chain = 0x0}, 
  language_specific = {obstack = 0x0, demangled_name = 0x0}, 
  language = language_cplus, ada_mangled = 0, section = -1}
(top-gdb) p (*listhead)->symbol[0]->ginfo
$2 = {name = 0x13cdc80 "md5_process_block(void const*, size_t, md5_ctx*)", 
  value = {ivalue = 21143856, block = 0x142a130, bytes = 0x142a130 "\220\v@", 
    address = 0x142a130, common_block = 0x142a130, chain = 0x142a130}, 
  language_specific = {obstack = 0x0, demangled_name = 0x0}, 
  language = language_c, ada_mangled = 0, section = -1}

where the languages are correct.

I suppose gdb isn't really up to handling uses of different "language"
symbols in a single CU :/

What's the issue with that anyways?  Why not simply _have_ different
language symbols in the symbol table?

For non-inline instances a solution on the GCC side might be to create separate CUs matching the abstract instances CU but that wouldn't work for inline instances nor for fixing 23713.

Eventually the assert()s can be weakened to allow "compatible enough" (same hash, etc.) languages?  I realize that allowing LTO-like mixed language environments looks like a very much larger change in gdb than I thought.

Though it looks like C and C++ use different symbol hashing...  in fact all but C++ seem to use default_search_name_hash ...

And what's the issue here?  Just store the hash alongside the entries and then
the "only" issue is that gdb doesn't properly switch languages automatically
(PR23713) so user name lookups might fail spuriously.

(In reply to Richard Biener from comment #4)
> Eventually the assert()s can be weakened to allow "compatible enough" (same
> hash, etc.) languages?  I realize that allowing LTO-like mixed language
> environments looks like a very much larger change in gdb than I thought.
> 
> Though it looks like C and C++ use different symbol hashing...  in fact all
> but C++ seem to use default_search_name_hash ...
> 
> And what's the issue here?  Just store the hash alongside the entries and
> then
> the "only" issue is that gdb doesn't properly switch languages automatically
> (PR23713) so user name lookups might fail spuriously.

I just wanted to let you know that I am looking at fixing this once and
for all, but I am on vacation until next week. I have the (good) beginnings
of a patch for this.

Alas, there is a recent patch on HEAD that is causing problems for me
and will require further investigation.

(In reply to Keith Seitz from comment #5)
> (In reply to Richard Biener from comment #4)
> > Eventually the assert()s can be weakened to allow "compatible enough" (same
> > hash, etc.) languages?  I realize that allowing LTO-like mixed language
> > environments looks like a very much larger change in gdb than I thought.
> > 
> > Though it looks like C and C++ use different symbol hashing...  in fact all
> > but C++ seem to use default_search_name_hash ...
> > 
> > And what's the issue here?  Just store the hash alongside the entries and
> > then
> > the "only" issue is that gdb doesn't properly switch languages automatically
> > (PR23713) so user name lookups might fail spuriously.
> 
> I just wanted to let you know that I am looking at fixing this once and
> for all, but I am on vacation until next week. I have the (good) beginnings
> of a patch for this.

That's great news!

Mark this one as needed for 8.2.1 (see discussion on gdb-patches: https://www.sourceware.org/ml/gdb-patches/2018-11/msg00096.html).

Btw, if GCC can do anything better here please speak up.  Currently the CUs generated at link-time (possibly combining different source language CUs) get
sth like

  Compilation Unit @ offset 0x138:
   Length:        0x8e (32-bit)
   Version:       4
   Abbrev Offset: 0xd2
   Pointer Size:  8
 <0><143>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <144>   DW_AT_producer    : (indirect string, offset: 0x22e): GNU GIMPLE 9.0.0 20181108 (experimental) -mtune=generic -march=x86-64 -mtune=generic -march=x86-64 -g -fno-openmp -fno-openacc -fno-pie -fltrans
    <148>   DW_AT_language    : 12      (ANSI C99)
    <149>   DW_AT_name        : (indirect string, offset: 0x221): <artificial>
    <14d>   DW_AT_comp_dir    : (indirect string, offset: 0x1ac): /abuild/rguenther/obj-slpcost-g/gcc
    <151>   DW_AT_low_pc      : 0x4004b2
    <159>   DW_AT_high_pc     : 0x3c
    <161>   DW_AT_stmt_list   : 0x10c

where DW_AT_language is either the single common language of the source CUs
contributing to this, the "newest" C/C++ when mixing C and C++ (where
C++ is "newer" than C), or as fallback DW_LANG_C (for example when combining
C and fortran).  The standard says a CU entry _may_ have a DW_AT_language
attribute - would gdb be more happy when we do not emit any DW_AT_language
attribute for link-time generated DIEs?  You can see we set a producer
of GNU GIMPLE which gdb may use to trigger special behavior (OTOH it
could do that whenever seeing a unit import with different DW_AT_language...)

