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

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.
+
 maint set backtrace-on-fatal-signal on|off
 maint show backtrace-on-fatal-signal
   This setting is 'on' by default.  When 'on' GDB will print a limited
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 9c0018ea5c1..44aba03fc44 100644
--- 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
+When calling a function in any language, some compilers might generate
+so-called trampoline functions, which wrap the actual function call (the
+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.
+
+@item set skip-trampoline-functions off
+Causes the @code{step} command to completely ignore any trampoline information
+a compiler might have emitted in its debug info.  Trampolines will be treated
+like any other function when stepping.
+
+@item show skip-trampoline-functions
+Show whether @value{GDBN} tries to skip trampolines or not.
+
 @kindex finish
 @kindex fin @r{(@code{finish})}
 @item finish
diff --git a/gdb/infrun.c b/gdb/infrun.c
index 9c81521683c..278393192a4 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -161,6 +161,12 @@  static ptid_t previous_inferior_ptid;
 
 static bool detach_fork = true;
 
+/* If set (default) GDB will step through functions/inlined subroutines marked
+   DW_AT_trampoline by the compiler.  If false, GDB will ignore the
+   attribute.  */
+
+static bool skip_trampoline_functions = true;
+
 bool debug_infrun = false;
 static void
 show_debug_infrun (struct ui_file *file, int from_tty,
@@ -7047,7 +7053,6 @@  process_event_stop_test (struct execution_control_state *ecs)
 		  != find_pc_function (ecs->event_thread->stop_pc ())))))
     {
       CORE_ADDR stop_pc = ecs->event_thread->stop_pc ();
-      CORE_ADDR real_stop_pc;
 
       infrun_debug_printf ("stepped into subroutine");
 
@@ -7119,8 +7124,41 @@  process_event_stop_test (struct execution_control_state *ecs)
 	 calling routine and the real function), locate the real
 	 function.  That's what tells us (a) whether we want to step
 	 into it at all, and (b) what prologue we want to run to the
-	 end of, if we do step into it.  */
-      real_stop_pc = skip_language_trampoline (frame, stop_pc);
+	 end of, if we do step into it.  For functions marked as
+	 trampoline functions we try to find their target and step
+	 towards it (if skip_trampoline_functions is not set to false by the
+	 user).  If no target can be determined we just step into the
+	 trampoline and hand control back to the user.  */
+      CORE_ADDR real_stop_pc = 0;
+      bool in_trampoline = skip_trampoline_functions
+			   && in_trampoline_function (stop_pc);
+
+      if (in_trampoline)
+	{
+	  real_stop_pc = find_function_trampoline_target (stop_pc);
+
+	  for (int i = 0; i < MAX_TRAMPOLINE_CHAIN_SIZE
+			  && in_trampoline_function (real_stop_pc); ++i)
+	    {
+		real_stop_pc = find_function_trampoline_target (real_stop_pc);
+		/* Exit if find_function_trampoline_target failed to find the
+		   trampoline target.  Do not try to resolve the trampolines
+		   in this case.  */
+		if (real_stop_pc == 0x0)
+		  break;
+	    }
+
+	  /* If we failed to find a target we will just single step in the
+	     hope of leaving the trampoline again soon.  */
+	  if (real_stop_pc == 0x0)
+	    {
+	      keep_going (ecs);
+	      return;
+	    }
+	}
+
+      if (real_stop_pc == 0)
+	real_stop_pc = skip_language_trampoline (frame, stop_pc);
       if (real_stop_pc == 0)
 	real_stop_pc = gdbarch_skip_trampoline_code (gdbarch, frame, stop_pc);
       if (real_stop_pc != 0)
@@ -7141,6 +7179,8 @@  process_event_stop_test (struct execution_control_state *ecs)
       /* If we have line number information for the function we are
 	 thinking of stepping into and the function isn't on the skip
 	 list, step into it.
+	 If we are about to step into a function marked trampoline with no
+	 line number information, we still want to enter it here.
 
