[Bug debug/66668] [6 regression] FAIL: gcc.dg/debug/dwarf2/stacked-qualified-types-3.c scan-assembler-times DIE \\([^\n]*\\) DW_TAG_(?:const|volatile|atomic|restrict)_type 8

[vogt at linux dot vnet.ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66668

--- Comment #5 from Dominik Vogt <vogt at linux dot vnet.ibm.com> ---
The linked page from comment #2 mentions a discussion about this issue which
I'm unable to find.  Any hints where it is?

Anyway, we've looked into it, and it seems that this bit of code generated a
duplicate DIE:

  char a; 
  char * _Atomic restrict h; 
  char * _Atomic i; 

When handling "h", the algorithm in
dwarf2out.c:get_nearest_type_subqualifiers() finds "char *" as the nearest
existing type.  Then the for-loop in modified_type_die (line 11254) generates
an "intermediate" DIE for "char * _Atomic", and the required DIE for "char *
_Atomic restrict".  Eventually, only the latter one is linked to it's type with
"equate_type_number_to_die()", but the intermediate type is not.

When the code handles "i", and tries to lookup an existing DIE for "char *
_Atomic" (line 11199) it does not find the "intermediate" DIE and generates
another one.  This is the additional "DW_TAG_atomic" that makes the test case
fail.

Without looking at the patches that cause this, I believe that with the "early
debug" stuff the DIEs are now generated before the list of used types is
available to the loop in get_nearest_type_subqualifiers:

  for (t = TYPE_MAIN_VARIANT (type); t && best_rank < max_rank; 
     t = TYPE_NEXT_VARIANT (t))

It's probably just called earlier now.  So, the algorithm to eliminate
duplicate type DIEs does not work because it requires a complete and correct
list of used types.