applet_tables: save space by removing applet name offsets

The array applet_nameofs consumes two bytes per applet.  It encodes

   nofork/noexec flags
   suid flags
   the offset of the applet name in the applet_name string

Change the applet_table build tool to store the flags in two separate
arrays (applet_flags and applet_suid).  Replace applet_nameofs[] with a
smaller version that only stores a limited number of offsets.

This requires changes to the macros APPLET_IS_NOFORK, APPLET_IS_NOEXEC
and APPLET_SUID.

   According to Valgrind the original find_applet_by_name required
   353 cycles per call, averaged over all names.  Adjusting the number
   of known offsets allows space to be traded off against execution time:

      KNOWN_OFFSETS     cycles     bytes (wrt KNOWN_OFFSETS = 0)

           0             9057        -
           2             4604       32
           4             2407       75
           8             1342       98
          16              908      130
          32              884      194

   This patch uses KNOWN_OFFSETS = 8.

v2:
   Remove some dead code from the applet_table tool;
   Treat the applet in the middle of the table as a special case.

v3:
   Use the middle applet to adjust the start of the linear search as
   well as the last applet found.

v4:
   Use an augmented linear search in find_applet_by_name.
   Drop the special treatment of the middle name from get_applet_name:
   most of the advantage now derives from the last stored value.

v5:
   Don't store index in applet_nameofs, it can be calculated.

v6:
   Tweaks by Denys

function                                             old     new   delta
find_applet_by_name                                   25     125    +100
applet_suid                                            -      92     +92
run_applet_no_and_exit                               452     460      +8
run_applet_and_exit                                  695     697      +2
applet_name_compare                                   31       -     -31
applet_nameofs                                       734      14    -720
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 3/1 up/down: 202/-751)         Total: -549 bytes
   text       data        bss        dec        hex    filename
 925464        906      17160     943530      e65aa    busybox_old
 924915        906      17160     942981      e6385    busybox_unstripped

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 60 insertions, 18 deletions

diff --git a/applets/applet_tables.c b/applets/applet_tables.c
index 92bf1e447..48544f08d 100644
--- a/applets/applet_tables.c
+++ b/applets/applet_tables.c
@@ -41,8 +41,6 @@ struct bb_applet {
 
 enum { NUM_APPLETS = ARRAY_SIZE(applets) };
 
-static int offset[NUM_APPLETS];
-
 static int cmp_name(const void *a, const void *b)
 {
         const struct bb_applet *aa = a;
@@ -60,22 +58,37 @@ static int str_isalnum_(const char *s)
         return 1;
 }
 
+// Before linear search, narrow it down by looking at N "equidistant" names:
+// KNOWN_APPNAME_OFFSETS  cycles  code_size
+//                     0    9057
+//                     2    4604        +32
+//                     4    2407        +75
+//                     8    1342        +98
+//                    16     908       +130
+//                    32     884       +194
+// With 8, applet_nameofs[] table has 7 elements.
+#define KNOWN_APPNAME_OFFSETS 8
+
 int main(int argc, char **argv)
 {
-        int i;
-        int ofs;
+        int i, j;
+        int ofs, offset[KNOWN_APPNAME_OFFSETS], index[KNOWN_APPNAME_OFFSETS];
 //      unsigned MAX_APPLET_NAME_LEN = 1;
 
         qsort(applets, NUM_APPLETS, sizeof(applets[0]), cmp_name);
 
+        for (i = 0; i < KNOWN_APPNAME_OFFSETS; i++)
+                index[i] = i * NUM_APPLETS / KNOWN_APPNAME_OFFSETS;
+
         ofs = 0;
         for (i = 0; i < NUM_APPLETS; i++) {
-                offset[i] = ofs;
+                for (j = 0; j < KNOWN_APPNAME_OFFSETS; j++)
+                        if (i == index[j])
+                                offset[j] = ofs;
                 ofs += strlen(applets[i].name) + 1;
         }
-        /* We reuse 4 high-order bits of offset array for other purposes,
-         * so if they are indeed needed, refuse to proceed */
-        if (ofs > 0xfff)
+        /* If the list of names is too long refuse to proceed */
+        if (ofs > 0xffff)
                 return 1;
         if (!argv[1])
                 return 1;
@@ -94,7 +107,17 @@ int main(int argc, char **argv)
                 printf("#define SINGLE_APPLET_STR \"%s\"\n", applets[0].name);
                 printf("#define SINGLE_APPLET_MAIN %s_main\n", applets[0].main);
         }
-        printf("\n");
+
+        if (KNOWN_APPNAME_OFFSETS > 0 && NUM_APPLETS > 2*KNOWN_APPNAME_OFFSETS) {
+                printf("#define KNOWN_APPNAME_OFFSETS %u\n\n", KNOWN_APPNAME_OFFSETS);
+                printf("const uint16_t applet_nameofs[] ALIGN2 = {\n");
+                for (i = 1; i < KNOWN_APPNAME_OFFSETS; i++)
+                        printf("%d,\n", offset[i]);
+                printf("};\n\n");
+        }
+        else {
+                printf("#define KNOWN_APPNAME_OFFSETS 0\n\n");
+        }
 
         //printf("#ifndef SKIP_definitions\n");
         printf("const char applet_names[] ALIGN1 = \"\"\n");
@@ -119,20 +142,39 @@ int main(int argc, char **argv)
         printf("};\n");
         printf("#endif\n\n");
 
-        printf("const uint16_t applet_nameofs[] ALIGN2 = {\n");
-        for (i = 0; i < NUM_APPLETS; i++) {
-                printf("0x%04x,\n",
-                        offset[i]
 #if ENABLE_FEATURE_PREFER_APPLETS
-                        + (applets[i].nofork << 12)
-                        + (applets[i].noexec << 13)
+        printf("const uint8_t applet_flags[] ALIGN1 = {\n");
+        i = 0;
+        while (i < NUM_APPLETS) {
+                int v = applets[i].nofork + (applets[i].noexec << 1);
+                if (++i < NUM_APPLETS)
+                        v |= (applets[i].nofork + (applets[i].noexec << 1)) << 2;
+                if (++i < NUM_APPLETS)
+                        v |= (applets[i].nofork + (applets[i].noexec << 1)) << 4;
+                if (++i < NUM_APPLETS)
+                        v |= (applets[i].nofork + (applets[i].noexec << 1)) << 6;
+                printf("0x%02x,\n", v);
+                i++;
+        }
+        printf("};\n\n");
 #endif
+
 #if ENABLE_FEATURE_SUID
-                        + (applets[i].need_suid << 14) /* 2 bits */
-#endif
-                );
+        printf("const uint8_t applet_suid[] ALIGN1 = {\n");
+        i = 0;
+        while (i < NUM_APPLETS) {
+                int v = applets[i].need_suid; /* 2 bits */
+                if (++i < NUM_APPLETS)
+                        v |= applets[i].need_suid << 2;
+                if (++i < NUM_APPLETS)
+                        v |= applets[i].need_suid << 4;
+                if (++i < NUM_APPLETS)
+                        v |= applets[i].need_suid << 6;
+                printf("0x%02x,\n", v);
+                i++;
         }
         printf("};\n\n");
+#endif
 
 #if ENABLE_FEATURE_INSTALLER
         printf("const uint8_t applet_install_loc[] ALIGN1 = {\n");