Re: [patch] measurements, numbers about CONFIG_OPTIMIZE_INLINING=y impact

Nicholas Miell
2009-01-09T20:17:56Z

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@comcast.net>