Add negative repeat count to 'x' command

This change adds support for specifying a negative repeat count to
all the formats of the 'x' command to examine memory backward.
A new testcase 'examine-backward' is added to cover this new feature.

Here's the example output from the new feature:

<format 'i'>
(gdb) bt
#0  Func1 (n=42, p=0x40432e "hogehoge") at main.cpp:5
#1  0x00000000004041fa in main (argc=1, argv=0x7fffffffdff8) at main.cpp:19
(gdb) x/-4i 0x4041fa
  0x4041e5 <main(int, char**)+11>: mov   %rsi,-0x10(%rbp)
  0x4041e9 <main(int, char**)+15>: lea   0x13e(%rip),%rsi
  0x4041f0 <main(int, char**)+22>: mov   $0x2a,%edi
  0x4041f5 <main(int, char**)+27>: callq 0x404147

<format 'x'>
(gdb) x/-4xw 0x404200
0x4041f0 <main(int, char**)+22>: 0x00002abf 0xff4de800 0x76e8ffff 0xb8ffffff
(gdb) x/-4
0x4041e0 <main(int, char**)+6>:  0x7d8910ec 0x758948fc 0x358d48f0 0x0000013e

gdb/ChangeLog:

	* NEWS: Mention that GDB now supports a negative repeat count in
	the 'x' command.
	* printcmd.c (decode_format): Allow '-' in the parameter
	"string_ptr" to accept a negative repeat count.
	(find_instruction_backward): New function.
	(read_memory_backward): New function.
	(integer_is_zero): New function.
	(find_string_backward): New function.
	(do_examine): Use new functions to examine memory backward.
	(_initialize_printcmd): Mention that 'x' command supports a negative
	repeat count.

gdb/doc/ChangeLog:

	* gdb.texinfo (Examining Memory): Document negative repeat
	count in the 'x' command.

gdb/testsuite/ChangeLog:

	* gdb.base/examine-backward.c: New file.
	* gdb.base/examine-backward.exp: New file.

