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Re: [PATCH] TRACING: Fix a copmile warning
- From: Ian Lance Taylor <iant at google dot com>
- To: Steven Rostedt <rostedt at goodmis dot org>
- Cc: Arnaud Lacombe <lacombar at gmail dot com>, gcc-help at gcc dot gnu dot org, stufever at gmail dot com, linux-kernel at vger dot kernel dot org, Wang Shaoyan <wangshaoyan dot pt at taobao dot com>, Frederic Weisbecker <fweisbec at gmail dot com>, Ingo Molnar <mingo at redhat dot com>
- Date: Mon, 25 Jul 2011 22:55:52 -0700
- Subject: Re: [PATCH] TRACING: Fix a copmile warning
- References: <1310982010-13849-1-git-send-email-wangshaoyan.pt@taobao.com> <1311618747.3526.32.camel@gandalf.stny.rr.com> <CACqU3MVJ9Oj4pXH9O_2jnqDVBqKA+AgoWHwP-n2HGD4LU0pMHQ@mail.gmail.com> <1311625197.3526.35.camel@gandalf.stny.rr.com> <CACqU3MUKU_0FbOQT+TKLnnaoFVS_QOgzzx-tvunQy94v-eykTw@mail.gmail.com> <CACqU3MXO_oR+8Ac2Z7wSji0sUeu-2FwskmxZxtY9NV+XpJXuwA@mail.gmail.com> <mcrfwltoov5.fsf@coign.corp.google.com> <1311642628.3526.58.camel@gandalf.stny.rr.com>
Steven Rostedt <rostedt@goodmis.org> writes:
> Compiling with -O2 (which gives no warning) (x86_64) produces:
>
> 0000000000000000 <fn>:
> 0: 48 83 ec 08 sub $0x8,%rsp
> 4: e8 00 00 00 00 callq 9 <fn+0x9>
> 5: R_X86_64_PC32 e-0x4
> 9: 48 85 c0 test %rax,%rax
> c: 74 1a je 28 <fn+0x28>
> e: e8 00 00 00 00 callq 13 <fn+0x13>
> f: R_X86_64_PC32 f-0x4
> 13: 48 85 c0 test %rax,%rax
> 16: 74 10 je 28 <fn+0x28>
> 18: 48 83 c4 08 add $0x8,%rsp
> 1c: e9 00 00 00 00 jmpq 21 <fn+0x21>
> 1d: R_X86_64_PC32 g-0x4
> 21: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
> 28: 48 83 c4 08 add $0x8,%rsp
> 2c: c3 retq
>
> and compiling with -Os:
>
> 0000000000000000 <fn>:
> 0: 55 push %rbp
> 1: 53 push %rbx
> 2: 51 push %rcx
> 3: e8 00 00 00 00 callq 8 <fn+0x8>
> 4: R_X86_64_PC32 e-0x4
> 8: 48 85 c0 test %rax,%rax
> b: 48 89 c3 mov %rax,%rbx
> e: 74 08 je 18 <fn+0x18>
> 10: e8 00 00 00 00 callq 15 <fn+0x15>
> 11: R_X86_64_PC32 f-0x4
> 15: 48 89 c5 mov %rax,%rbp
> 18: 48 85 ed test %rbp,%rbp
> 1b: 74 0d je 2a <fn+0x2a>
> 1d: 48 85 db test %rbx,%rbx
> 20: 74 08 je 2a <fn+0x2a>
> 22: 5a pop %rdx
> 23: 5b pop %rbx
> 24: 5d pop %rbp
> 25: e9 00 00 00 00 jmpq 2a <fn+0x2a>
> 26: R_X86_64_PC32 g-0x4
> 2a: 58 pop %rax
> 2b: 5b pop %rbx
> 2c: 5d pop %rbp
> 2d: c3 retq
>
> Which is 1 byte more than -O2. I would think that -Os would be smaller.
Ideally, it should be, yes. The -Os code would be smaller except that
it needs to save a register across a function call, which forces it to
push and pop %rbx, which in turn means that stack alignment adds yet
another push and two pop instructions. It's a heuristic failure.
Ian