gdb behavior on tight loop

[Ender Dai <xdai at uoregon dot edu>]

Hi,

I just find some counterintuitive (to me) behavior of gdb on tight
loop. Please see the gdb session below. When I run "next" on line 5, the
tight loop, it will take several seconds before gdb stop on line 6 and I
regain control. Strace shows that it looks like gdb is doing some magic
in every iteration.

If I set an additional breackpoint on line 6, and "continue" on line 5
instead of "next", everything looks good.

Is this supposed behavior? What is gdb doing undercover?

(There is a warning message about vdso, but google tell me that it
should be fine...)

Thanks,
Ender

$ gdb a.out 
GNU gdb (GDB) 7.6 (Debian 7.6-5)
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show
copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/ender/src/tmp/a.out...done.
(gdb) l
1     int
2     main(void)
3     {
4       int i;
5       for (i=0; i<100000; i++);
6       return 0;
7     }
(gdb) b 5
Breakpoint 1 at 0x4004b1: file loop.c, line 5.
(gdb) r
Starting program: /home/xdai/src/tmp/a.out 
warning: no loadable sections found in added symbol-file system-supplied
DSO at 0x7ffff7ffa000
warning: Could not load shared library symbols for linux-vdso.so.1.
Do you need "set solib-search-path" or "set sysroot"?

Breakpoint 1, main () at loop.c:5
5      for (i=0; i<100000; i++);
(gdb) n
6      return 0;
(gdb) 

$ uname -a
Linux cortana 3.9-1-amd64 #1 SMP Debian 3.9.8-1 x86_64 GNU/Linux

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.8.1-7'
--with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs
--enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.8 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib
--enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--enable-gnu-unique-object --enable-plugin --with-system-zlib
--disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre
--enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64
--with-arch-directory=amd64
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc
--enable-multiarch --with-arch-32=i586 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.1 (Debian 4.8.1-7)