[debug] Handle debug references to skipped params

When compiling guality/vla-1.c with -O3 -g, vla a in f1 is optimized away, but
f1 still contains a debug expression describing the upper bound of the vla
(D.1914):
...
     __attribute__((noinline))
     f1 (intD.6 iD.1900)
     {
       <bb 2>
       saved_stack.1_2 = __builtin_stack_save ();
       # DEBUG BEGIN_STMT
       # DEBUG D#3 => i_1(D) + 1
       # DEBUG D#2 => (long intD.8) D#3
       # DEBUG D#1 => D#2 + -1
       # DEBUG D.1914 => (sizetype) D#1
...

Then f1 is cloned to a version f1.constprop with no parameters, eliminating
parameter i, and 'DEBUG D#3 => i_1(D) + 1' turns into 'D#3 => NULL'.

This patch fixes that by defining debug expressions for default defs of
eliminated parameters in remap_ssa_name:
...
     __attribute__((noinline))
     f1.constprop ()
     {
       intD.6 iD.1949;

       <bb 3>
       # DEBUG D#8 s=> iD.1900
       # DEBUG iD.1949 => D#8

       <bb 2>
+      # DEBUG D#6 s=> iD.1949
       saved_stack.1_1 = __builtin_stack_save ();
       # DEBUG BEGIN_STMT
-      # DEBUG D#3 => NULL
+      # DEBUG D#3 => D#6 + 1
       # DEBUG D#2 => (long intD.8) D#3
       # DEBUG D#1 => D#2 + -1
       # DEBUG D.1951 => (sizetype) D#1
...

Bootstrapped and reg-tested on x86_64.

2018-07-09  Tom de Vries  <tdevries@suse.de>

	* cfgexpand.c (expand_debug_source_expr): Handle VAR_DECL.
	* tree-inline.c (remap_ssa_name): Handle default def ssa_name mapping
	onto VAR_DECL with abstract origin.

	* gcc.dg/vla-1.c: New test.

From-SVN: r262510

gcc/ChangeLog

+2018-07-09  Tom de Vries  <tdevries@suse.de>
+
+	* cfgexpand.c (expand_debug_source_expr): Handle VAR_DECL.
+	* tree-inline.c (remap_ssa_name): Handle default def ssa_name mapping
+	onto VAR_DECL with abstract origin.
+
 2018-07-07  Jim Wilson  <jimw@sifive.com>
 
 	* config/riscv/riscv.c (TARGET_CUSTOM_FUNCTION_DESCRIPTORS): New.

gcc/cfgexpand.c

@@ -5141,6 +5141,10 @@ expand_debug_source_expr (tree exp)
 
   switch (TREE_CODE (exp))
     {
+    case VAR_DECL:
+      if (DECL_ABSTRACT_ORIGIN (exp))
+	return expand_debug_source_expr (DECL_ABSTRACT_ORIGIN (exp));
+      break;
     case PARM_DECL:
       {
 	mode = DECL_MODE (exp);

gcc/testsuite/ChangeLog

+2018-07-09  Tom de Vries  <tdevries@suse.de>
+
+	* gcc.dg/vla-1.c: New test.
+
 2018-07-07  Tom de Vries  <tdevries@suse.de>
 
 	* gcc.misc-tests/options.exp (check_for_all_options): Clean up dump

gcc/testsuite/gcc.dg/vla-1.c

+/* { dg-do compile } */
+/* { dg-options "-g -O3 -fdump-tree-optimized -fvar-tracking-assignments -fno-selective-scheduling -fno-selective-scheduling2" } */
+
+int __attribute__((noinline))
+f1 (int i)
+{
+  char a[i + 1];
+  char b[i + 2];
+  b[1] = 3;
+  a[0] = 5;
+  return a[0] + b[1];
+}
+
+int
+main ()
+{
+  volatile int j;
+  int x = 5;
+  j = f1 (x);
+  return 0;
+}
+
+/* One debug source bind is generated for the parameter, and two to describe the
+   sizes of a and b.  */
+/* { dg-final { scan-tree-dump-times " s=> i" 3 "optimized" } } */

gcc/tree-inline.c

@@ -208,7 +208,8 @@ remap_ssa_name (tree name, copy_body_data *id)
 	  n = id->decl_map->get (val);
 	  if (n != NULL)
 	    val = *n;
-	  if (TREE_CODE (val) != PARM_DECL)
+	  if (TREE_CODE (val) != PARM_DECL
+	      && !(VAR_P (val) && DECL_ABSTRACT_ORIGIN (val)))
 	    {
 	      processing_debug_stmt = -1;
 	      return name;