[PATCH] Handle many consecutive location notes more efficiently in dwarf2.

[David Miller]

Tests such as limits-fndefn.c spend %97 of their time in
next_real_insn() when -g is on the command line.

It's the invocation from dwarf2out_var_location().  These test cases
are basically 100,000 consecutive var location notes, followed by a
real instruction.

This is similar to the reorg issue I dealt with yesterday, we
basically scan the entire function over and over again.

How I handle this here is if we see that the next insn is yet another
var location note we cache the next_real_insn() result.

Now limits-fndefn.c with "-O3 -g" takes about 20 seconds instead of
several minutes on my Niagara-T3 box.

Committed to trunk.

gcc/

        * dwarf2out.c (dwarf2out_var_location): When processing several
        consecutive location notes, cache the result of next_real_insn().

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@180695 
138bc75d-0d04-0410-961f-82ee72b054a4
---
 gcc/ChangeLog   |    5 +++++
 gcc/dwarf2out.c |   34 ++++++++++++++++++++++++++++++++--
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 26bb1a9..037138a 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+2011-10-30  David S. Miller  <da...@davemloft.net>
+
+       * dwarf2out.c (dwarf2out_var_location): When processing several
+       consecutive location notes, cache the result of next_real_insn().
+
 2011-10-30  Uros Bizjak  <ubiz...@gmail.com>
 
        * config/i386/i386.md (avx2_vec_dup<mode>): Macroize insn from
diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index 8d5a9f0..478952f 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -20127,10 +20127,12 @@ dwarf2out_var_location (rtx loc_note)
 {
   char loclabel[MAX_ARTIFICIAL_LABEL_BYTES + 2];
   struct var_loc_node *newloc;
-  rtx next_real;
+  rtx next_real, next_note;
   static const char *last_label;
   static const char *last_postcall_label;
   static bool last_in_cold_section_p;
+  static rtx expected_next_loc_note;
+  static rtx cached_next_real_insn;
   tree decl;
   bool var_loc_p;
 
@@ -20149,7 +20151,35 @@ dwarf2out_var_location (rtx loc_note)
   if (var_loc_p && !DECL_P (NOTE_VAR_LOCATION_DECL (loc_note)))
     return;
 
-  next_real = next_real_insn (loc_note);
+  /* Optimize processing a large consecutive sequence of location
+     notes so we don't spend too much time in next_real_insn.  If the
+     next insn is another location note, remember the next_real_insn
+     calculation for next time.  */
+  next_real = cached_next_real_insn;
+  if (next_real)
+    {
+      if (expected_next_loc_note != loc_note)
+       next_real = NULL_RTX;
+    }
+
+  next_note = NEXT_INSN (loc_note);
+  if (! next_note
+      || INSN_DELETED_P (next_note)
+      || GET_CODE (next_note) != NOTE
+      || (NOTE_KIND (next_note) != NOTE_INSN_VAR_LOCATION
+         && NOTE_KIND (next_note) != NOTE_INSN_CALL_ARG_LOCATION))
+    next_note = NULL_RTX;
+
+  if (! next_real)
+    next_real = next_real_insn (loc_note);
+
+  if (next_note)
+    {
+      expected_next_loc_note = next_note;
+      cached_next_real_insn = next_real;
+    }
+  else
+    cached_next_real_insn = NULL_RTX;
 
   /* If there are no instructions which would be affected by this note,
      don't do anything.  */
-- 
1.7.6.401.g6a319