[gcc r9-9397] dwarf2out: Fix up add_scalar_info not to create invalid DWARF

[gcc r9-9397] dwarf2out: Fix up add_scalar_info not to create invalid DWARF

[Jakub Jelinek]

https://gcc.gnu.org/g:27f94064f8c678f99aa145bdcab2047433819f51

commit r9-9397-g27f94064f8c678f99aa145bdcab2047433819f51
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Dec 3 00:29:46 2020 +0100

    dwarf2out: Fix up add_scalar_info not to create invalid DWARF
    
    As discussed in https://sourceware.org/bugzilla/show_bug.cgi?id=26987 ,
    for very large bounds (which don't fit into HOST_WIDE_INT) GCC emits invalid
    DWARF.
    In DWARF2, DW_AT_{lower,upper}_bound were constant reference class.
    In DWARF3 they are block constant reference and the
    Static and Dynamic Properties of Types
    chapter says:
    "For a block, the value is interpreted as a DWARF expression; evaluation of the expression
    yields the value of the attribute."
    In DWARF4/5 they are constant exprloc reference class.
    Now, for add_AT_wide we use DW_FORM_data16 (valid in constant class)
    when -gdwarf-5, but otherwise just use DW_FORM_block1, which is not constant
    class, but block.
    For DWARF3 this means emitting clearly invalid DWARF, because the
    DW_FORM_block1 should contain a DWARF expression, not random bytes
    containing the constant directly.
    For DWARF2/DWARF4/5 it could be considered a GNU extension, but a very badly
    designed one when it means something different in DWARF3.
    
    The following patch uses add_AT_wide only if we know we'll be using
    DW_FORM_data16, and otherwise wastes 2 extra bytes and emits in there
    DW_OP_implicit_value <size> before the constant.
    
    2020-12-03  Jakub Jelinek  <jakub@redhat.com>
    
            * dwarf2out.c (add_scalar_info): Only use add_AT_wide for 128-bit
            constants and only in dwarf-5 or later, where DW_FORM_data16 is
            available.  Otherwise use DW_FORM_block*/DW_FORM_exprloc with
            DW_OP_implicit_value to describe the constant.
    
    (cherry picked from commit a08add4b115800281669d24e445342bd782de0cb)

Diff:
---
 gcc/dwarf2out.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index d7c3e389e4b..a83ea5c54cd 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -20805,12 +20805,23 @@ add_scalar_info (dw_die_ref die, enum dwarf_attribute attr, tree value,
 	  else
 	    add_AT_int (die, attr, TREE_INT_CST_LOW (value));
 	}
-      else
+      else if (dwarf_version >= 5
+	       && TREE_INT_CST_LOW (TYPE_SIZE (TREE_TYPE (value))) == 128)
 	/* Otherwise represent the bound as an unsigned value with
 	   the precision of its type.  The precision and signedness
 	   of the type will be necessary to re-interpret it
 	   unambiguously.  */
 	add_AT_wide (die, attr, wi::to_wide (value));
+      else
+	{
+	  rtx v = immed_wide_int_const (wi::to_wide (value),
+					TYPE_MODE (TREE_TYPE (value)));
+	  dw_loc_descr_ref loc
+	    = loc_descriptor (v, TYPE_MODE (TREE_TYPE (value)),
+			      VAR_INIT_STATUS_INITIALIZED);
+	  if (loc)
+	    add_AT_loc (die, attr, loc);
+	}
       return;
     }