Jim


On Monday, July 22, 2002, at 05:36  PM, 
gdb-patches-digest-help@sources.redhat.com wrote:

>
>
> I would like to have your opinion on the following issue. This is more
> spectacular on Tru64, but the idea behind this can be reproduced on any
> system.
>
> Given the following simple program:
>           1       #include <stdio.h>
>           2
>           3       void
>           4       hello (void)
>           5       {
>           6         printf ("Hello world.\n");
>           7       }
>           8
>           9       int
>           10      main (void)
>           11      {
>           12        hello ();
>           13        return 0;
>           14      }
> (compiled using "gcc -g hello.c -o hello")
>
> The program does not stop if I put a breakpoint at line 4 before 
> running it:
>
>          (gdb) b hello.c:4
>          Breakpoint 1 at 0x120001150: file hello.c, line 4.
>          (gdb) run
>          Starting program: /usr/prague.a/brobecke/skip_prologue/hello
>          Hello world.
>
>          Program exited normally.
>          (gdb)
>
> On the other hand, if I use the function name to put the breakpoint,
> then the program stops:
>
>          (gdb) b hello
>          Breakpoint 2 at 0x120001168: file hello.c, line 6.
>          (gdb) run
>          Starting program: /usr/prague.a/brobecke/skip_prologue/hello
>
>          Breakpoint 2, hello () at hello.c:6
>          6         printf ("Hello world.\n");
>          (gdb)
>
> It is more spectacular in the Tru64 case, because the Tru64 linker
> performs some optimization by default that often cause the first few
> instructions to be skipped (usually the instruction loading the gp).
> A disass in function main() shows this:
>
>          0x1200011b0 <main+24>:  bsr     ra,0x120001158 <hello+8>
>
> But the problem is a bit more general that this:
>
>   - "break function-name" causes GDB to skip the function prologue
>   - On the other hand, "break file:line_num" does not cause GDB to skip
>     the function prologue
>
> Some of our users have been confused by this, mostly because they use a
> graphical front-end where it is so easy to click to put a breakpoint on
> a given line that they sometimes don't know or want to know that there
> are other ways to insert breakpoints.
>
> Some of our users thought that breakpoint 1 and 2 above where
> equivalent, and where therefore surprised to see that their function
> parameters had junk values. Once you know that in case of breakpoint 1,
> the prologue has not been executed, it is easy to figure out that the
> parameter homing had not taken place yet, hence the incorrect values.
>
> In our experience, the only case when a user don't want to skip the
> function prologue is when doing instruction-level debugging. So, we are
> considering changing the behavior of the "break file:line-num" command
> to behave like "break function-name", that is slightly offset the
> breakpoint address to skip the prologue.
>
> That will fix the Tru64 problem, and we believe that it will make GDB
> more user-friendly by making breakpoint 1 and 2 equivalent. I would 
> like
> to have your opinion on this.
>
> Interestingly enough, I had made a prototype change that was adding 
> this
> capability to GDB due to another problem (it was a compiler deficiency
> that I wanted to work-around in GDB). I never finished this work 
> because
> I convinced myself that it was better to fix the compiler (which, due 
> to
> lack of time, I haven't done yet :-). I am attaching this patch to this
> mail just as a reference. It probably needs a bit of dusting off, and
> some touch-ups before it is suitable for submission if the GDB 
> community
> like the idea.
>
> -- 
> Joel
>
--
Jim Ingham                                   jingham@apple.com
Developer Tools - gdb
Apple Computer