Re-confirmed on master.  Any progress here?  It's quite a blocker for LTO adoption to us.

Hi Richard. I'm not personally involved in this ticket, but I know there is movement, and it's not as trivial as we would have hoped. There is a 5-patches series being discussed at the moment. Here are some links:

last month -> https://www.sourceware.org/ml/gdb-patches/2018-11/msg00138.html
this month -> https://www.sourceware.org/ml/gdb-patches/2018-12/msg00187.html

On Wed, 19 Dec 2018, brobecker at gnat dot com wrote:

> https://sourceware.org/bugzilla/show_bug.cgi?id=23712
> 
> --- Comment #10 from Joel Brobecker <brobecker at gnat dot com> ---
> Hi Richard. I'm not personally involved in this ticket, but I know there is
> movement, and it's not as trivial as we would have hoped.

I figured that when trying to poke myself...

> There is a 5-patches
> series being discussed at the moment. Here are some links:
> 
> last month -> https://www.sourceware.org/ml/gdb-patches/2018-11/msg00138.html
> this month -> https://www.sourceware.org/ml/gdb-patches/2018-12/msg00187.html

That's good news - I hope it will make the next release!

Too late now, for 8.2.1. Hoping we will be able to fix it for 8.3.

The master branch has been updated by Keith Seitz <kseitz@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=c7748ee9ceb5a394658cd07aeb0445924599e442

commit c7748ee9ceb5a394658cd07aeb0445924599e442
Author: Keith Seitz <keiths@redhat.com>
Date:   Thu Jan 10 13:57:08 2019 -0800

    gdb/23712: Introduce multidictionary's
    
    gdb/23712 is a new manifestation of the now-infamous (at least to me)
    symtab/23010 assertion failure (DICT_LANGUAGE == SYMBOL_LANGAUGE).
    
    An example of the problem (using test case from symtab/23010):
    
    Reading symbols from /home/rdiez/rdiez/arduino/JtagDue/BuildOutput/JtagDue-obj-release/firmware.elf...done.
    (gdb) p SysTick_Handler
    dwarf2read.c:9715: internal-error: void dw2_add_symbol_to_list(symbol*, pending**): Assertion `(*listhead) == NULL || (SYMBOL_LANGUAGE ((*listhead)->symbol[0]) == SYMBOL_LANGUAGE (symbol))' failed.
    A problem internal to GDB has been detected,
    further debugging may prove unreliable.
    Quit this debugging session? (y or n)
    
    This assertion was added specifically to catch this condition (of adding
    symbols of different languages to a single pending list).
    
    The problems we're now seeing on systems utilizing DWARF debugging seem to
    be caused by the use of LTO, which adds a CU with an artificial DIE of
    language C99 which references DIEs in other CUs of language C++.
    
    Thus, we create a dictionary containing symbols of C99 but end up
    stuffing C++ symbols into it, and the dw2_add_symbol_to_list triggers.
    
    The approach taken here to fix this is to introduce multi-language
    dictionaries to "replace" the standard, single-language dictionaries
    used today.
    
    Note to reviewers: This patch introduces some temporary functions to
    aide with review.  This and other artifacts (such as "See dictionary.h"
    which appear incorrect) will all be valid at the end of the series.
    
    This first patch introduces the new multidictionary and its API (which
    is, by design, identical to the old dictionary interface).  It also
    mutates dict_create_hashed and dict_create_linear so that they take
    a std::vector instead of the usual struct pending linked list.  This will
    be needed later on.
    
    This patch does /not/ actually enable multidictionary's.  That is left
    for a subsequent patch in the series.
    
    I've done exhaustive performance testing with this approach, and I've
    attempted to minimize the overhead for the (overwhelmingly) most common
    one-language scenario.
    
    On average, a -g3 -O0 GDB (the one we developers use) will see
    approximately a 4% slowdown when initially reading symbols. [I've
    tested only GDB and firefox with -readnow.]  When using -O2, this
    difference shrinks to ~0.5%.  Since a number of runs with these
    patches actually run /faster/ than unpatched GDB, I conclude that
    these tests have at least a 0.5% error margin.
    
