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[oweals/busybox.git] / docs / keep_data_small.txt

diff --git a/docs/keep_data_small.txt b/docs/keep_data_small.txt
index 72de2d1ef24b7e1b16b9f18efd6b7df596504582..2ddbefa10aa3cde76a3700d9353432319953a761 100644 (file)
--- a/docs/keep_data_small.txt
+++ b/docs/keep_data_small.txt
@@ -43,6 +43,16 @@ takes 55k of memory on 64-bit x86 kernel.
 
 On 32-bit kernel we need ~26k per applet.
 
+Script:
+
+i=1000; while test $i != 0; do
+        echo -n .
+        busybox sleep 30 &
+        i=$((i - 1))
+done
+echo
+wait
+
 (Data from NOMMU arches are sought. Provide 'size busybox' output too)
 
 
@@ -65,7 +75,7 @@ archival/libunarchive/decompress_unzip.c:
 (see the rest of the file to get the idea)
 
 This example completely eliminates globals in that module.
-Required memory is allocated in inflate_gunzip() [its main module]
+Required memory is allocated in unpack_gz_stream() [its main module]
 and then passed down to all subroutines which need to access 'globals'
 as a parameter.
 
@@ -89,9 +99,9 @@ and then declare that ptr_to_globals is a pointer to it:
 
 ptr_to_globals is declared as constant pointer.
 This helps gcc understand that it won't change, resulting in noticeably
-smaller code. In order to assign it, use PTR_TO_GLOBALS macro:
+smaller code. In order to assign it, use SET_PTR_TO_GLOBALS macro:
 
-       PTR_TO_GLOBALS = xzalloc(sizeof(G));
+       SET_PTR_TO_GLOBALS(xzalloc(sizeof(G)));
 
 Typically it is done in <applet>_main().
 
@@ -112,7 +122,7 @@ Be careful, though, and use it only if globals fit into bb_common_bufsiz1.
 Since bb_common_bufsiz1 is BUFSIZ + 1 bytes long and BUFSIZ can change
 from one libc to another, you have to add compile-time check for it:
 
-if(sizeof(struct globals) > sizeof(bb_common_bufsiz1))
+if (sizeof(struct globals) > sizeof(bb_common_bufsiz1))
        BUG_<applet>_globals_too_big();
 
 
@@ -163,14 +173,44 @@ ridiculously large values. asm output diff for above example:
 
 gcc doesn't seem to have options for altering this behaviour.
 
-gcc 3.4.3:
+gcc 3.4.3 and 4.1.1 tested:
+char c = 1;
 // gcc aligns to 32 bytes if sizeof(struct) >= 32
-struct st {
-    int c_iflag,c_oflag,c_cflag,c_lflag;
-    int i1,i2,i3;    // struct will be aligned to 4 bytes
-//  int i1,i2,i3,i4; // struct will be aligned to 32 bytes
-};
-struct st t = { 1 };
+struct {
+    int a,b,c,d;
+    int i1,i2,i3;
+} s28 = { 1 };    // struct will be aligned to 4 bytes
+struct {
+    int a,b,c,d;
+    int i1,i2,i3,i4;
+} s32 = { 1 };    // struct will be aligned to 32 bytes
 // same for arrays
 char vc31[31] = { 1 }; // unaligned
 char vc32[32] = { 1 }; // aligned to 32 bytes
+
+-fpack-struct=1 reduces alignment of s28 to 1 (but probably
+will break layout of many libc structs) but s32 and vc32
+are still aligned to 32 bytes.
+
+I will try to cook up a patch to add a gcc option for disabling it.
+Meanwhile, this is where it can be disabled in gcc source:
+
+gcc/config/i386/i386.c
+int
+ix86_data_alignment (tree type, int align)
+{
+#if 0
+  if (AGGREGATE_TYPE_P (type)
+       && TYPE_SIZE (type)
+       && TREE_CODE (TYPE_SIZE (type)) == INTEGER_CST
+       && (TREE_INT_CST_LOW (TYPE_SIZE (type)) >= 256
+           || TREE_INT_CST_HIGH (TYPE_SIZE (type))) && align < 256)
+    return 256;
+#endif
+
+Result (non-static busybox built against glibc):
+
+# size /usr/srcdevel/bbox/fix/busybox.t0/busybox busybox
+   text    data     bss     dec     hex filename
+ 634416    2736   23856  661008   a1610 busybox
+ 632580    2672   22944  658196   a0b14 busybox_noalign