[RFA] performance improvement of lookup_partial_symtab

[Jerome Guitton]

Hello,

Debugging an executable build with more than 3000 source files, it
can take several seconds to GDB to output some source file information.
A reproducer can be generated with the following script (It generated an
Ada program):

---

rm main.adb
rm -rf many_files
mkdir many_files
for i in 0 1 2 3 ; do
   for j in 0 1 2 3 4 5 6 7 8 9 ; do
      for k in 0 1 2 3 4 5 6 7 8 9 ; do
         for l in 0 1 2 3 4 5 6 7 8 9 ; do
echo "with Pack_${i}${j}${k}${l}_Test;" >> main.adb
echo "package Pack_${i}$j${k}${l}_Test is" \
	>> many_files/pack_${i}${j}${k}${l}_test.ads
echo "   pragma Pure;" \
	>> many_files/pack_${i}${j}${k}${l}_test.ads
echo "   Var_${i}$j${k}${l}_Test : constant String := \"${i}${j}${k}${l}\";" \
	>> many_files/pack_${i}${j}${k}${l}_test.ads
echo "end Pack_${i}$j${k}${l}_Test;" \
	>> many_files/pack_${i}${j}${k}${l}_test.ads
         done
      done
   done
done
echo "procedure Main is" >> main.adb
echo "begin" >> main.adb
echo "   null;" >> main.adb
echo "end Main;" >> main.adb

---

Compiling with GNAT 3.16a, and debugging with gdb 5.3:


(gdb) l /home/guitton/bug/C411-003/many_files/pack_0001_test.ads:2

[after several seconds:]

1       package Pack_0001_Test is
2          pragma Pure;
3          Var_0001_Test : constant String := "0001";
4       end Pack_0001_Test;
(gdb)

The culprit is lookup_partial_symtab, that may do a call
to source_full_path_of for every file referenced. I suggest trying
to avoid as much as possible this function call by doing some preliminary
testing: if the basenames are not equal, no need to fetch the
fullname and make the fullname comparison. With this modification
GDB reacts almost instantly. 

I have tested this patch against the testsuite on linux; no regression
appeared. There seems to be no test using an absolute path, I can
also propose one.

Joel Brobecker volunteered to commit this path if/when it is approved.


2003-05-26  J. Guitton <guitton@gnat.com>

        * symtab.c (lookup_partial_symbol): To improve the performance,
        try to avoid as much as possible to call source_full_path_of,
	by doing some preliminary testing.

Index: symtab.c
===================================================================
RCS file: /cvs/src/src/gdb/symtab.c,v
retrieving revision 1.109
diff -c -r1.109 symtab.c
*** symtab.c	20 May 2003 01:55:17 -0000	1.109
--- symtab.c	10 Jun 2003 13:22:43 -0000
***************
*** 273,280 ****
        }
  
      /* If the user gave us an absolute path, try to find the file in
!        this symtab and use its absolute path.  */
!     if (full_path != NULL)
        {
  	if (pst->fullname == NULL)
  	  source_full_path_of (pst->filename, &pst->fullname);
--- 273,283 ----
        }
  
      /* If the user gave us an absolute path, try to find the file in
!        this symtab and use its absolute path.  However, if the
!        base names are different, the two files are different and there
!        is no need to do a full comparison.  */
!     if (full_path != NULL &&
! 	strcmp (lbasename (full_path), lbasename (pst->filename)) == 0)
        {
  	if (pst->fullname == NULL)
  	  source_full_path_of (pst->filename, &pst->fullname);
***************
*** 285,291 ****
  	  }
        }
  
!     if (real_path != NULL)
        {
          char *rp = NULL;
  	if (pst->fullname == NULL)
--- 288,295 ----
  	  }
        }
  
!     if (real_path != NULL &&
! 	strcmp (lbasename (real_path), lbasename (pst->filename)) == 0)
        {
          char *rp = NULL;
  	if (pst->fullname == NULL)