1 files changed, 324 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.base/examine-backward.exp b/gdb/testsuite/gdb.base/examine-backward.exp
new file mode 100644
index 0000000000..e03cbfd752
--- /dev/null
+++ b/gdb/testsuite/gdb.base/examine-backward.exp
@@ -0,0 +1,324 @@
+# Copyright 2015-2016 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 testsuite is to test examining memory backward by specifying a negative
+# number in the 'x' command.
+
+standard_testfile
+if { [prepare_for_testing "failed to prepare for examine-backward" \
+        ${testfile} ${srcfile}] } {
+    return -1
+}
+
+if ![runto_main] {
+    untested "could not run to main"
+    return -1
+}
+
+proc get_first_mapped_address {} {
+    global gdb_prompt
+
+    set addr "0"
+    gdb_test_multiple "info proc mappings" "info proc mappings" {
+        -re "objfile\[\r\n\t \]+(0x\[0-9a-fA-F\]+).*\[\r\n\]*$gdb_prompt $" {
+            set addr $expect_out(1,string)
+        }
+        -re "$gdb_prompt $" {
+            unsupported "Current target does not support 'info proc mappings'"
+        }
+    }
+    return ${addr}
+}
+
+with_test_prefix "invalid format" {
+    gdb_test "x/- 10xb main" "Invalid number \"10xb\"\." \
+        "a whitespace after a leading hyphen"
+    gdb_test "x/--10xb main" "Invalid number \"10xb\"\." \
+        "double hyphen"
+    gdb_test "x/-a10xb main" "Invalid number \"10xb\"\." \
+        "an alphabet after a leading hyphen"
+    gdb_test_no_output "x/-0i main" "zero with backward disassemble"
+    gdb_test_no_output "x/-0sh main" "zero with backward examine string"
+}
+
+with_test_prefix "memory page boundary" {
+    set boundary [get_first_mapped_address]
+    if {![is_address_zero_readable] && $boundary != 0} {
+        gdb_test_no_output "set print elements 0"
+        gdb_test_sequence "x/3s ${boundary}" "take 3 strings forward" {
+            "0x"
+            "0x"
+            "0x"
+        }
+        gdb_test_sequence "x/-4s" "take 4 strings backward" {
+            "Cannot access memory at address 0x"
+            "0x"
+            "0x"
+            "0x"
+        }
+        gdb_test_sequence "x/3s ${boundary}" "take 3 strings forward again" {
+            "0x"
+            "0x"
+            "0x"
+        }
+        gdb_test_sequence "x/-3s" "take 3 strings backward" {
+            "Cannot access memory at address 0x"
+            "0x"
+            "0x"
+            "0x"
+        }
+    }
+}
+
+with_test_prefix "address zero boundary" {
+    if {[is_address_zero_readable]} {
+        set address_zero "0x0"
+        set byte "\t0x\[0-9a-f\]+"
+        gdb_test "x/3xb ${address_zero}" \
+            "0x\[0-9a-f\]+00.*:${byte}${byte}${byte}" \
+            "examine 3 bytes forward from ${address_zero}"
+        gdb_test "x/-6x" \
+            "0x\[0-9a-f\]+fd.*:${byte}${byte}${byte}${byte}${byte}${byte}" \
+            "examine 6 bytes backward"
+        gdb_test "x/-3x ${address_zero}" \
+            "0x\[0-9a-f\]+fd.*:${byte}${byte}${byte}" \
+            "examine 3 bytes backward from ${address_zero}"
+    }
+}
+
+gdb_test_no_output "set charset ASCII"
+
+with_test_prefix "char-width=1, print-max=20" {
+    gdb_test_no_output "set print elements 20"
+    gdb_test_sequence "x/6s &TestStrings" "take 6 strings forward" {
+        "\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
+        "\"UVWXYZ\""
+        "\"\""
+        "\"\""
+        "\"[^\"]+\""
+        "\"01234567890123456789\"\.\.\."
+    }
+    gdb_test "x/-1xb" "0x39" "take 1 char backward"
+    gdb_test_sequence "x/-6s" "take 6 strings backward" {
+        "\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