 	 If there are several symtabs at that PC (e.g. with include
 	 files), just want to know whether *any* of them have line
@@ -7149,7 +7189,9 @@  process_event_stop_test (struct execution_control_state *ecs)
 	struct symtab_and_line tmp_sal;
 
 	tmp_sal = find_pc_line (ecs->stop_func_start, 0);
-	if (tmp_sal.line != 0
+	if ((tmp_sal.line != 0
+	     || (skip_trampoline_functions
+		 && in_trampoline_function (ecs->stop_func_start)))
 	    && !function_name_is_marked_for_skip (ecs->stop_func_name,
 						  tmp_sal)
 	    && !inline_frame_is_marked_for_skip (true, ecs->event_thread))
@@ -7284,6 +7326,19 @@  process_event_stop_test (struct execution_control_state *ecs)
       return;
     }
 
+  /* If we ended up in a function trampoline without stepping into a new
+     function we are either in some inlined trampoline or returning through a
+     trampoline function.  In either case we continue to single step until we
+     are out of the trampoline code again.  This check has to be done before
+     stop_pc_sal.line == 0 below, as trampolines usually don't have source
+     line information associated with them.  */
+  if (skip_trampoline_functions && in_trampoline_function (stop_pc_sal.pc))
+    {
+      infrun_debug_printf ("stepped into trampoline code");
+      keep_going (ecs);
+      return;
+    }
+
   if (stop_pc_sal.line == 0)
     {
       /* We have no line number information.  That means to stop
@@ -9504,6 +9559,14 @@  show_exec_direction_func (struct ui_file *out, int from_tty,
   }
 }
 
+static void
+show_skip_trampoline_functions (ui_file *file, int from_tty,
+				cmd_list_element *c,
+				const char *value)
+{
+  gdb_printf (file, _("Skipping trampoline functions is %s.\n"), value);
+}
+
 static void
 show_schedule_multiple (struct ui_file *file, int from_tty,
 			struct cmd_list_element *c, const char *value)
@@ -9849,6 +9912,16 @@  Options are 'forward' or 'reverse'."),
 			set_exec_direction_func, show_exec_direction_func,
 			&setlist, &showlist);
 
+  add_setshow_boolean_cmd ("skip-trampoline-functions", class_run,
+			  &skip_trampoline_functions, _("\
+Set whether gdb attempts to hide trampolines marked in the debug info."), _("\
+Show whether gdb attempts to hide trampolines marked in the debug info."), _("\
+If on, while stepping gdb will skip through functions and inlined functions\n\
+marked as trampolines by the compiler.  If off, gdb will ignore such function\n\
+trampolines."),
+			  nullptr, show_skip_trampoline_functions, &setlist,
+			  &showlist);
+
   /* Set/show detach-on-fork: user-settable mode.  */
 
   add_setshow_boolean_cmd ("detach-on-fork", class_run, &detach_fork, _("\
diff --git a/gdb/infrun.h b/gdb/infrun.h
index 43fd1b44f5a..55dd0672895 100644
--- a/gdb/infrun.h
+++ b/gdb/infrun.h
@@ -76,6 +76,10 @@  infrun_debug_show_threads (const char *title, ThreadRange threads)
 }
 
