[Bug debug/47471] [4.6/4.7 Regression] stdarg functions extraneous too-early prologue end

["dodji at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47471

--- Comment #2 from Dodji Seketeli <dodji at gcc dot gnu.org> 2011-03-29 15:24:07 UTC ---
I believe the issue is that for that source code, GCC emits two
identical .loc asm directives for line 3.  In theory I don't thing
doing that would be wrong.

But in practise the second .loc directive triggers a special opcode in
the generated line program that increments the line with an increment
of zero; it also increments the current program address with a
increment of zero.  And that happens before the first line that
doesn't belong to the prologue.

That seems to break GDB's heuristic, and is also a bit of bloat.

The patch below avoids emitting two identical consecutive line number
debug information and seems to fix the issue for me.  I am currently
regression-testing it.

>From eb1450a263a4e50b43132ef9b914f49a971c8e9d Mon Sep 17 00:00:00 2001
From: Dodji Seketeli <dodji@redhat.com>
Date: Tue, 29 Mar 2011 16:56:20 +0200
Subject: [PATCH] PR debug/47471

gcc/
    * dwarf2out.c (dwarf2out_source_line):  Avoid emitting two
    identical contiguous .loc asm directive.

gcc/testsuite/
    * gcc.dg/debug/dwarf2/line-prog-1.c: New test.
---
 gcc/dwarf2out.c                                 |   33 +++++++++++++++-------
 gcc/testsuite/gcc.dg/debug/dwarf2/line-prog-1.c |   25 +++++++++++++++++
 2 files changed, 47 insertions(+), 11 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/debug/dwarf2/line-prog-1.c

diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index 7803ab7..b350f43 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -22068,6 +22068,8 @@ dwarf2out_source_line (unsigned int line, const char
*filename,
       && line != 0)
     {
       int file_num = maybe_emit_file (lookup_filename (filename));
+      static int prev_file_num;
+      static unsigned int prev_line;

       switch_to_section (current_function_section ());

@@ -22078,19 +22080,28 @@ dwarf2out_source_line (unsigned int line, const char
*filename,

       if (DWARF2_ASM_LINE_DEBUG_INFO)
     {
-      /* Emit the .loc directive understood by GNU as.  */
-      fprintf (asm_out_file, "\t.loc %d %d 0", file_num, line);
-      if (is_stmt != last_is_stmt)
+      static int prev_file_num;
+      static unsigned int prev_line;
+      if (prev_file_num != file_num
+          || prev_line != line)
         {
-          fprintf (asm_out_file, " is_stmt %d", is_stmt ? 1 : 0);
-          last_is_stmt = is_stmt;
-        }
-      if (SUPPORTS_DISCRIMINATOR && discriminator != 0)
-        fprintf (asm_out_file, " discriminator %d", discriminator);
-      fputc ('\n', asm_out_file);
+          /* Emit the .loc directive understood by GNU as.  */
+          fprintf (asm_out_file, "\t.loc %d %d 0", file_num, line);
+          if (is_stmt != last_is_stmt)
+        {
+          fprintf (asm_out_file, " is_stmt %d", is_stmt ? 1 : 0);
+          last_is_stmt = is_stmt;
+        }
+          if (SUPPORTS_DISCRIMINATOR && discriminator != 0)
+        fprintf (asm_out_file, " discriminator %d", discriminator);
+          fputc ('\n', asm_out_file);
+
+          /* Indicate that line number info exists.  */
+          line_info_table_in_use++;

-      /* Indicate that line number info exists.  */
-      line_info_table_in_use++;
+          prev_file_num = file_num;
+          prev_line = line;
+        }
     }
       else if (function_section (current_function_decl) != text_section)
     {
diff --git a/gcc/testsuite/gcc.dg/debug/dwarf2/line-prog-1.c
b/gcc/testsuite/gcc.dg/debug/dwarf2/line-prog-1.c
new file mode 100644
index 0000000..63637dc
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/debug/dwarf2/line-prog-1.c
@@ -0,0 +1,25 @@
+/*
+  Origin: PR debug/47471
+  { dg-options "-g -dA" }
+ */
+
+int v;
+void f (int i, ...)
+{
+  v++;
+}
+
+int
+main (void)
+{
+  f (1);
+  return 0;
+}
+
+/* We want to have only one .loc directive that points to the opening
+   curly bracket of the definition of function f, at line 8.  */
+
+/*
+  { dg-final { scan-assembler-times "\.loc 1 8 0" 1 } }
+
+ */
-- 
1.7.3.4