[patch] Improve debug info for small structures passed by reference

[Eric Botcazou]

Hi,

the subject is slightly misleading since it's actually about structures with 
integral modes which are passed by reference on some platforms, e.g. SPARC or 
PowerPC 32-bit.  There are 3 issues:

 1. At -O0, the parameters of this kind are not homed on the stack, which 
means that backtraces can display <optimized out> for them.

 2. Since the fix for PR debug/48163, the locations emitted at -O1 and above 
for them are totally bogus (there is a bogus additional dereference).

 3. The location lists are quickly wrong for them because they fail to take 
into account that the register parameter is clobbered.

The attached patch addresses 1. and 2. fully and 3. partially (for now).  It 
has been tested on x86-64 and PowerPC/Linux.  OK for the mainline?


2013-06-16  Eric Botcazou  <ebotca...@adacore.com>

        * function.c (assign_parm_setup_reg): For a parameter passed by pointer
        and which can live in a register, always retrieve the value on entry.
        * var-tracking.c (add_stores): Treat the copy on entry for a parameter
        passed by invisible reference specially.
        (emit_notes_in_bb) <MO_VAL_USE>: Emit notes before the instruction.
        (vt_add_function_parameter): Correctly deal with a parameter passed by
        invisible reference.


-- 
Eric Botcazou
Index: function.c
===================================================================
--- function.c	(revision 200122)
+++ function.c	(working copy)
@@ -3084,17 +3084,27 @@ assign_parm_setup_reg (struct assign_par
     emit_move_insn (parmreg, validated_mem);
 
   /* If we were passed a pointer but the actual value can safely live
-     in a register, put it in one.  */
-  if (data->passed_pointer
-      && TYPE_MODE (TREE_TYPE (parm)) != BLKmode
-      /* If by-reference argument was promoted, demote it.  */
-      && (TYPE_MODE (TREE_TYPE (parm)) != GET_MODE (DECL_RTL (parm))
-	  || use_register_for_decl (parm)))
+     in a register, retrieve it and use it directly.  */
+  if (data->passed_pointer && TYPE_MODE (TREE_TYPE (parm)) != BLKmode)
     {
       /* We can't use nominal_mode, because it will have been set to
 	 Pmode above.  We must use the actual mode of the parm.  */
-      parmreg = gen_reg_rtx (TYPE_MODE (TREE_TYPE (parm)));
-      mark_user_reg (parmreg);
+      if (use_register_for_decl (parm))
+	{
+	  parmreg = gen_reg_rtx (TYPE_MODE (TREE_TYPE (parm)));
+	  mark_user_reg (parmreg);
+	}
+      else
+	{
+	  int align = STACK_SLOT_ALIGNMENT (TREE_TYPE (parm),
+					    TYPE_MODE (TREE_TYPE (parm)),
+					    TYPE_ALIGN (TREE_TYPE (parm)));
+	  parmreg
+	    = assign_stack_local (TYPE_MODE (TREE_TYPE (parm)),
+				  GET_MODE_SIZE (TYPE_MODE (TREE_TYPE (parm))),
+				  align);
+	  set_mem_attributes (parmreg, parm, 1);
+	}
 
       if (GET_MODE (parmreg) != GET_MODE (DECL_RTL (parm)))
 	{
Index: var-tracking.c
===================================================================
--- var-tracking.c	(revision 200122)
+++ var-tracking.c	(working copy)
@@ -5836,7 +5836,24 @@ add_stores (rtx loc, const_rtx expr, voi
 	    {
 	      rtx xexpr = gen_rtx_SET (VOIDmode, loc, src);
 	      if (same_variable_part_p (src, REG_EXPR (loc), REG_OFFSET (loc)))
-		mo.type = MO_COPY;
+		{
+		  /* If this is an instruction copying (part of) a parameter
+		     passed by invisible reference to its register location,
+		     pretend it's a SET so that the initial memory location
+		     is discarded, as the parameter register can be reused
+		     for other purposes and we do not track locations based
+		     on generic registers.  */
+		  if (MEM_P (src)
+		      && REG_EXPR (loc)
+		      && TREE_CODE (REG_EXPR (loc)) == PARM_DECL
+		      && DECL_MODE (REG_EXPR (loc)) != BLKmode
+		      && MEM_P (DECL_INCOMING_RTL (REG_EXPR (loc)))
+		      && XEXP (DECL_INCOMING_RTL (REG_EXPR (loc)), 0)
+			 != arg_pointer_rtx)
+		    mo.type = MO_SET;
+		  else
+		    mo.type = MO_COPY;
+		}
 	      else
 		mo.type = MO_SET;
 	      mo.u.loc = xexpr;
@@ -9086,7 +9103,7 @@ emit_notes_in_bb (basic_block bb, datafl
 	      else
 		var_mem_set (set, loc, VAR_INIT_STATUS_UNINITIALIZED, NULL);
 
-	      emit_notes_for_changes (insn, EMIT_NOTE_AFTER_INSN, set->vars);
+	      emit_notes_for_changes (insn, EMIT_NOTE_BEFORE_INSN, set->vars);
 	    }
 	    break;
 
@@ -9533,12 +9550,11 @@ vt_add_function_parameter (tree parm)
 
   if (!vt_get_decl_and_offset (incoming, &decl, &offset))
     {
-      if (REG_P (incoming) || MEM_P (incoming))
+      if (MEM_P (incoming))
 	{
 	  /* This means argument is passed by invisible reference.  */
 	  offset = 0;
 	  decl = parm;
-	  incoming = gen_rtx_MEM (GET_MODE (decl_rtl), incoming);
 	}
       else
 	{