[PATCH][PR84213] DWARF: no location for non-definition DECLs with non-trivial DECL_VALUE_EXPR

[Pierre-Marie de Rodat <derodat at adacore dot com>]

This patch restricts the set of cases in which we allow the generation 
of location attributes for variables that are not defined in the current 
unit. For such variables with complex DECL_VALUE_EXPR trees, generating 
a location attribute can end up creating relocations to text symbols in 
the debug section of LTO object files, which is not valid.

Richard originally suggested to revert r248792 and then enhance 
tree_add_const_value_attribute_for_decl to generate a DW_AT_location 
attribute in the specific case of a DECL_VALUE_EXPR that is an 
INDIRECT_REF(NOP_EXPR(INTEGER_CST)). I ended up with this patch because 
changing a function called "tree_add_const_value_attribute*" to generate 
instead a location attribute whereas there already exists a function 
called "add_location_or_const_value_attribute" felt really wrong. ;-) 
Especially if this would either re-implement part of the latter function 
in the former one.

What I did instead was to make dwarf2out_late_global_decl call 
add_location_or_const_value_attribute only when we know it's safe to do 
so (i.e. when it won't generate relocations to text symbols). I have the 
 feeling that it is cleaner and from what I could see, it fixes the 
reported issue with no regression.

Bootstrapped and regtested on x86_64-linux. Ok to commit to trunk?

gcc/
	PR lto/84213
	* dwarf2out.c (is_trivial_indirect_ref): New function.
	(dwarf2out_late_global_decl): Do not generate a location attribute for
	variables that have a non-trivial DECL_VALUE_EXPR and that are not
	defined in the current unit.
---
 gcc/dwarf2out.c | 31 +++++++++++++++++++++++++++----
 1 file changed, 27 insertions(+), 4 deletions(-)

diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index 948b3cb..6538596 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -25716,6 +25716,23 @@ dwarf2out_early_global_decl (tree decl)
   symtab->global_info_ready = save;
 }
 +/* Return whether EXPR is an expression with the following pattern:
+   INDIRECT_REF (NOP_EXPR (INTEGER_CST)).  */
+
+static bool
+is_trivial_indirect_ref (tree expr)
+{
+  if (expr == NULL_TREE || TREE_CODE (expr) != INDIRECT_REF)
+    return false;
+
+  tree nop = TREE_OPERAND (expr, 0);
+  if (nop == NULL_TREE || TREE_CODE (nop) != NOP_EXPR)
+    return false;
+
+  tree int_cst = TREE_OPERAND (nop, 0);
+  return int_cst != NULL_TREE && TREE_CODE (int_cst) == INTEGER_CST;
+}
+
 /* Output debug information for global decl DECL.  Called from
    toplev.c after compilation proper has finished.  */
 @@ -25740,11 +25757,17 @@ dwarf2out_late_global_decl (tree decl)
       if (die)
 	{
 	  /* We get called via the symtab code invoking late_global_decl
-	     for symbols that are optimized out.  Do not add locations
-	     for those, except if they have a DECL_VALUE_EXPR, in which case
-	     they are relevant for debuggers.  */
+	     for symbols that are optimized out.
+
+	     Do not add locations for those, except if they have a
+	     DECL_VALUE_EXPR, in which case they are relevant for debuggers.
+	     Still don't add a location if the DECL_VALUE_EXPR is not a trivial
+	     INDIRECT_REF expression, as this could generate relocations to
+	     text symbols in LTO object files, which is invalid.  */
 	  varpool_node *node = varpool_node::get (decl);
-	  if ((! node || ! node->definition) && ! DECL_HAS_VALUE_EXPR_P (decl))
+	  if ((! node || ! node->definition)
+	      && ! (DECL_HAS_VALUE_EXPR_P (decl)
+		    && is_trivial_indirect_ref (DECL_VALUE_EXPR (decl))))
 	    tree_add_const_value_attribute_for_decl (die, decl);
 	  else
 	    add_location_or_const_value_attribute (die, decl, false);
--
2.1.4