linux-kernel - Re: [patch] measurements, numbers about CONFIG_OPTIMIZE

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Date:	Fri, 09 Jan 2009 12:17:56 -0800
From:	Nicholas Miell <nmiell@...cast.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, jim owens <jowens@...com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	paulmck@...ux.vnet.ibm.com, Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [patch] measurements, numbers about CONFIG_OPTIMIZE_INLINING=y
 impact

On Fri, 2009-01-09 at 08:28 -0800, Linus Torvalds wrote:
> 
> We get oopses that have a nice symbolic back-trace, and it reports an 
> error IN TOTALLY THE WRONG FUNCTION, because gcc "helpfully" inlined 
> things to the point that only an expert can realize "oh, the bug was 
> actually five hundred lines up, in that other function that was just 
> called once, so gcc inlined it even though it is huge".
> 
> See? THIS is the problem with gcc heuristics. It's not about quality of 
> code, it's about RELIABILITY of code. 

[bt]$ cat backtrace.c 
#include <stdlib.h>

static void called_once()
{
	abort();
}

int main(int argc, char* argv[])
{
	called_once();
	return 0;
}
[bt]$ gcc -Wall -O2 -g backtrace.c -o backtrace
[bt]$ gdb --quiet backtrace
(gdb) disassemble main 
Dump of assembler code for function main:
0x00000000004004d0 <main+0>:	sub    $0x8,%rsp
0x00000000004004d4 <called_once+0>:	callq  0x4003b8 <abort@plt>
End of assembler dump.
(gdb) run
Starting program: /home/nicholas/src/bitbucket/bt/backtrace 

Program received signal SIGABRT, Aborted.
0x0000003d9dc32f05 in raise (sig=<value optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
64	  return INLINE_SYSCALL (tgkill, 3, pid, selftid, sig);
(gdb) bt
#0  0x0000003d9dc32f05 in raise (sig=<value optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
#1  0x0000003d9dc34a73 in abort () at abort.c:88
#2  0x00000000004004d9 in called_once () at backtrace.c:5
#3  main (argc=3989, argv=0xf95) at backtrace.c:10
(gdb)


Maybe the kernel's backtrace code should be fixed instead of blaming
gcc.

-- 
Nicholas Miell <nmiell@...cast.net>

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