[PATCH 1/2] libcpp: Avoid unnecessary ad-hoc uses for large source files

[David Malcolm]

When debugging PR c++/68819 I noticed an inefficiency during
the handling of locations > LINE_MAP_MAX_LOCATION_WITH_COLS.
When combining ranges for locations with
  start == caret == finish (with a NULL data ptr)
get_combined_adhoc_loc was always building an ad-hoc entry
rather than simply returning the location.

Normally can_be_stored_compactly_p returns true for such triples,
but it returns false for locations above
LINE_MAP_MAX_LOCATION_WITH_COLS (as the range-packing code isn't
designed to handle the latter).  There's a followup test within
get_combined_adhoc_loc for detecting the start == caret == finish
case, but I'd conditionalized it on the location being one of the
two reserved locations:
   locus < RESERVED_LOCATION_COUNT

That condition has been there since I introduced range-packing
(as part ofr230331).  I believe at the time I didn't realize there
was any other way for such a location triple to fail
can_be_stored_compactly_p.

These location triples occur a lot where we're in this mode:
every token, and every expression that fits on one line,
so this is wasteful.

Removing the locus < RESERVED_LOCATION_COUNT part of the
condition allows us to avoid using the ad-hoc table for these
cases.

Successfully bootstrapped&regrtested on x86_64-pc-linux-gnu
in combination with the following patch.

libcpp/ChangeLog:
        * line-map.c (get_combined_adhoc_loc): Remove condition
        on locus < RESERVED_LOCATION_COUNT when considering
        whether a caret == start == finish location can be
        simply stored as the caret location.
---
 libcpp/line-map.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/libcpp/line-map.c b/libcpp/line-map.c
index 209d0fb..c20a32b 100644
--- a/libcpp/line-map.c
+++ b/libcpp/line-map.c
@@ -196,10 +196,9 @@ get_combined_adhoc_loc (struct line_maps *set,
        }
     }
 
-  /* We can also compactly store the reserved locations
+  /* We can also compactly store locations
      when locus == start == finish (and data is NULL).  */
-  if (locus < RESERVED_LOCATION_COUNT
-      && locus == src_range.m_start
+  if (locus == src_range.m_start
       && locus == src_range.m_finish
       && !data)
     return locus;
-- 
1.8.5.3