    On our own gdb.perf test suite, again, results appear to be pretty
    negligible.  Differences to unpatched GDB range from -7.8% (yes,
    patched version is again faster than unpatched) to 27%.  All tests
    lying outside "negligible," such as the 27% slowdown, involve a total
    run time of 0.0007 (or less) with smaller numbers of CUs/DSOs (usually 10
    or 100).  In all cases, the follow-up tests with more CUs/DSOs is never
    more than 3% difference to the baseline, unpatched GDB.
    
    In my opinion, these results are satisfactory.
    
    gdb/ChangeLog:
    
    	PR gdb/23712
    	PR symtab/23010
    	* dictionary.c: Include unordered_map.
    	(pending_to_vector): New function.
    	(dict_create_hashed_1, dict_create_linear_1, dict_add_pending_1):
    	Rewrite the non-"_1" functions to take vector instead
    	of linked list.
    	(dict_create_hashed, dict_create_linear, dict_add_pending): Use the
    	"new" _1 versions of the same name.
    	(multidictionary): Define.
    	(std::hash<enum language): New definition.
    	(collate_pending_symbols_by_language, mdict_create_hashed)
    	(mdict_create_hashed_expandable, mdict_create_linear)
    	(mdict_create_linear_expandable, mdict_free)
    	(find_language_dictionary, create_new_language_dictionary)
    	(mdict_add_symbol, mdict_add_pending, mdict_iterator_first)
    	(mdict_iterator_next, mdict_iter_match_first, mdict_iter_match_next)
    	(mdict_size, mdict_empty): New functions.
    	* dictionary.h (mdict_iterator): Define.

The master branch has been updated by Keith Seitz <kseitz@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=b026f59345a336cabf74719fce9f96cab7c7ab4d

commit b026f59345a336cabf74719fce9f96cab7c7ab4d
Author: Keith Seitz <keiths@redhat.com>
Date:   Thu Jan 10 13:57:08 2019 -0800

    gdb/23712: Use new multidictionary API
    
    This patch builds on the previous by enabling the `new' multidictionary
    API.  A lot of the hunks are simply textual replacements of "dict_"
    with "mdict_" and similar transformations.
    
    A word of warning, even with the use of multidictionaries, the code
    still does not satisfactorily fix the reported problems with gdb/23712
    (or gdb/23010). We still have additional changes to make before that
    happens.
    
    gdb/ChangeLog:
    
    	PR gdb/23712
    	PR symtab/23010
    	* dictionary.h (struct dictionary): Replace declaration with
    	multidictionary.
    	(dict_create_hashed, dict_create_hashed_expandable)
    	(dict_create_linear, dict_create_linear_expandable)
    	(dict_free, dict_add_symbol, dict_add_pending, dict_empty)
    	(dict_iterator_first, dict_iterator_next, dict_iter_match_first)
    	(dict_iter_match_next, dict_size): Rename to "mdict_" versions
    	taking multidictionary argument.
    	[ALL_DICT_SYMBOLS]: Update for multidictionary.
    	* block.h (struct block) <dict>: Change to multidictionary
    	and rename `multidict'.
    	* block.c, buildsym.c, jit.c, mdebugread.c, objfiles.c,
    	symmisc.c: Update all dictionary references to multidictionary.

The master branch has been updated by Keith Seitz <kseitz@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=63a20375b401e24c30987367a10b47b289612e1c

commit 63a20375b401e24c30987367a10b47b289612e1c
Author: Keith Seitz <keiths@redhat.com>
Date:   Thu Jan 10 13:57:08 2019 -0800

    gdb/23712: Cleanup/Remove temporary dictionary functions
    
    Now that multidictionary's are being used, there is no longer any need
    to retain the four temporary functions introduced in the beginning of
    this series.
    
    This patch removes them.
    
    As an additional cleanup, since the single-language dictionaries are
    no longer used outside dictionary.c, make all of those functions
    static.
    
    gdb/ChangeLog:
    
    	PR gdb/23712
    	PR symtab/23010
    	* dictionary.c (pending_to_vector): Remove.
    	(dict_create_hashed_1, dict_create_linear_1, dict_add_pending_1):
    	Remove _1 suffix, replacing functions of the same name.  Update
    	all callers.
    	(dict_create_hashed, dict_create_hashed_expandable)
    	(dict_create_linear, dict_create_linear_expandable, dict_free)
    	(dict_add_symbol, dict_add_pending, dict_size, dict_empty):
    	Make functions static.