 
+/* Maximum size of trampoline chain to process while resolving
+   trampolines.  */
+#define MAX_TRAMPOLINE_CHAIN_SIZE 10
+
 /* Nonzero if we want to give control to the user when we're notified
    of shared library events by the dynamic linker.  */
 extern int stop_on_solib_events;
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-function-trampolines.c b/gdb/testsuite/gdb.dwarf2/dw2-function-trampolines.c
new file mode 100644
index 00000000000..0336710e130
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-function-trampolines.c
@@ -0,0 +1,80 @@ 
+/* Copyright 2022 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 test relies on inlined_trampoline being inlined into main and the other
+   functions not.  All functions except target will be marked via
+   DW_AT_trampoline in the debug info and we'll check whether one can step
+   through the trampolines towards target.  */
+volatile int global_var;
+
+int __attribute__ ((noinline))
+target ()					/* target decl line */
+{						/* target prologue */
+  asm ("target_label: .globl target_label");
+  ++global_var;					/* target add */
+  asm ("target_label2: .globl target_label2");
+  return 9 + 10;				/* target return */
+}						/* target end */
+
+int __attribute__ ((noinline))
+trampoline ()
+{						/* trampoline prologue */
+  asm ("trampoline_label: .globl trampoline_label");
+  ++global_var;
+  return target ();				/* trampoline target call */
+}						/* trampoline end */
+
+static inline int __attribute__ ((always_inline))
+inlined_trampoline ()
+{						/* inlined_trampoline prologue */
+  asm ("inlined_trampoline_label: .globl inlined_trampoline_label");
+  ++global_var;					/* inlined_trampoline add */
+  asm ("inlined_trampoline_label2: .globl inlined_trampoline_label2");
+  return target ();				/* inlined_trampoline target call */
+}						/* inlined_trampoline end */
+
+int __attribute__ ((noinline))
+chained_trampoline ()
+{						/* chained_trampoline prologue */
+  asm ("chained_trampoline_label: .globl chained_trampoline_label");
+  ++global_var;
+  return trampoline ();				/* chained_trampoline trampoline call */
+}						/* chained_trampoline end */
+
+int __attribute__ ((noinline))
+doubly_chained_trampoline ()
+{						/* doubly_chained_trampoline prologue */
+  asm ("doubly_chained_trampoline_label: .globl doubly_chained_trampoline_label");
+  ++global_var;
+  return chained_trampoline ();			/* doubly_chained_trampoline chained_trampoline call */
+}						/* doubly_chained_trampoline end */
+
+int
+main ()						/* main decl line */
+{						/* main prologue */
+  int ans;
+  asm ("main_label: .globl main_label");
+  global_var = 0;				/* main set global_var */
+  asm ("main_label2: .globl main_label2");
+  ans = inlined_trampoline ();			/* main call inlined_trampoline */
+  asm ("main_label3: .globl main_label3");
+  ans = trampoline ();				/* main call trampoline */
+  asm ("main_label4: .globl main_label4");
+  ans = chained_trampoline ();			/* main call chained_trampoline */
+  asm ("main_label5: .globl main_label5");
+  ans = doubly_chained_trampoline ();		/* main call doubly_chained_trampoline */
+  asm ("main_label6: .globl main_label6");
+  return ans;					/* main call return */
+}						/* main end */
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-function-trampolines.exp b/gdb/testsuite/gdb.dwarf2/dw2-function-trampolines.exp
new file mode 100644
index 00000000000..0d1126c226c
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-function-trampolines.exp
@@ -0,0 +1,245 @@ 
+# Copyright 2019-2022 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 test checks GDB's handling of functions/inlined functions marked
+# DW_AT_trampoline by the compiler.  A function marked as trampoline should
+# generally be hidden from the user.  We check whether we can step through
+# trampolines.  Every trampoline is defined using a different type for its
