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Date: Thu, 27 Nov 2014 10:35:21 -0800 From: Joe Perches <joe@...ches.com> To: David Laight <David.Laight@...LAB.COM> Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Quentin Lambert <lambert.quentin@...il.com>, "David S. Miller" <davem@...emloft.net>, Alexey Kuznetsov <kuznet@....inr.ac.ru>, James Morris <jmorris@...ei.org>, Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, Patrick McHardy <kaber@...sh.net>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH] x86: bpf_jit_comp: simplify trivial boolean return On Thu, 2014-11-27 at 12:25 +0000, David Laight wrote: > From: Joe Perches > > On Wed, 2014-11-26 at 10:34 -0800, Alexei Starovoitov wrote: > > > On Wed, Nov 26, 2014 at 10:02 AM, Joe Perches <joe@...ches.com> wrote: > > > > On Wed, 2014-11-26 at 09:23 -0800, Alexei Starovoitov wrote: > > > >> On Wed, Nov 26, 2014 at 8:58 AM, Joe Perches <joe@...ches.com> wrote: > > > > > > > >> > Is there any value in reordering these tests for frequency > > > >> > or maybe using | instead of || to avoid multiple jumps? > > > >> > > > >> probably not. It's not a critical path. > > > >> compiler may fuse conditions depending on values anyway. > > > >> If it was a critical path, we could have used > > > >> (1 << reg) & mask trick. > > > >> I picked explicit 'return true' else 'return false' here, > > > >> because it felt easier to read. Just a matter of taste. > > > > > > > > There is a size difference though: (allyesconfig) > > > > > > > > $ size arch/x86/net/built-in.o* > > > > text data bss dec hex filename > > > > 12999 1012 4336 18347 47ab arch/x86/net/built-in.o.new > > > > 13177 1076 4592 18845 499d arch/x86/net/built-in.o.old > > > > > > interesting. Compiler obviously thinks that 178 byte increase > > > with -O2 is the right trade off. Which I agree with :) > > > > > > If I think dropping 'inline' and using -Os will give bigger savings... > > > > This was allyesconfig which already uses -Os > > > > Using -O2, there is no difference using inline > > or not, but the size delta with the bitmask is > > much larger > > > > $ size arch/x86/net/built-in.o* (allyesconfig, but not -Os) > > text data bss dec hex filename > > 13410 820 3624 17854 45be arch/x86/net/built-in.o.new > > 16130 884 4200 21214 52de arch/x86/net/built-in.o.old > > 16130 884 4200 21214 52de arch/x86/net/built-in.o.static > > That is quite a big % change in the code size. > Why the change in data? $ objdump -t arch/x86/net/bpf_jit_comp.o.new > new $ objdump -t arch/x86/net/bpf_jit_comp.o.old > old $ diff -urN old new --- old 2014-11-27 10:31:36.654373756 -0800 +++ new 2014-11-27 10:31:31.254373453 -0800 @@ -1,5 +1,5 @@ -arch/x86/net/bpf_jit_comp.o.old: file format elf64-x86-64 +arch/x86/net/bpf_jit_comp.o.new: file format elf64-x86-64 SYMBOL TABLE: 0000000000000000 l df *ABS* 0000000000000000 bpf_jit_comp.c @@ -8,28 +8,26 @@ 0000000000000000 l d .bss 0000000000000000 .bss 0000000000000000 l d .text.unlikely 0000000000000000 .text.unlikely 0000000000000000 l F .text 000000000000001f jit_fill_hole -0000000000000098 l O .bss 0000000000000008 __gcov0.jit_fill_hole +0000000000000060 l O .bss 0000000000000008 __gcov0.jit_fill_hole 0000000000000000 l d .rodata.str1.1 0000000000000000 .rodata.str1.1 0000000000000000 l d .rodata.str1.8 0000000000000000 .rodata.str1.8 -0000000000000020 l F .text 00000000000030a2 do_jit -00000000000000c0 l O .bss 0000000000000b68 __gcov0.do_jit +0000000000000020 l F .text 000000000000260b do_jit +0000000000000080 l O .bss 0000000000000970 __gcov0.do_jit 00000000000006e0 l O .rodata 0000000000000034 reg2hex 0000000000000000 l d .rodata 0000000000000000 .rodata -0000000000000060 l O .bss 0000000000000038 __gcov0.add_2mod -0000000000000c28 l O .bss 0000000000000008 __gcov0.bpf_jit_compile -0000000000000c40 l O .bss 00000000000003f0 __gcov0.bpf_int_jit_compile +00000000000009f0 l O .bss 0000000000000008 __gcov0.bpf_jit_compile +0000000000000a00 l O .bss 00000000000003f0 __gcov0.bpf_int_jit_compile 0000000000000040 l O .bss 0000000000000020 __gcov0.bpf_jit_dump -0000000000001040 l O .bss 0000000000000028 __gcov0.bpf_jit_free +0000000000000e00 l O .bss 0000000000000028 __gcov0.bpf_jit_free 0000000000000010 l O .bss 0000000000000018 __gcov0.bpf_prog_unlock_free 0000000000000000 l d .text.startup 0000000000000000 .text.startup 0000000000000000 l F .text.startup 0000000000000012 _GLOBAL__sub_I_65535_0_bpf_jit_compile 0000000000000000 l d .init_array 0000000000000000 .init_array -0000000000000340 l O .data 0000000000000028 __gcov_.bpf_jit_free -0000000000000260 l O .data 0000000000000028 __gcov_.bpf_int_jit_compile -0000000000000220 l O .data 0000000000000028 __gcov_.bpf_jit_compile -00000000000001e0 l O .data 0000000000000028 __gcov_.do_jit -00000000000001a0 l O .data 0000000000000028 __gcov_.jit_fill_hole -0000000000000160 l O .data 0000000000000028 __gcov_.add_2mod +0000000000000300 l O .data 0000000000000028 __gcov_.bpf_jit_free +0000000000000220 l O .data 0000000000000028 __gcov_.bpf_int_jit_compile +00000000000001e0 l O .data 0000000000000028 __gcov_.bpf_jit_compile +00000000000001a0 l O .data 0000000000000028 __gcov_.do_jit +0000000000000160 l O .data 0000000000000028 __gcov_.jit_fill_hole 0000000000000120 l O .data 0000000000000028 __gcov_.bpf_jit_dump 00000000000000e0 l O .data 0000000000000028 __gcov_.bpf_prog_unlock_free 00000000000000a0 l O .data 0000000000000028 __gcov_.__get_order @@ -43,17 +41,17 @@ 0000000000000000 *UND* 0000000000000000 memset 0000000000000000 *UND* 0000000000000000 sk_load_half 0000000000000000 *UND* 0000000000000000 printk +0000000000000000 *UND* 0000000000000000 sk_load_byte +0000000000000000 *UND* 0000000000000000 sk_load_word 0000000000000000 *UND* 0000000000000000 sk_load_half_positive_offset 0000000000000000 *UND* 0000000000000000 sk_load_half_negative_offset -0000000000000000 *UND* 0000000000000000 sk_load_word_positive_offset -0000000000000000 *UND* 0000000000000000 sk_load_byte 0000000000000000 *UND* 0000000000000000 sk_load_byte_positive_offset 0000000000000000 *UND* 0000000000000000 sk_load_byte_negative_offset -0000000000000000 *UND* 0000000000000000 sk_load_word +0000000000000000 *UND* 0000000000000000 sk_load_word_positive_offset 0000000000000000 *UND* 0000000000000000 __bpf_call_base 0000000000000000 *UND* 0000000000000000 sk_load_word_negative_offset -00000000000030d0 g F .text 0000000000000013 bpf_jit_compile -00000000000030f0 g F .text 0000000000000352 bpf_int_jit_compile +0000000000002630 g F .text 0000000000000013 bpf_jit_compile +0000000000002650 g F .text 0000000000000352 bpf_int_jit_compile 0000000000000000 g O .data..read_mostly 0000000000000004 bpf_jit_enable 0000000000000000 *UND* 0000000000000000 __kmalloc 0000000000000000 *UND* 0000000000000000 bpf_jit_binary_alloc @@ -62,7 +60,7 @@ 0000000000000000 *UND* 0000000000000000 set_memory_ro 0000000000000000 *UND* 0000000000000000 kfree 0000000000000000 *UND* 0000000000000000 bpf_jit_binary_free -0000000000003450 g F .text 000000000000008c bpf_jit_free +00000000000029b0 g F .text 000000000000008c bpf_jit_free 0000000000000000 *UND* 0000000000000000 set_memory_rw 0000000000000000 *UND* 0000000000000000 __bpf_prog_free 0000000000000000 *UND* 0000000000000000 __gcov_init -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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