Re: [PATCH] Fix gdb.dwarf2/dwz.exp for thumb mode

[Doug Evans <dje@google.com>]

On Tue, Sep 2, 2014 at 6:15 PM, Yao Qi <yao@codesourcery.com> wrote:
> Hi,
> We see the fail in gdb.dwarf2/dwz.exp in thumb mode,
>
> p the_int^M
> $2 = 99^M
> (gdb) FAIL: gdb.dwarf2/dwz.exp: p the_int
>
> In thumb mode, the lsb of references to 'main' in the assembly
> (produced by dwarf assember) is set, so the generated debug
> information is incorrect.
>
> This patch copies the approach from
>
>   [PATCH 4/4] Fix gdb.trace/entry-values.exp for thumb mode
>   https://sourceware.org/ml/gdb-patches/2014-07/msg00041.html
>
> introduce new label 'main_start' which is the correct
> function address (without lsb set).  This patch fixes these
> fails we've seen.
>
> Regression tested on arm-none-eabi and x86_64-linux.
>
> gdb/testsuite:
>
> 2014-09-03  Yao Qi  <yao@codesourcery.com>
>
>         * gdb.dwarf2/dwz.exp: Use main_start instead of main.
>         * gdb.dwarf2/main.c: Define label 'main_start' at the beginning
>         function 'main'.
> ---
>  gdb/testsuite/gdb.dwarf2/dwz.exp | 4 ++--
>  gdb/testsuite/gdb.dwarf2/main.c  | 4 ++++
>  2 files changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/gdb/testsuite/gdb.dwarf2/dwz.exp b/gdb/testsuite/gdb.dwarf2/dwz.exp
> index 9175f9e..ef19631 100644
> --- a/gdb/testsuite/gdb.dwarf2/dwz.exp
> +++ b/gdb/testsuite/gdb.dwarf2/dwz.exp
> @@ -79,8 +79,8 @@ Dwarf::assemble $asm_file {
>         partial_label: partial_unit {} {
>             subprogram {
>                 {name main}
> -               {low_pc main addr}
> -               {high_pc "main + $main_length" addr}
> +               {low_pc main_start addr}
> +               {high_pc "main_start + $main_length" addr}
>             }
>         }
>      }
> diff --git a/gdb/testsuite/gdb.dwarf2/main.c b/gdb/testsuite/gdb.dwarf2/main.c
> index 3ddd194..5a4cbc3 100644
> --- a/gdb/testsuite/gdb.dwarf2/main.c
> +++ b/gdb/testsuite/gdb.dwarf2/main.c
> @@ -17,6 +17,10 @@
>
>  /* Dummy main function.  */
>
> +asm (".section \".text\"");
> +asm (".balign 8");
> +asm ("main_start: .globl main_start");
> +
>  int
>  main()
>  {

This approach is fragile w.r.t. clang.
IIUC, Clang makes no guarantees about the ordering of global asms and
other objects, or even other objects w.r.t. each other (even at -O0).

The email you refer to above refers to another email,
https://sourceware.org/ml/gdb-patches/2014-03/msg00202.html
and that email does a similar thing to fix dw2-ifort-parameter.exp
(not for thumb, but for similar reasons).

dw2-ifort-parameter.exp currently fails with clang on amd64-linux because
func gets moved after main, but the asms that wrap func stay put.

Loosely speaking,

asm ("func_start:");
void func () {}
asm ("func_end:");
int main () { return 0; }

gets emitted as:

asm ("func_start:");
asm ("func_end:");
int main () { return 0; }
void func () {}

I don't have a solution yet, I'm just giving a "heads up".