+# target: a string, an address, a DIE reference or a flag.
+# Setting skip-trampoline-functions to false inside GDB should make it return
+# to its 'normal' behavior, ignore the DW_AT_trampoline, and skip all of the
+# non-inlined trampoline calls (as their DIEs don't have any source
+# information).
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile .c .S
+
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    global srcdir subdir srcfile
+    declare_labels lines_label trampoline_label
+
+    get_func_info target
+    get_func_info trampoline
+    get_func_info chained_trampoline
+    get_func_info doubly_chained_trampoline
+    get_func_info main
+
+    set target_decl_line [gdb_get_line_number "target decl line"]
+    set main_decl_line [gdb_get_line_number "main decl line"]
+    set main_call_inlined_trampoline_line [gdb_get_line_number "main call inlined_trampoline"]
+
+    cu {} {
+	compile_unit {
+	    {language @DW_LANG_C}
+	    {name dw2-function-trampolines.c}
+	    {low_pc 0 addr}
+	    {stmt_list ${lines_label} DW_FORM_sec_offset}
+	} {
+	    subprogram {
+		{name target}
+		{low_pc $target_start addr}
+		{high_pc "$target_start + $target_len" addr}
+		{decl_file 1 data1}
+		{decl_line $target_decl_line data1}
+	    }
+	    # The 'trampoline' subprogram declares its target by name.
+	    trampoline_label: subprogram {
+		    {name trampoline}
+		    {low_pc $trampoline_start addr}
+		    {high_pc "$trampoline_start + $trampoline_len" addr}
+		    {trampoline target string}
+		}
+	    # The 'chained_trampoline' subprogram declares its target as die
+	    # reference.
+	    subprogram {
+		{name chained_trampoline}
+		{low_pc $chained_trampoline_start addr}
+		{high_pc "$chained_trampoline_start + $chained_trampoline_len" addr}
+		{trampoline %$trampoline_label}
+	    }
+	    # The 'doubly_chained_trampoline' subprogram declares no target.
+	    # Its DW_AT_trampoline is a flag set to true.
+	    subprogram {
+		{name doubly_chained_trampoline}
+		{low_pc $doubly_chained_trampoline_start addr}
+		{high_pc "$doubly_chained_trampoline_start + $doubly_chained_trampoline_len" addr}
+		{trampoline 1 flag}
+	    }
+	    subprogram {
+		{external 1 flag}
+		{name main}
+		{main_subprogram 1 flag}
+		{low_pc $main_start addr}
+		{high_pc "$main_start + $main_len" addr}
+		{decl_file 1 data1}
+		{decl_line $main_decl_line data1}
+	    } {
+		# The 'inlined_trampoline' subroutine declares its target as
+		# an address.
+		inlined_subroutine {
+		    {name inlined_trampoline}
+		    {low_pc main_label2 addr}
+		    {high_pc main_label3 addr}
+		    {trampoline $target_start addr}
+		    {call_file 1 data1}
+		    {call_line $main_call_inlined_trampoline_line data1}
+		}
+	    }
+	}
+    }
+
+    lines {version 2} lines_label {
+	include_dir "${srcdir}/${subdir}"
+	file_name "$srcfile" 1
+
+	program {
+	    DW_LNE_set_address $main_start
+	    line [gdb_get_line_number "main set global_var"]
+	    DW_LNS_copy
+	    DW_LNE_set_address main_label
+	    line [gdb_get_line_number "main set global_var"]
+	    DW_LNS_copy
+	    DW_LNE_set_address main_label2
+	    line [gdb_get_line_number "main call inlined_trampoline"]
+	    DW_LNS_copy
+	    DW_LNE_set_address inlined_trampoline_label
+	    line [gdb_get_line_number "inlined_trampoline add"]
+	    DW_LNS_copy
+	    DW_LNE_set_address inlined_trampoline_label2
+	    line [gdb_get_line_number "inlined_trampoline target call"]
+	    DW_LNS_copy
+	    DW_LNE_set_address main_label3
+	    line [gdb_get_line_number "main call trampoline"]
+	    DW_LNS_copy
+	    DW_LNE_set_address main_label4
+	    line [gdb_get_line_number "main call chained_trampoline"]
+	    DW_LNS_copy
+	    DW_LNE_set_address main_label5
+	    line [gdb_get_line_number "main call doubly_chained_trampoline"]
+	    DW_LNS_copy
+	    DW_LNE_set_address main_label6
