gdb/testsuite: Add test to step through function epilogue

The testsuite implicitly tests GDB's ability to step through epilogues
in multiple tests, without doing it explicitly anywhere.  This is
unfortunate, as clang does not emit epilogue information, so using clang
on our testsuite makes many tests fail.  This patch adds a central,
explicit test for walking through the epilogue so we can safely remove
this from other tests and have them working with clang.

The test created attempts to step through a simple epilogue, an
epilogue that ends on another epilogue, and epilogues leading to other
function calls.

gdb/testsuite/gdb.base/step-through-epilogue.c

@@ -0,0 +1,38 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 1992-2022 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/>.  */
+
+int
+multiply (int a, int b)
+{
+  return a * b;
+} /* Epilogue line of multiply.  */
+
+int
+square (int x)
+{
+  return multiply (x, x);
+} /* Epilogue line of square.  */
+
+int
+main(void)
+{
+  int x;
+  x = multiply (1, 2);
+  x = square (2);
+  x = multiply (square (1), square (2));
+  return 0;
+}

gdb/testsuite/gdb.base/step-through-epilogue.exp

@@ -0,0 +1,98 @@
+#   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 is used to confirm that GDB is able to step, stopping at an
+# epilogue line, then step out of the function.
+
+standard_testfile
+
+if { ![have_epilogue_line_info] } {
+    untested "This test doesn't work with this compiler"
+    return
+}
+
+if { [prepare_for_testing "failed to prepare" $testfile \
+			  {step-through-epilogue.c}] } {
+    untested "failed to prepare"
+    return
+}
+
+if { ![runto_main] } {
+    untested "couldn't run to main"
+    return
+}
+
+proc step_till_epilogue_multiply {} {
+    gdb_test "step" ".*return a . b;.*" "step into multiply"
+    gdb_test "step" \
+	     "$::decimal\\s+\\\}\[^\r\n\]+Epilogue line of multiply.*" \
+	     "stop at the epilogue of multiply"
+}
+
+proc step_till_epilogue_square {} {
+    gdb_test "step" ".*return multiply.*" "step into square"
+    step_till_epilogue_multiply
+    gdb_test "step" \
+	     "$::decimal\\s+\\\}\[^\r\n\]+Epilogue line of square.*" \
+	     "stop at epilogue of square"
+}
+
+with_test_prefix "multiply" {
+    step_till_epilogue_multiply
+    gdb_test "step" "x = square \\(2\\);" "leave function"
+}
+
+with_test_prefix "square" {
+    step_till_epilogue_square
+    gdb_test "step" ".*x = multiply \\(square \\(1\\), square \\(2\\)\\);.*"\
+	"leave function"
+}
+
+# Some gcc versions can mess this test by requiring extra steps in a
+# few locations.  We dynamically test if we're in one such versions in the
+# the first argument, and use that to see if extra steps are needed to
+# finish the second argument
+set midway_return false
+
+with_test_prefix "square, first argument" {
+    step_till_epilogue_square
+    gdb_test_multiple "step" "step back to main" {
+	-re -wrap "multiply \\(square \\(1\\), square \\(2\\)\\);" {
+	    set midway_return true
+	    gdb_send "step\n"
+	    exp_continue
+	}
+
+	-re -wrap "return multiply.*" {
+	    pass $gdb_test_name
+	}
+    }
+}
+with_test_prefix "square, second argument" {
+    step_till_epilogue_multiply
+    gdb_test "step" \
+	     "$::decimal\\s+\\\}\[^\r\n\]+Epilogue line of square.*" \
+	     "stop at epilogue of square"
+    if {$midway_return} {
+	gdb_test "step"\
+	    ".*x = multiply \\(square \\(1\\), square \\(2\\)\\);.*"\
+	    "step back to main"
+    }
+}
+
+with_test_prefix "multiply with function args" {
+    step_till_epilogue_multiply
+    gdb_test "step" ".*return 0;.*" "leave last function"
+}