+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 SET_PTR_TO_GLOBALS macro:
+
+ SET_PTR_TO_GLOBALS(xzalloc(sizeof(G)));
+
+Typically it is done in <applet>_main().
+
+Now you can reference "globals" by G.a, G.buf and so on, in any function.
+
+
+ bb_common_bufsiz1
+
+There is one big common buffer in bss - bb_common_bufsiz1. It is a much
+earlier mechanism to reduce bss usage. Each applet can use it for
+its needs. Library functions are prohibited from using it.
+
+'G.' trick can be done using bb_common_bufsiz1 instead of malloced buffer:
+
+#define G (*(struct globals*)&bb_common_bufsiz1)
+
+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))
+ BUG_<applet>_globals_too_big();
+
+
+ Drawbacks
+
+You have to initialize it by hand. xzalloc() can be helpful in clearing
+allocated storage to 0, but anything more must be done by hand.
+
+All global variables are prefixed by 'G.' now. If this makes code
+less readable, use #defines:
+
+#define dev_fd (G.dev_fd)
+#define sector (G.sector)
+
+
+ Word of caution
+
+If applet doesn't use much of global data, converting it to use
+one of above methods is not worth the resulting code obfuscation.
+If you have less than ~300 bytes of global data - don't bother.
+
+
+ gcc's data alignment problem
+
+The following attribute added in vi.c:
+
+static int tabstop;
+static struct termios term_orig __attribute__ ((aligned (4)));
+static struct termios term_vi __attribute__ ((aligned (4)));
+
+reduces bss size by 32 bytes, because gcc sometimes aligns structures to
+ridiculously large values. asm output diff for above example:
+
+ tabstop:
+ .zero 4
+ .section .bss.term_orig,"aw",@nobits
+- .align 32
++ .align 4
+ .type term_orig, @object
+ .size term_orig, 60
+ term_orig:
+ .zero 60
+ .section .bss.term_vi,"aw",@nobits
+- .align 32
++ .align 4
+ .type term_vi, @object
+ .size term_vi, 60
+
+gcc doesn't seem to have options for altering this behaviour.
+
+gcc 3.4.3 and 4.1.1 tested:
+char c = 1;
+// gcc aligns to 32 bytes if sizeof(struct) >= 32
+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:
projects / oweals / busybox.git / blobdiff