Re: [PATCH] gdb/record: print frame information when exiting a recursive call

[Kevin Buettner <kevinb@redhat.com>]

Hi Guinevere,

Just a few nits.  See below...

On Sat, 23 Sep 2023 12:34:58 +0200
Guinevere Larsen via Gdb-patches <gdb-patches@sourceware.org> wrote:

> Currently,  when GDB is reverse stepping out of a function into the same
> function due to a recursive call, it doesn't print frame information, as
> reported by PR record/29178. This happens because when the inferior
> leaves the current frame, GDB decides to refresh the step information,
> clobbering the original step_frame_id, making it impossible to figure
> out later on that the frame has been changed.
> 
> This commit changes GDB so that, if we notice we're in this exact
> situation, we won't refresh the step information.
> 
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29178
> ---
>  gdb/infrun.c                            | 18 +++++++++
>  gdb/testsuite/gdb.reverse/recursion.c   | 38 +++++++++++++++++++
>  gdb/testsuite/gdb.reverse/recursion.exp | 49 +++++++++++++++++++++++++
>  3 files changed, 105 insertions(+)
>  create mode 100644 gdb/testsuite/gdb.reverse/recursion.c
>  create mode 100644 gdb/testsuite/gdb.reverse/recursion.exp
> 
> diff --git a/gdb/infrun.c b/gdb/infrun.c
> index 4730d290442..00e6215ebc8 100644
> --- a/gdb/infrun.c
> +++ b/gdb/infrun.c
> @@ -7679,6 +7679,11 @@ process_event_stop_test (struct execution_control_state *ecs)
>      }
>  
>    bool refresh_step_info = true;
> +
> +  /* shorthand to make if statements smaller.  */

Capitalize "shorthand".

> +  struct frame_id original_frame_id
> +    = ecs->event_thread->control.step_frame_id;
> +  struct frame_id curr_frame_id = get_frame_id (get_current_frame ());

I think these could be used to simplify at least one other, already
existing, if-statement too.  Perhaps post another patch with that
change?  (Or make it a two-part series with the above addition w/
updates to existing code as part 1.)

>    if ((ecs->event_thread->stop_pc () == stop_pc_sal.pc)
>        && (ecs->event_thread->current_line != stop_pc_sal.line
>  	  || ecs->event_thread->current_symtab != stop_pc_sal.symtab))
> @@ -7722,6 +7727,19 @@ process_event_stop_test (struct execution_control_state *ecs)
>  			       "it's not the start of a statement");
>  	}
>      }
> +  else if (execution_direction == EXEC_REVERSE
> +	  && curr_frame_id != original_frame_id
> +	  && original_frame_id.code_addr_p && curr_frame_id.code_addr_p
> +	  && original_frame_id.code_addr == curr_frame_id.code_addr)
> +    {
> +      /* If we enter here, we're leaving a recursive function call.  In this
> +	 situation, we shouldn't refresh the step information, because if we
> +	 do, we'll lose the frame_id of when we started stepping, and this
> +	 will make GDB not know we need to print frame information.  */
> +      refresh_step_info = false;
> +      infrun_debug_printf ("reverse stepping, left a recursive call, don't "
> +			   "update step info so we remember we left a frame");
> +    }
>  
>    /* We aren't done stepping.
>  
> diff --git a/gdb/testsuite/gdb.reverse/recursion.c b/gdb/testsuite/gdb.reverse/recursion.c
> new file mode 100644
> index 00000000000..747404ce22c
> --- /dev/null
> +++ b/gdb/testsuite/gdb.reverse/recursion.c
> @@ -0,0 +1,38 @@
> +/* This testcase is part of GDB, the GNU debugger.
> +
> +   Copyright 2023 Free Software Foundation, Inc.
> +
> +   This program is free software; you can redistribute it and/or modify
> +   it under the terms of the GNU General Public License as published by
> +   the Free Software Foundation; either version 3 of the License, or
> +   (at your option) any later version.
> +
> +   This program is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +   GNU General Public License for more details.
> +
> +   You should have received a copy of the GNU General Public License
> +   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
> +
> +/* Test GDB's ability to handle recursive functions when executing
> +   in reverse.  */
> +
> +int
> +foo (int x) {
> +    if (x) return foo(x-1);
> +    return 0;
> +}
> +
> +int
> +bar(int x){
> +    int r = foo(x);
> +    return 2*r;
> +}
> +
> +int
> +main() {
> +    int i = bar(5);
> +    int j = foo(5);
> +    return 0;			/* END OF MAIN */
> +}

Unless there's a good reason not to do so, I'd like to see the above
C code follow the GNU coding standard.

> diff --git a/gdb/testsuite/gdb.reverse/recursion.exp b/gdb/testsuite/gdb.reverse/recursion.exp
> new file mode 100644
> index 00000000000..331113bee0a
> --- /dev/null
> +++ b/gdb/testsuite/gdb.reverse/recursion.exp
> @@ -0,0 +1,49 @@
> +# Copyright 2008-2023 Free Software Foundation, Inc.
> +
> +# This program is free software; you can redistribute it and/or modify
> +# it under the terms of the GNU General Public License as published by
> +# the Free Software Foundation; either version 3 of the License, or
> +# (at your option) any later version.
> +#
> +# This program is distributed in the hope that it will be useful,
> +# but WITHOUT ANY WARRANTY; without even the implied warranty of
> +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +# GNU General Public License for more details.
> +#
> +# You should have received a copy of the GNU General Public License
> +# along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
> +
> +# This file is part of the GDB testsuite.  It tests reverse stepping.
> +# Lots of code borrowed from "step-test.exp".
> +
> +#
> +# Test step and next in reverse
> +#
> +
> +require supports_reverse
> +
> +standard_testfile
> +
> +if { [prepare_for_testing "failed to prepare" $testfile $srcfile] } {
> +    return -1
> +}
> +
> +runto_main
> +
> +if [supports_process_record] {
> +    # Activate process record/replay
> +    gdb_test_no_output "record" "turn on process record"
> +}
> +
> +set end_of_program [gdb_get_line_number "END OF MAIN" "$srcfile"]
> +gdb_breakpoint $end_of_program
> +gdb_continue_to_breakpoint ".*$srcfile/$end_of_program.*"
> +
> +## test if GDB can reverse over a recursive program
> +gdb_test "reverse-next" ".*int j = foo.*" "Skipping recursion from outside"
> +## setup and next over a recursion for inside a recursive call
> +repeat_cmd_until "reverse-step" ".*" ".*foo .x=4.*"
> +gdb_test "reverse-next" ".*return foo.*" "Skipping recursion from inside"
> +gdb_test "reverse-next" ".*foo .x=5.*" "print frame when stepping out"
> +gdb_test "reverse-next" ".*bar .x=5.*" "stepping into a different function"
> +gdb_test "reverse-next" "main .. at .*" "stepping back to main"
> -- 
> 2.41.0
>