Re: [PATCH v2 3/4] gdb/infrun: handle stepping through functions with DW_AT_trampoline

[Eli Zaretskii <eliz@gnu.org>]

> Cc: abdul.b.ijaz@intel.com, JiniSusan.George@amd.com, tom@tromey.com,
>  Nils-Christian Kempke <nils-christian.kempke@intel.com>
> Date: Mon,  5 Jun 2023 13:04:09 +0200
> From: Abdul Basit Ijaz via Gdb-patches <gdb-patches@sourceware.org>
> 
>  gdb/NEWS                                      |  14 +
>  gdb/doc/gdb.texinfo                           |  36 +++
>  gdb/infrun.c                                  |  81 +++++-
>  gdb/infrun.h                                  |   4 +
>  .../gdb.dwarf2/dw2-function-trampolines.c     |  80 ++++++
>  .../gdb.dwarf2/dw2-function-trampolines.exp   | 245 ++++++++++++++++++
>  6 files changed, 456 insertions(+), 4 deletions(-)
>  create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-function-trampolines.c
>  create mode 100644 gdb/testsuite/gdb.dwarf2/dw2-function-trampolines.exp

Thanks.

> diff --git a/gdb/NEWS b/gdb/NEWS
> index c0aac212e30..d349791de62 100644
> --- a/gdb/NEWS
> +++ b/gdb/NEWS
> @@ -342,6 +342,20 @@ GDB now supports floating-point on LoongArch GNU/Linux.
>  
>  * New commands
>  
> +set skip-trampoline-functions on|off
> +show skip-trampoline-functions
> +  This controls whether GDB's stepping behavior will recognize function calls
> +  that have been marked as trampolines in the debug info.  It improves
> +  stepping behavior in that it steps through trampoline code and hides it from
> +  the user.  GDB can now step through trampolines that are correctly marked as
> +  such in the compiler's debug info.  If the target of a trampoline is unknown,
> +  GDB will continue until the trampoline section is left again and only then
> +  hand control back to the user.  GDB does this even if the trampoline has no
> +  associated line info.  If this is turned off, GDB will step into trampolines
> +  if there is line table information for them or step over the trampoline calls
> +  if there is no line table information.  Currently, only DWARF trampolines
> +  are supported.

This part is OK, but I'd suggest to say what is the default (I'm
guessing ON).

> --- a/gdb/doc/gdb.texinfo
> +++ b/gdb/doc/gdb.texinfo
> @@ -6366,6 +6366,42 @@ debug information.  This is the default.
>  Show whether @value{GDBN} will stop in or step over functions without
>  source line debug information.
>  
> +@kindex set skip-trampoline-functions
> +@item set skip-trampoline-functions
> +@cindex trampoline functions
> +@cindex stepping through trampoline functions
> +@itemx set skip-trampoline-functions on

Please move all the index entries before all the @item/@itemx.  This
is so index-search in an Info reader will place the reader before the
text, not in its middle.

> +When calling a function in any language, some compilers might generate
> +so-called trampoline functions, which wrap the actual function call (the

"trampoline functions" should be in @dfn, since you are introducing
new terminology.

> +target of the trampoline).  The compiler might mark such a trampoline in its
> +debug information.  Often, such trampolines do not have any source line
> +information associated with them which will lead the @code{step} command to
> +behave like a @code{next} and skip the function call completely.
> +
> +The @code{set skip-trampoline-functions on} command will cause the @code{step}
> +command to treat these trampolines differently.  When issuing a @code{step} at
> +the call site of a trampoline function if @code{skip-trampoline-functions} is
> +set @value{GDBN} will attempt to determine the target of the trampoline and
> +then step through the trampoline stopping at the target.  If the target could
> +not be found or was not given in the debug info, @value{GDBN} will simply
> +continue execution until it leaves the trampoline code again, even if the
> +trampoline has no line info associated with it.  When returning from a target
> +function call and stepping back into the trampoline, @value{GDBN} will again
> +step through the trampoline towards the call site.
> +Additionally, even if stopped in a trampoline function with source line
> +information, issuing a @code{step} will prompt @value{GDBN} to resume execution
> +until leaving the trampoline region again.  The @code{stepi} command is not
> +affected by the setting which is enabled by default.  Currently, only
> +DWARF trampolines marked via DW_AT_trampoline are supported by this.

Again, this should say what is the default of this setting.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>