Re: Re: Wrong debug info for argc at -O2

[杨已彪 <2019010291@hust.edu.cn>]

I am very sorry that I forgot to reply this. 

I am using Ubuntu 20.04 with x86_64. 

$ uname -a
Linux Lab 5.4.0-26-generic #30-Ubuntu SMP Mon Apr 20 16:58:30 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

"Luis Machado" <luis.machado@linaro.org>wrote:
> On 5/29/20 12:58 PM, 杨已彪 wrote:
> > 
> > 
> > Consider test-case:
> > ...
> > $ cat small.c
> > #include <stdio.h>
> > 
> > int main(int argc, char **argv) {
> >    char buf[6];
> >    char c[] = "abc";
> >    sprintf(buf, (char *)c, 1);
> > 
> >    return 0;
> > }
> > ...
> > 
> > 
> > Stepping with step and print the values of arguments:
> > ...
> > $ gcc -O2 -g small.c; gdb -q a.out
> > Reading symbols from a.out...
> > (gdb) b main
> > Breakpoint 1 at 0x401040: file small.c, line 5.
> > (gdb) r
> > Starting program: /home/yibiao/Debugger/a.out
> > 
> > Breakpoint 1, main (argc=1, argv=0x7fffffffdff8) at small.c:5
> > 5      char c[] = "abc";
> > (gdb) info args argc
> > argc = 1
> > (gdb) step
> > 6      sprintf(buf, (char *)c, 1);
> > (gdb) info args argc
> > argc = -8454
> > (gdb)
> 
> What architecture is this?
> 
> If you want precise debug information, you should use -O0 instead. With 
> O2 you are likely to run into situations where the debug info has been 
> lost or is just incorrect.
> 
> With that said, GCC has improved over the past few years in terms of 
> debug info generation for O2+.