+	    line [gdb_get_line_number "main call return"]
+	    DW_LNS_copy
+	    DW_LNE_set_address $main_end
+	    DW_LNE_end_sequence
+
+	    DW_LNE_set_address $target_start
+	    line [gdb_get_line_number "target prologue"]
+	    DW_LNS_negate_stmt
+	    DW_LNS_copy
+	    DW_LNE_set_address target_label
+	    line [gdb_get_line_number "target add"]
+	    DW_LNS_negate_stmt
+	    DW_LNS_copy
+	    DW_LNE_set_address target_label2
+	    line [gdb_get_line_number "target return"]
+	    DW_LNS_copy
+	    DW_LNE_set_address $target_end
+	    DW_LNE_end_sequence
+	}
+    }
+}
+
+if {[prepare_for_testing "failed to prepare" ${testfile} \
+	[list $srcfile $asm_file] {nodebug additional_flags=-O0}]} {
+    return -1
+}
+
+set target_first_line_pattern ".*target add.*"
+set target_second_line_pattern ".*target return.*"
+
+if ![runto_main] {
+    return -1
+}
+
+gdb_test "show skip-trampoline-functions" \
+    "Skipping trampoline functions is on\." \
+    "check skip-trampoline-functions is enabled"
+
+with_test_prefix "with trampoline handling" {
+    foreach {trampoline return_line} { "inlined_trampoline" "trampoline" \
+	"trampoline" "chained_trampoline" \
+	"chained_trampoline" "doubly_chained_trampoline" } {
+
+	gdb_test "s" "$target_first_line_pattern" "step through $trampoline"
+	gdb_test "s" "$target_second_line_pattern" \
+	    "step target second line from $trampoline"
+	gdb_test "s" ".*main call $return_line.*" \
+	    "step back through $trampoline"
+    }
+
+    # The doubly_chained_trampoline has only been marked as trampoline but no
+    # target was given.  In this case GDB steps into the trampoline and then
+    # continues until the trampoline section is left again.
+
+    # When compiled with gcc 7.5 (and possibly others) on a 32 bit system, the
+    # trampoline function contains a call to __x86.get_pc_thunk.ax before the
+    # actual target call.  So, we end up in __x86.get_pc_thunk.ax.  Issuing a
+    # second step command will return from the function call back into the
+    # trampoline and go on inside the trampoline towards the actual target call.
+    # On other targets we step directly towards the target call.
+    gdb_test_multiple "s" "step through double_chained_trampoline" {
+	-re -wrap "$target_first_line_pattern" {
+	    pass $gdb_test_name
+	}
+	-re -wrap ".*__x86.get_pc_thunk.ax.*" {
+	    gdb_test "s" "$target_first_line_pattern" \
+		"step through double_chained_trampoline 2nd try"
+	}
+    }
+    gdb_test "s" "$target_second_line_pattern" \
+	"step target second line fromdoubly_chained_trampoline"
+    gdb_test "s" ".*main call return.*" \
+	"step back through doubly_chained_trampoline"
+}
+
+clean_restart ${testfile}
+
+if ![runto_main] {
+    return -1
+}
+
+gdb_test_no_output "set skip-trampoline-functions off" \
+    "disable trampoline handling"
+gdb_test "show skip-trampoline-functions" \
+    "Skipping trampoline functions is off." \
+    "check skip-trampoline-functions is disabled"
+
+with_test_prefix "without trampoline handling" {
+    gdb_test "s" ".*main call inlined_trampoline.*"
+    gdb_test "s" ".*inlined_trampoline add.*" \
+	"step into inlined_trampoline with skip-trampoline off"
+    gdb_test "s" ".*inlined_trampoline target call.*" \
+	"step in inlined_trampoline with skip-trampoline off"
+    gdb_test "s" "$target_first_line_pattern" \
+	"step into target with skip-trampoline off"
+    gdb_test "s" "$target_second_line_pattern" \
+	"step second line in target with skip-trampoline off"
+    gdb_test "s" ".*main call trampoline.*" \
+	"step brack from target with skip-trampoline off"
+    gdb_test "s" ".*main call chained_trampoline.*" \
+	"skip trampoline call with no line info"
+    gdb_test "s" ".*main call doubly_chained_trampoline.*" \
+	"skip chained_trampoline call with no line info"
+    gdb_test "s" ".*main call return.*" \
+	"skip doubly_chained_trampoline call with no line info"
+}