+        "\"UVWXYZ\""
+        "\"\""
+        "\"\""
+        "\"[^\"]+\""
+        "\"01234567890123456789\"\.\.\."
+    }
+    gdb_test_sequence "x/6s &TestStrings" "take 6 strings forward again" {
+        "\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
+        "\"UVWXYZ\""
+        "\"\""
+        "\"\""
+        "\"[^\"]+\""
+        "\"01234567890123456789\"\.\.\."
+    }
+    gdb_test "x/-xb" "0x39" "take 1 char backward again"
+    gdb_test "x/-s" "\"01234567890123456789\"\.\.\." \
+        "take 1 string backward (1/6)"
+    gdb_test "x/-s" "\".+\"" \
+        "take 1 string backward (2/6)"
+    gdb_test "x/-s" "\"\"" \
+        "take 1 string backward (3/6)"
+    gdb_test "x/-s" "\"\"" \
+        "take 1 string backward (4/6)"
+    gdb_test "x/-s" "\"GHIJKLMNOPQRSTUVWXYZ\"" \
+        "take 1 string backward (5/6)"
+    gdb_test "x/-s" "\"ABCDEFGHIJKLMNOPQRST\"\.\.\." \
+        "take 1 string backward (6/6)"
+}
+
+with_test_prefix "char-width=2, print-max=20" {
+    gdb_test_no_output "set print elements 20"
+    gdb_test_sequence "x/6sh &TestStringsH" "take 6 strings forward" {
+        "u\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
+        "u\"UVWXYZ\""
+        "u\"\""
+        "u\"\""
+        "u\"[^\"]+\""
+        "u\"01234567890123456789\"\.\.\."
+    }
+    gdb_test "x/-1xh" "0x0039" "take 1 char backward"
+    gdb_test_sequence "x/-6sh" "take 6 strings backward" {
+        "u\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
+        "u\"UVWXYZ\""
+        "u\"\""
+        "u\"\""
+        "u\"[^\"]+\""
+        "u\"01234567890123456789\"\.\.\."
+    }
+    gdb_test_sequence "x/6sh &TestStringsH" "take 6 strings forward again" {
+        "u\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
+        "u\"UVWXYZ\""
+        "u\"\""
+        "u\"\""
+        "u\"[^\"]+\""
+        "u\"01234567890123456789\"\.\.\."
+    }
+    gdb_test "x/-xh" "0x0039" "take 1 char backward again"
+    gdb_test "x/-sh" "u\"01234567890123456789\"\.\.\." \
+        "take 1 string backward (1/6)"
+    gdb_test "x/-sh" "u\".+\"" \
+        "take 1 string backward (2/6)"
+    gdb_test "x/-sh" "u\"\"" \
+        "take 1 string backward (3/6)"
+    gdb_test "x/-sh" "u\"\"" \
+        "take 1 string backward (4/6)"
+    gdb_test "x/-sh" "u\"GHIJKLMNOPQRSTUVWXYZ\"" \
+        "take 1 string backward (5/6)"
+    gdb_test "x/-sh" "u\"ABCDEFGHIJKLMNOPQRST\"\.\.\." \
+        "take 1 string backward (6/6)"
+}
+
+with_test_prefix "char-width=4, print-max=20" {
+    gdb_test_no_output "set print elements 20"
+    gdb_test_sequence "x/6sw &TestStringsW" "take 6 strings forward" {
+        "U\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
+        "U\"UVWXYZ\""
+        "U\"\""
+        "U\"\""
+        "U\"[^\"]+\""
+        "U\"01234567890123456789\"\.\.\."
+    }
+    gdb_test "x/-1xw" "0x00000039" "take 1 char backward"
+    gdb_test_sequence "x/-6sw" "take 6 strings backward" {
+        "U\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
+        "U\"UVWXYZ\""
+        "U\"\""
+        "U\"\""
+        "U\"[^\"]+\""
+        "U\"01234567890123456789\"\.\.\."
+    }
+    gdb_test_sequence "x/6sw &TestStringsW" "take 6 strings forward again" {
+        "U\"ABCDEFGHIJKLMNOPQRST\"\.\.\."
+        "U\"UVWXYZ\""
+        "U\"\""
+        "U\"\""
+        "U\"[^\"]+\""
+        "U\"01234567890123456789\"\.\.\."
+    }
+    gdb_test "x/-xw" "0x00000039" "take 1 char backward again"
+    gdb_test "x/-sw" "U\"01234567890123456789\"\.\.\." \
+        "take 1 string backward (1/6)"
+    gdb_test "x/-sw" "U\".+\"" \
+        "take 1 string backward (2/6)"
