avoid early reference to debug-only symbol

Alexandre Oliva oliva@adacore.com
Tue Jul 13 03:12:50 GMT 2021


If some IPA pass replaces the only reference to a constant non-public
non-automatic variable with its initializer, namely the address of
another such variable, the former becomes unreferenced and it's
discarded by symbol_table::remove_unreachable_nodes.  It calls
debug_hooks->late_global_decl while at that, and this expands the
initializer, which assigs RTL to the latter variable and forces it to
be retained by remove_unreferenced_decls, and eventually be output
despite not being referenced.  Without debug information, it's not
output.

This has caused a bootstrap-debug compare failure in
libdecnumber/decContext.o, while developing a transformation that ends
up enabling the above substitution in constprop.

This patch makes reference_to_unused slightly more conservative about
such variables at the end of IPA passes, falling back onto
expand_debug_expr for expressions referencing symbols that might or
might not be output, avoiding the loss of debug information when the
symbol is output, while avoiding a symbol output only because of debug
info.

Regstrapped on x86_64-linux-gnu.  Ok to install?


for  gcc/ChangeLog

	* dwarf2out.c (add_const_value_attribute): Return false if
	resolve_one_addr fails.
	(reference_to_unused): Don't assume local symbol presence
	while it can still be optimized out.
	(rtl_for_decl_init): Fallback to expand_debug_expr.
	* cfgexpand.c (expand_debug_expr): Export.
	* expr.h (expand_debug_expr): Declare.
---
 gcc/cfgexpand.c |   12 +++++++++---
 gcc/dwarf2out.c |   15 +++++++++++++--
 gcc/expr.h      |    2 ++
 3 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/gcc/cfgexpand.c b/gcc/cfgexpand.c
index 3edd53c37dcb3..b731a5598230c 100644
--- a/gcc/cfgexpand.c
+++ b/gcc/cfgexpand.c
@@ -91,8 +91,6 @@ struct ssaexpand SA;
    of comminucating the profile info to the builtin expanders.  */
 gimple *currently_expanding_gimple_stmt;
 
-static rtx expand_debug_expr (tree);
-
 static bool defer_stack_allocation (tree, bool);
 
 static void record_alignment_for_reg_var (unsigned int);
@@ -4413,7 +4411,7 @@ expand_debug_parm_decl (tree decl)
 
 /* Return an RTX equivalent to the value of the tree expression EXP.  */
 
-static rtx
+rtx
 expand_debug_expr (tree exp)
 {
   rtx op0 = NULL_RTX, op1 = NULL_RTX, op2 = NULL_RTX;
@@ -5285,6 +5283,14 @@ expand_debug_expr (tree exp)
       else
 	goto flag_unsupported;
 
+    case COMPOUND_LITERAL_EXPR:
+      exp = COMPOUND_LITERAL_EXPR_DECL_EXPR (exp);
+      /* DECL_EXPR is a tcc_statement, which expand_debug_expr does
+	 not expect, so instead of recursing we take care of it right
+	 away.  */
+      exp = DECL_EXPR_DECL (exp);
+      return expand_debug_expr (exp);
+
     case CALL_EXPR:
       /* ??? Maybe handle some builtins?  */
       return NULL;
diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index 82783c4968b85..bb7e2b8dc4e2c 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -20170,7 +20170,8 @@ add_const_value_attribute (dw_die_ref die, rtx rtl)
       if (dwarf_version >= 4 || !dwarf_strict)
 	{
 	  dw_loc_descr_ref loc_result;
-	  resolve_one_addr (&rtl);
+	  if (!resolve_one_addr (&rtl))
+	    return false;
 	rtl_addr:
           loc_result = new_addr_loc_descr (rtl, dtprel_false);
 	  add_loc_descr (&loc_result, new_loc_descr (DW_OP_stack_value, 0, 0));
@@ -20255,6 +20256,12 @@ reference_to_unused (tree * tp, int * walk_subtrees,
       varpool_node *node = varpool_node::get (*tp);
       if (!node || !node->definition)
 	return *tp;
+      /* If it's local, it might still be optimized out, unless we've
+	 already committed to outputting it by assigning RTL to it.  */
+      if (! TREE_PUBLIC (*tp) && ! TREE_ASM_WRITTEN (*tp)
+	  && symtab->state <= IPA_SSA_AFTER_INLINING
+	  && ! DECL_RTL_SET_P (*tp))
+	return *tp;
     }
   else if (TREE_CODE (*tp) == FUNCTION_DECL
 	   && (!DECL_EXTERNAL (*tp) || DECL_DECLARED_INLINE_P (*tp)))
@@ -20279,6 +20286,7 @@ static rtx
 rtl_for_decl_init (tree init, tree type)
 {
   rtx rtl = NULL_RTX;
+  bool valid_p = false;
 
   STRIP_NOPS (init);
 
@@ -20322,7 +20330,7 @@ rtl_for_decl_init (tree init, tree type)
   /* If the initializer is something that we know will expand into an
      immediate RTL constant, expand it now.  We must be careful not to
      reference variables which won't be output.  */
-  else if (initializer_constant_valid_p (init, type)
+  else if ((valid_p = initializer_constant_valid_p (init, type))
 	   && ! walk_tree (&init, reference_to_unused, NULL, NULL))
     {
       /* Convert vector CONSTRUCTOR initializers to VECTOR_CST if
@@ -20367,6 +20375,9 @@ rtl_for_decl_init (tree init, tree type)
       /* If expand_expr returns a MEM, it wasn't immediate.  */
       gcc_assert (!rtl || !MEM_P (rtl));
     }
+  else if (valid_p)
+    /* Perhaps we could just use this and skip all of the above?  */
+    rtl = expand_debug_expr (init);
 
   return rtl;
 }
diff --git a/gcc/expr.h b/gcc/expr.h
index a4f44265759ce..7b060462020be 100644
--- a/gcc/expr.h
+++ b/gcc/expr.h
@@ -307,6 +307,8 @@ expand_normal (tree exp)
   return expand_expr_real (exp, NULL_RTX, VOIDmode, EXPAND_NORMAL, NULL, false);
 }
 
+/* This one is defined in cfgexpand.c.  */
+extern rtx expand_debug_expr (tree);
 
 /* Return STRING_CST and set offset, size and decl, if the first
    argument corresponds to a string constant.  */

-- 
Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
   Free Software Activist                       GNU Toolchain Engineer
Disinformation flourishes because many people care deeply about injustice
but very few check the facts.  Ask me about <https://stallmansupport.org>


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