Date: Fri, 20 Dec 2002 14:55:39 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> To: Peter Wemm <peter@wemm.org> Cc: "David O'Brien" <obrien@FreeBSD.org>, Poul-Henning Kamp <phk@FreeBSD.org>, <cvs-committers@FreeBSD.org>, <cvs-all@FreeBSD.org> Subject: Re: cvs commit: src/sys/boot/i386/boot2 Makefile boot1.s Message-ID: <20021220135958.C34646-100000@gamplex.bde.org> In-Reply-To: <20021220000810.139972A7EA@canning.wemm.org>
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On Thu, 19 Dec 2002, Peter Wemm wrote: > Bruce Evans wrote: > > On Mon, 16 Dec 2002, David O'Brien wrote: > > > > > On Sun, Dec 15, 2002 at 08:31:45PM +1100, Bruce Evans wrote: > > > > Please remove the alignment hacks (Makefile rev.1.25) while you are there > . > > > > Breakage of -[fm]align-* seems to have been fixed in gcc, so the hacks > > > > make no difference except to reduce the size of boot2.o's data by 3 bytes > > > > (I think the 3 bytes is wasted at link time). > > > > > > I'd rather leave them in to get the 3 bytes -- believe it or not it has > > > made a difference in the past. > > > > It doesn't any difference because the 3 bytes is wasted at link time. > > The data section has size 29, but sections are padded to 4-byte boundaries, > > at least for ELF objects with the default linker scripts, so 29 becomes 32 > > the same as if it had been padded internally for aligment. > > In my case, it was a *final* object size difference of 3 bytes in /boot/ > boot2. I found that turning off the sed caused quite a bit of bloat, so > obviously we dont have all the necessary -[fm]align-*'s in the Makefile > that you have. But I don't have any extra ones. It's possible that I have some pollution in the environment (no pentiumpro optimizations ...), but I don't think it affects boot2. Here the sed removes just 5 .p2aligns, which reduces the size of boot2.o by a whole 3 bytes (from 32 to 29). There are no .p2align's in the text section since -falign-* now works. There are .p2align's in the .rodata* and .data sections because gcc doesn't have any flags to reduce alignment outside of the text section AFAIK (except for my -mno-align-long-strings which is not used, and the __packed attribute at the source level which is not used either). Most of the data is aligned anyway so the .p2align's have no effect. The potentially most wasteful one is: .section .rodata.str1.32,"aMS",@progbits,1 .p2align 5 .LC13: .string "\nFreeBSD/i386 boot\nDefault: %u:%s(%u,%c)%s\nboot: " This can be killed using -mno-align-long-strings. The .p2align 5 string seems to be what is wasteful in practice. boot2 no longer fits despite the size of boot2.o as reported by size(1) being essentially no different. objdump -h show the problem. The following dif is of objdump -h output on boot2.o with all alignment stripped (orig) and with no alignment stripped: 2c2 < orig/boot2.o: file format elf32-i386-freebsd --- > boot2.o: file format elf32-i386-freebsd 8c8 < 1 .data 0000001d 00000000 00000000 0000139c 2**2 --- > 1 .data 00000020 00000000 00000000 0000139c 2**2 12c12 < 3 .rodata 00000027 00000000 00000000 000013c0 2**0 --- > 3 .rodata 00000027 00000000 00000000 000013c0 2**2 16c16 < 5 .rodata.str1.32 00000032 00000000 00000000 00001494 2**0 --- > 5 .rodata.str1.32 00000032 00000000 00000000 000014a0 2**5 18c18 < 6 .comment 0000002f 00000000 00000000 000014c6 2**0 --- > 6 .comment 0000002f 00000000 00000000 000014d2 2**0 size(1) says that the text size is 5228 = sum of sizes of .text and .rodata* sections in both cases. But the text size is actually larger in the non-sed'ed version since some padding is needed to align the .rodata and .rodata.str1.32 sections (especially the latter). This results in the non-sed'ed boot2 being 8 bytes too large here. With -mno-align-long-strings, there are 12 bytes to spare. The sed'ed version has 15 bytes to spare. It really is 3 bytes smaller, presumably because the section order is such that an unaligned section is packed next to the data section. Summary: - you save more space than -falign-* can, by hacking out most of the alignment for data sections. - the saving is quite small except for the .rodata.str1.32 section and this can be handled less hackishly using -mno-align-long-strings. - size(1) is broken. It apparently doesn't understand alignment of .rodata*. Bruce To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe cvs-all" in the body of the message
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