+    gdb_test "x/-sw" "U\"\"" \
+        "take 1 string backward (3/6)"
+    gdb_test "x/-sw" "U\"\"" \
+        "take 1 string backward (4/6)"
+    gdb_test "x/-sw" "U\"GHIJKLMNOPQRSTUVWXYZ\"" \
+        "take 1 string backward (5/6)"
+    gdb_test "x/-sw" "U\"ABCDEFGHIJKLMNOPQRST\"\.\.\." \
+        "take 1 string backward (6/6)"
+}
+
+with_test_prefix "char-width=2, print-max=0" {
+    gdb_test_no_output "set print elements 0"
+    gdb_test_sequence "x/6sh &TestStringsH" "take 6 strings forward" {
+        "u\"ABCDEFGHIJKLMNOPQRSTUVWXYZ\""
+        "u\"\""
+        "u\"\""
+        "u\"\\\\x307b\\\\x3052\\\\x307b\\\\x3052\""
+        "u\"012345678901234567890123456789\""
+        "u\"!!!!!!\""
+    }
+    gdb_test "x/-4xh" "0x0021\[\t \]+0x0021\[\t \]+0x0021\[\t \]+0x0000" \
+        "take 4 characters backward"
+    gdb_test_sequence "x/-6sh" "take 6 strings backward" {
+        "u\"ABCDEFGHIJKLMNOPQRSTUVWXYZ\""
+        "u\"\""
+        "u\"\""
+        "u\"[^\"]+\""
+        "u\"012345678901234567890123456789\""
+        "u\"!!!!!!\""
+    }
+    gdb_test_sequence "x/6sh &TestStringsH" "take 6 strings forward again" {
+        "u\"ABCDEFGHIJKLMNOPQRSTUVWXYZ\""
+        "u\"\""
+        "u\"\""
+        "u\"\\\\x307b\\\\x3052\\\\x307b\\\\x3052\""
+        "u\"012345678901234567890123456789\""
+        "u\"!!!!!!\""
+    }
+    gdb_test "x/-xh" "0x0000" "take 1 char backward"
+    gdb_test "x/-sh" "u\"!!!!!!\"" \
+        "take 1 string backward (1/6)"
+    gdb_test "x/-sh" "u\"012345678901234567890123456789\"" \
+        "take 1 string backward (2/6)"
+    gdb_test "x/-sh" "u\".+\"" \
+        "take 1 string backward (3/6)"
+    gdb_test "x/-sh" "u\"\"" \
+        "take 1 string backward (4/6)"
+    gdb_test "x/-sh" "u\"\"" \
+        "take 1 string backward (5/6)"
+    gdb_test "x/-sh" "u\"ABCDEFGHIJKLMNOPQRSTUVWXYZ\"" \
+        "take 1 string backward (6/6)"
+}
+
+with_test_prefix "char-width=1, print-max=4" {
+    gdb_test_no_output "set print elements 4"
+    gdb_test_sequence "x/9s &TestStrings" "take 9 strings forward" {
+        "\"ABCD\"\.\.\."
+        "\"EFGH\"\.\.\."
+        "\"IJKL\"\.\.\."
+        "\"MNOP\"\.\.\."
+        "\"QRST\"\.\.\."
+        "\"UVWX\"\.\.\."
+        "\"YZ\""
+        "\"\""
+        "\"\""
+    }
+    gdb_test "x/-xb" "0x00" "take 1 byte backward"
+    gdb_test_sequence "x/-4s" "take 4 strings backward (1/2)" {
+        "\"TUVW\"\.\.\."
+        "\"XYZ\""
+        "\"\""
+        "\"\""
+    }
+    gdb_test_sequence "x/-4s" "take 4 strings backward (2/2)" {
+        "\"CDEF\"\.\.\."
+        "\"GHIJ\"\.\.\."
+        "\"KLMN\"\.\.\."
+        "\"OPQR\"\.\.\."
+    }
+}
+
+with_test_prefix "backward disassemble general" {
+    set length_to_examine {1 2 3 4 10}
+    set disassmbly {}
+
+    gdb_test "x/i main" "0x\[0-9a-fA-F\]+ <main>:\t.*" \
+        "move the current position to main (x/i)"
+    gdb_test "x/-i" "0x\[0-9a-fA-F\]+ <main>:\t.*" \
+        "move the current position to main (x/-i)"
+    for {set i 0} {$i < [llength $length_to_examine]} {incr i} {
+        set len [lindex $length_to_examine $i]
+        set instructions [capture_command_output "x/${len}i" ""]
+        lappend disassmbly $instructions
+    }
+    for {set i 0} {$i < [llength $length_to_examine]} {incr i} {
+        set idx [expr [llength $length_to_examine] - $i - 1]
+        set len [lindex $length_to_examine $idx]
+        set actual [capture_command_output "x/-${len}i" ""]
+        set expected [lindex $disassmbly $idx]
+        if {$actual == $expected} {
+            pass "inst:$idx"
+        } else {
+            fail "inst:$idx"
+        }
+    }
+}