Add file offsets to the source cache

Currently, gdb stores the number of lines and an array of file offsets
for the start of each line in struct symtab.  This patch moves this
information to the source cache.  This has two benefits.

First, it allows gdb to read a source file less frequently.
Currently, a source file may be read multiple times: once when
computing the file offsets, once when highlighting, and then pieces
may be read again while printing source lines.  With this change, the
file is read once for its source text and file offsets; and then
perhaps read again if it is evicted from the cache.

Second, if multiple symtabs cover the same source file, then this will
share the file offsets between them.  I'm not sure whether this
happens in practice.

gdb/ChangeLog
2019-08-06  Tom Tromey  <tromey@adacore.com>

	* annotate.c (annotate_source_line): Use g_source_cache.
	* source-cache.c (source_cache::get_plain_source_lines): Change
	parameters.  Populate m_offset_cache.
	(source_cache::ensure): New method.
	(source_cache::get_line_charpos): New method.
	(extract_lines): Move lower.  Change parameters.
	(source_cache::get_source_lines): Move lower.
	* source-cache.h (class source_cache): Update comment.
	<get_line_charpos>: New method.
	<get_source_lines>: Update comment.
	<clear>: Clear m_offset_cache.
	<get_plain_source_lines>: Change parameters.
	<ensure>: New method
	<m_offset_cache>: New member.
	* source.c (forget_cached_source_info_for_objfile): Update.
	(info_source_command): Use g_source_cache.
	(find_source_lines, open_source_file_with_line_charpos): Remove.
	(print_source_lines_base, search_command_helper): Use g_source_cache.
	* source.h (open_source_file_with_line_charpos): Don't declare.
	* symtab.h (struct symtab) <nlines, line_charpos>: Remove.
	* tui/tui-source.c (tui_source_window::do_scroll_vertical):
	Use g_source_cache.

1 files changed, 41 insertions, 11 deletions

diff --git a/gdb/source-cache.h b/gdb/source-cache.h
index 0c8b14e..b6e8690 100644
--- a/gdb/source-cache.h
+++ b/gdb/source-cache.h
@@ -19,12 +19,20 @@
 #ifndef SOURCE_CACHE_H
 #define SOURCE_CACHE_H
 
-/* This caches highlighted source text, keyed by the source file's
-   full name.  A size-limited LRU cache is used.
+#include <unordered_map>
+#include <unordered_set>
+
+/* This caches two things related to source files.
+
+   First, it caches highlighted source text, keyed by the source
+   file's full name.  A size-limited LRU cache is used.
 
    Highlighting depends on the GNU Source Highlight library.  When not
-   available, this cache will fall back on reading plain text from the
-   appropriate file.  */
+   available or when highlighting fails for some reason, this cache
+   will instead store the un-highlighted source text.
+
+   Second, this will cache the file offsets corresponding to the start
+   of each line of a source file.  This cache is not size-limited.  */
 class source_cache
 {
 public:
@@ -33,11 +41,23 @@ public:
   {
   }
 
+  /* This returns the vector of file offsets for the symtab S,
+     computing the vector first if needed.
+
+     On failure, returns false.
+
+     On success, returns true and sets *OFFSETS.  This pointer is not
+     guaranteed to remain valid across other calls to get_source_lines
+     or get_line_charpos.  */
+  bool get_line_charpos (struct symtab *s,
+			 const std::vector<off_t> **offsets);
+
   /* Get the source text for the source file in symtab S.  FIRST_LINE
      and LAST_LINE are the first and last lines to return; line
-     numbers are 1-based.  If the file cannot be read, false is
-     returned.  Otherwise, LINES_OUT is set to the desired text.  The
-     returned text may include ANSI terminal escapes.  */
+     numbers are 1-based.  If the file cannot be read, or if the line
+     numbers are out of range, false is returned.  Otherwise,
+     LINES_OUT is set to the desired text.  The returned text may
+     include ANSI terminal escapes.  */
   bool get_source_lines (struct symtab *s, int first_line,
 			 int last_line, std::string *lines_out);
 
@@ -45,6 +65,7 @@ public:
   void clear ()
   {
     m_source_map.clear ();
+    m_offset_cache.clear ();
   }
 
 private:
@@ -59,12 +80,21 @@ private:
   };
 
   /* A helper function for get_source_lines reads a source file.
-     Returns false on error.  If no error, the contents of the file
-     are put into *LINES_OUT, and returns true.  */
-  bool get_plain_source_lines (struct symtab *s, std::string *lines_out);
+     Returns the contents of the file; or throws an exception on
+     error.  This also updates m_offset_cache.  */
+  std::string get_plain_source_lines (struct symtab *s,
+				      const std::string &fullname);
 
-  /* The contents of the cache.  */
+  /* A helper function that the data for the given symtab is entered
+     into both caches.  Returns false on error.  */
+  bool ensure (struct symtab *s);
+
+  /* The contents of the source text cache.  */
   std::vector<source_text> m_source_map;
+
+  /* The file offset cache.  The key is the full name of the source
+     file.  */
+  std::unordered_map<std::string, std::vector<off_t>> m_offset_cache;
 };
 
 /* The global source cache.  */