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Date: Mon, 18 Jan 2016 14:45:48 -0700 From: Jeff Merkey <linux.mdb@...il.com> To: LKML <linux-kernel@...r.kernel.org> Cc: Thomas Gleixner <tglx@...utronix.de>, Andy Lutomirski <luto@...capital.net>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Andy Lutomirski <luto@...nel.org>, Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>, Steven Rostedt <rostedt@...dmis.org>, Borislav Petkov <bp@...en8.de>, Jiri Olsa <jolsa@...nel.org> Subject: Re: x86_64 Compiler Output Kernel Bloat v4.4 On 1/18/16, Jeff Merkey <linux.mdb@...il.com> wrote: > On 1/18/16, Jeff Merkey <linux.mdb@...il.com> wrote: >> Hi, >> >> I noticed that in the assembler output for the x86_64 builds almost >> every single function originating from C code has a nop instruction >> that prefaces the function call. I guess the concern with this is >> the wasted space issue as each one of these placeholders takes up a >> bunch of bytes at the head of each function. Is there a reason this >> assembler header is there in the first place to anyones knowledge? >> Since every single function just about is prefaced by this inert 5 >> byte instruction it adds up to quite a bit of bloat in the size of the >> linux executable. >> >> 0xffffffffa073e010 0F1F440000 nop DWORD PTR [rax+rax]=0x0 >> >> The intel assembler format shows the bytes that comprise each >> instruction. The GDB format does not. Both are provided. >> >> 0xffffffffa073e050 4155 push r13 >> (0)> id mdb_watchdogs >> mdb|mdb_watchdogs: >> 0xffffffffa073e010 mdb_watchdogs: nopl 0x0(%rax,%rax,1)) << >> 0xffffffffa073e015 mdb_watchdogs+0x5: push %rbp >> 0xffffffffa073e016 mdb_watchdogs+0x6: mov %rsp,%rbp >> 0xffffffffa073e019 mdb_watchdogs+0x9: callq 0xffffffff811337e0 >> touch_softlockup_watchdog_sync >> 0xffffffffa073e01e mdb_watchdogs+0xe: callq 0xffffffff810f0ba0 >> clocksource_touch_watchdog >> 0xffffffffa073e023 mdb_watchdogs+0x13: callq 0xffffffff810dea20 >> rcu_cpu_stall_reset >> 0xffffffffa073e028 mdb_watchdogs+0x18: callq 0xffffffff811337c0 >> touch_nmi_watchdog >> 0xffffffffa073e02d mdb_watchdogs+0x1d: pop %rbp >> 0xffffffffa073e02e mdb_watchdogs+0x1e: data16 >> 0xffffffffa073e030 mdb_watchdogs+0x20: retq >> 0xffffffffa073e031 mdb_watchdogs+0x21: nopw %cs:0x0(%rax,%rax,1)) >> mdb|mdb: >> 0xffffffffa073e040 mdb: nopl 0x0(%rax,%rax,1)) << >> 0xffffffffa073e045 mdb+0x5: push %rbp >> 0xffffffffa073e046 mdb+0x6: mov %rsp,%rbp >> 0xffffffffa073e049 mdb+0x9: push %r15 >> 0xffffffffa073e04b mdb+0xb: push %r14 >> 0xffffffffa073e04d mdb+0xd: mov %rdi,%r14 >> 0xffffffffa073e050 mdb+0x10: push %r13 >> (0)> u mdb_watchdogs >> mdb|mdb_watchdogs: >> 0xffffffffa073e010 0F1F440000 nop DWORD PTR [rax+rax]=0x0 << >> 0xffffffffa073e015 55 push rbp >> 0xffffffffa073e016 4889E5 mov rbp,rsp >> 0xffffffffa073e019 E8C2579FE0 call touch_softlockup_watchdog_sync >> 0xffffffffa073e01e E87D2B9BE0 call clocksource_touch_watchdog >> 0xffffffffa073e023 E8F8099AE0 call rcu_cpu_stall_reset >> 0xffffffffa073e028 E893579FE0 call touch_nmi_watchdog >> 0xffffffffa073e02d 5D pop rbp >> 0xffffffffa073e02e 6690 data16 >> 0xffffffffa073e030 C3 ret >> 0xffffffffa073e031 6666666666662E0F1F840000000000 nop cs:WORD PTR >> [rax+rax]=0x0000 >> mdb|mdb: >> 0xffffffffa073e040 0F1F440000 nop DWORD PTR [rax+rax]=0x0 << >> 0xffffffffa073e045 55 push rbp >> 0xffffffffa073e046 4889E5 mov rbp,rsp >> 0xffffffffa073e049 4157 push r15 >> 0xffffffffa073e04b 4156 push r14 >> 0xffffffffa073e04d 4989FE mov r14,rdi >> 0xffffffffa073e050 4155 push r13 >> (0)> g >> >> Jeff >> > > I think xor eax,eax is a lot shorter and fewer bytes. > > Jeff > That's xor rax,rax
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