[PATCH] Add a comment briefly explaining partial symbols

["Christian Biesinger via gdb-patches" <gdb-patches@sourceware.org>]

Based on an explanation by tromey on IRC.

gdb/ChangeLog:

2019-07-31  Christian Biesinger  <cbiesinger@google.com>

	* objfiles.h (objfile): Add a comment describing partial symbols.
---
 gdb/objfiles.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/gdb/objfiles.h b/gdb/objfiles.h
index a0c106be3d..c7e0be1955 100644
--- a/gdb/objfiles.h
+++ b/gdb/objfiles.h
@@ -382,7 +382,13 @@ private:
    2.  Additional symbol files added by the add-symbol-file command,
    3.  Shared library objfiles, added by ADD_SOLIB,  4.  symbol files
    for modules that were loaded when GDB attached to a remote system
-   (see remote-vx.c).  */
+   (see remote-vx.c).
+   
+   GDB typically reads symbols twice -- first an initial scan which just
+   reads "partial symbols"; these are partial information for the
+   static/global symbols in a symbol file.  When later looking up symbols,
+   objfile->sf->qf->lookup_symbol is used to check if we only have a partial
+   symbol and if so, read and expand the full compunit.  */
 
 struct objfile
 {
-- 
2.22.0.709.g102302147b-goog