Fix ARM stepping over Thumb-mode "bx pc" or "blx pc"

["Joseph S. Myers" <joseph@codesourcery.com>]

arm-tdep.c has code to determine the next instruction for use in
single stepping.  This code fails to handle a Thumb-mode "bx pc" or
"blx pc" correctly; it acts as if the branch target (four bytes after
the current instruction) should be in Thumb mode, when actually these
instructions switch to ARM mode (results being UNPREDICTABLE if the bx
instruction is not at a four-byte aligned address).  In particular,
this breaks stepping through PLT entries called from Thumb-mode code
(those start with a "bx pc; nop" to switch to ARM mode for the rest of
the PLT entry).

This patch fixes this by masking off the low bits from a value that
was computed with the low bit set (I followed another case that masks
off two low bits, though as noted above the results are UNPREDICTABLE
if the address has bit 1 set).  Tested with no regressions with cross
to arm-none-linux-gnueabi; the new test fails with unpatched GDB and
passes with patched GDB.  OK to commit?

2012-08-10  Joseph Myers  <joseph@codesourcery.com>

	* arm-tdep.c (thumb_get_next_pc_raw): Mask off low bits for bx pc
	and blx pc.

testsuite:
2012-08-10  Joseph Myers  <joseph@codesourcery.com>

	* gdb.arch/thumb-bx-pc.S: New file.
	* gdb.arch/thumb-bx-pc.exp: New file.

Index: gdb/arm-tdep.c
===================================================================
RCS file: /cvs/src/src/gdb/arm-tdep.c,v
retrieving revision 1.365
diff -u -r1.365 arm-tdep.c
--- gdb/arm-tdep.c	25 Jun 2012 12:32:45 -0000	1.365
+++ gdb/arm-tdep.c	10 Aug 2012 15:18:37 -0000
@@ -4541,7 +4541,7 @@
   else if ((inst1 & 0xff00) == 0x4700)	/* bx REG, blx REG */
     {
       if (bits (inst1, 3, 6) == 0x0f)
-	nextpc = pc_val;
+	nextpc = pc_val & 0xfffffffc;
       else
 	nextpc = get_frame_register_unsigned (frame, bits (inst1, 3, 6));
     }
Index: gdb/testsuite/gdb.arch/thumb-bx-pc.S
===================================================================
RCS file: gdb/testsuite/gdb.arch/thumb-bx-pc.S
diff -N gdb/testsuite/gdb.arch/thumb-bx-pc.S
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ gdb/testsuite/gdb.arch/thumb-bx-pc.S	10 Aug 2012 15:18:38 -0000
@@ -0,0 +1,34 @@
+/* Test PC adjustment from Thumb-mode "bx pc" instruction.
+
+   Copyright 2012 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   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/>.  */
+
+	.syntax unified
+	.thumb
+	.text
+	.p2align 2
+	.global	main
+	.thumb
+	.thumb_func
+	.type main, %function
+main:
+	bx	pc
+	nop
+.code 32
+	mov	r0, #0
+	bx	lr
+	.size	main, .-main
Index: gdb/testsuite/gdb.arch/thumb-bx-pc.exp
===================================================================
RCS file: gdb/testsuite/gdb.arch/thumb-bx-pc.exp
diff -N gdb/testsuite/gdb.arch/thumb-bx-pc.exp
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ gdb/testsuite/gdb.arch/thumb-bx-pc.exp	10 Aug 2012 15:18:38 -0000
@@ -0,0 +1,41 @@
+# Copyright 2012 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/>.
+
+# Test PC adjustment from Thumb-mode "bx pc" instruction.
+
+if {![istarget arm*-*]} then {
+    verbose "Skipping ARM tests."
+    return
+}
+
+set testfile "thumb-bx-pc"
+set srcfile ${testfile}.S
+set opts {}
+
+if [prepare_for_testing ${testfile}.exp ${testfile} ${srcfile} $opts] {
+    untested ${testfile}.exp
+    return -1
+}
+
+if ![runto_main] then {
+    untested ${testfile}.exp
+    return -1
+}
+
+gdb_test "stepi" "0x\[0-9a-fA-F\]+ in main \\(\\)" "stepi for bx pc"
+
+gdb_test "x /i \$pc" \
+    "0x\[0-9a-fA-F\]+ <main\\+4>:\[ \t\]+mov\[ \t\]+r0,\[ \t\]+#0.*" \
+    "stepi reached correct instruction"

-- 
Joseph S. Myers
joseph@codesourcery.com