linux-kernel - Re: [PATCH] TRACING: Fix a copmile warning

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Date:	Mon, 25 Jul 2011 22:55:52 -0700
From:	Ian Lance Taylor <iant@...gle.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Arnaud Lacombe <lacombar@...il.com>, gcc-help@....gnu.org,
	stufever@...il.com, linux-kernel@...r.kernel.org,
	Wang Shaoyan <wangshaoyan.pt@...bao.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] TRACING: Fix a copmile warning

Steven Rostedt <rostedt@...dmis.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
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