gdb: add 'maintenance print record-instruction' command

While chasing some reverse debugging bugs, I found myself wondering what
was recorded by GDB to undo and redo a certain instruction. This commit
implements a simple way of printing that information.

If there isn't enough history to print the desired instruction (such as
when the user hasn't started recording yet or when they request 2
instructions back but only 1 was recorded), GDB warns the user like so:

(gdb) maint print record-instruction
Not enough recorded history

If there is enough, GDB prints the instruction like so:

(gdb) maint print record-instruction
4 bytes of memory at address 0x00007fffffffd5dc changed from: 01 00 00 00
Register eflags changed: [ IF ]
Register rip changed: (void (*)()) 0x401115 <main+15>

Approved-by: Eli Zaretskii <eliz@gnu.org>
Reviewed-by: Alexandra Hajkova <ahajkova@redhat.com>
Reviewed-by: Lancelot Six <lsix@lancelotsix.com>
Approved-by: Tom Tromey <tom@tromey.com>

2 files changed, 100 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.reverse/maint-print-instruction.c b/gdb/testsuite/gdb.reverse/maint-print-instruction.c
new file mode 100644
index 0000000..292af6d
--- /dev/null
+++ b/gdb/testsuite/gdb.reverse/maint-print-instruction.c
@@ -0,0 +1,25 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2022-2023 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/>.  */
+
+int
+main ()
+{
+  int x = 0;
+  x ++;
+  x --;
+  return x;
+}
diff --git a/gdb/testsuite/gdb.reverse/maint-print-instruction.exp b/gdb/testsuite/gdb.reverse/maint-print-instruction.exp
new file mode 100644
index 0000000..41b2817
--- /dev/null
+++ b/gdb/testsuite/gdb.reverse/maint-print-instruction.exp
@@ -0,0 +1,75 @@
+#   Copyright 2022-2023 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 the functionality of
+# the maintenance print record-instruction command, but does not check the
+# syntax, only if the command finds or fails to find recorded history.
+# This is done by putting the inferior in mulpitle states with and without
+# history to be printed, then checking if GDB is able to print an
+# instruction or not.
+# To identify if GDB has printed an instruction, we can see if some
+# change is printed, since any instruction must have at least a change
+# to the PC.
+
+if ![supports_reverse] {
+    return
+}
+
+standard_testfile
+
+if { [prepare_for_testing "failed to prepare" $testfile $srcfile] } {
+    return -1
+}
+
+proc test_print { has_history level test_name } {
+    gdb_test_multiple "maint print record-instruction $level" $test_name {
+	-re -wrap ".*Not enough recorded history.*" {
+	    gdb_assert !$has_history $test_name
+	}
+
+    -re -wrap ".*changed.*" {
+	    gdb_assert $has_history $test_name
+	}
+    }
+}
+
+if { ![runto_main] } {
+    return 0
+}
+
+#confirm that GDB doesn't go crazy if recording isn't enabled
+test_print false "" "print before starting to record"
+
+if ![supports_process_record] {
+    # No point in testing the rest if we can't record anything
+    return
+}
+
+gdb_test_no_output "record" "turn on process record"
+
+test_print false "" "print before any instruction"
+
+gdb_test "stepi 3" ".*" "collecting history"
+test_print true "" "print current after executing a bit"
+test_print true "-1" "print previous after executing a bit"
+test_print false "1" "print following after executing a bit"
+
+gdb_test "reverse-stepi" ".*" "moving back"
+test_print true "" "print current after reversing"
+test_print true "-1" "print previous after reversing"
+test_print true "1" "print following after reversing"
+
+test_print false "-10" "trying to print too far back"
+test_print false "10" "trying to print too far forward"