The master branch has been updated by Keith Seitz <kseitz@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=d3cb68081112a4976979df3f8eae7ca926e76519

commit d3cb68081112a4976979df3f8eae7ca926e76519
Author: Keith Seitz <keiths@redhat.com>
Date:   Thu Jan 10 13:57:08 2019 -0800

    gdb/23712: Remove dw2_add_symbol_to_list
    
    Finally, we can remove dw2_add_symbol_to_list since the wrapper function
    originally introduced to catch this multi-language scenario is no longer
    needed.  With multi-language dictionaries, we can now support adding
    symbols of multiple languages, negating the need for the assertion
    entirely.
    
    This patch should now fix gdb/23712 (and symtab/23010).  At least it will
    if the NULL buildsym_compunit problem doesn't strike first (see gdb/23773).
    
    gdb/ChangeLog:
    
    	PR gdb/23712
    	PR symtab/23010
    	* dwarf2read.c (dw2_add_symbol_to_list): Remove.
    	(fixup_go_packaging, new_symbol): Use add_symbol_to_list.

The master branch has been updated by Keith Seitz <kseitz@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=b56f80d8b27dffd0f8c02b8b11068b71b9fec375

commit b56f80d8b27dffd0f8c02b8b11068b71b9fec375
Author: Keith Seitz <keiths@redhat.com>
Date:   Thu Jan 10 13:57:09 2019 -0800

    gdb/23712: Test case for multidictionary
    
    This is a test derived from one of the reproducers in symtab/23010.
    The DIE tree used here is typical of compilations with LTO, where an
    artificial parent DIE of language C99 imports DIEs of other languages.
    
    gdb/testsuite/ChangeLog:
    
    	PR gdb/23712
    	PR symtab/23010
    	* gdb.dwarf2/multidictionary.exp: New file.

debugging cc1 now seems to work without crashing gdb.  debugging experience is
quite bad though - gdb doesn't seem to be able to do breakpoints on inlined
instances.

Btw, how can I debug

(gdb) b choose_multiplier
Breakpoint 2 at 0x143f8d0: file /space/rguenther/src/svn/trunk2/gcc/hwint.h, line 245.

which shows bogus source line info - it's of course in libiberties hashtab.c,
hwint.h at this place has

static inline int
ceil_log2 (unsigned HOST_WIDE_INT x)
{
  return x == 0 ? 0 : floor_log2 (x - 1) + 1;

full dwarf is of course big...

But this bug can be closed as from my side unless you are aware of missing pieces of the fix.

(In reply to Richard Biener from comment #18)
> Btw, how can I debug
> 
> (gdb) b choose_multiplier
> Breakpoint 2 at 0x143f8d0: file /space/rguenther/src/svn/trunk2/gcc/hwint.h,
> line 245.

My Jan 16-dated build (from master) behaves differently:

$ ./gdb -nx -q gcc/cc1 -ex "b choose_multiplier" -ex "i b"
Reading symbols from gcc/cc1...
Breakpoint 1 at 0xb2f94f: file ../../gcc/gcc/expmed.c, line 3649.
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x0000000000b2f94f in choose_multiplier(unsigned long, int, int, unsigned long*, int*, int*) at ../../gcc/gcc/expmed.c:3649
(gdb) list choose_multiplier
3639	
3640	unsigned HOST_WIDE_INT
3641	choose_multiplier (unsigned HOST_WIDE_INT d, int n, int precision,
3642			   unsigned HOST_WIDE_INT *multiplier_ptr,
3643			   int *post_shift_ptr, int *lgup_ptr)
3644	{
3645	  int lgup, post_shift;
3646	  int pow, pow2;
3647	
3648	  /* lgup = ceil(log2(divisor)); */
(gdb) info func choose_multiplier
All functions matching regular expression "choose_multiplier":

File ../../gcc/gcc/expmed.c:
3641:	unsigned long choose_multiplier(unsigned long, int, int, unsigned long*, int*, int*);
(gdb) 

In my sources, choose_multiplier in hashtab.c is wrapped with #if 0...#endif.

I'm closing this bug as resolved/fixed.

If you continue to have usage problems debugging cc1, please open a new bug so that we don't lose track of it.

*** Bug 24145 has been marked as a duplicate of this bug. ***