[arm] Fix displaced stepping for thumb alu reg instruction

Recent patch series "V2 All-stop on top of non-stop" causes a SIGSEGV
in the test case,

> -PASS: gdb.base/info-shared.exp: continue to breakpoint: library function #4
> +FAIL: gdb.base/info-shared.exp: continue to breakpoint: library function #4
>
> continue^M
> Continuing.^M
> ^M
> Program received signal SIGSEGV, Segmentation fault.^M
> 0x40021564 in ?? () gdb/testsuite/gdb.base/info-shared-solib1.so^M
> (gdb) FAIL: gdb.base/info-shared.exp: continue to breakpoint: library function #4

and an ARM displaced stepping bug is exposed.  It can be reproduced by
the modified gdb.arch/arm-disp-step.exp as below,

continue^M
Continuing.^M
^M
Program received signal SIGSEGV, Segmentation fault.^M
0xa713cfcc in ?? ()^M
(gdb) FAIL: gdb.arch/arm-disp-step.exp: continue to breakpoint: continue to test_add_rn_pc_end

This patch is to fix it.

gdb:

2015-04-10  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.c (install_alu_reg): Update comment.
	(thumb_copy_alu_reg): Remove local variable rn.  Update
	debugging message.  Use r2 instead of r1 in the modified
	instruction.

gdb/testsuite:

2015-04-10  Yao Qi  <yao.qi@linaro.org>

	* gdb.arch/arm-disp-step.S (main): Call test_add_rn_pc.
	(test_add_rn_pc): New function.
	* gdb.arch/arm-disp-step.exp (test_add_rn_pc): New proc.
	(top level): Invoke test_add_rn_pc.

1 files changed, 41 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.arch/arm-disp-step.exp b/gdb/testsuite/gdb.arch/arm-disp-step.exp
index 7eac4d1..e191f73 100644
--- a/gdb/testsuite/gdb.arch/arm-disp-step.exp
+++ b/gdb/testsuite/gdb.arch/arm-disp-step.exp
@@ -371,6 +371,45 @@ proc test_str_pc {} {
 	".*bx lr.*"
 }
 
+# Test 16 bit thumb instruction 'add rd, pc'.
+
+proc test_add_rn_pc {} {
+    global srcfile gdb_prompt
+
+    set test "break test_add_rn_pc"
+    gdb_test_multiple "break *test_add_rn_pc" $test {
+	-re "Breakpoint.*at.* file .*$srcfile, line.*\r\n$gdb_prompt $" {
+	    pass $test
+	}
+	-re "No symbol.*\r\n$gdb_prompt $" {
+	    return
+	}
+    }
+
+    gdb_continue_to_breakpoint "continue to test_add_rn_pc" \
+	".*mov.*r3, 4.*"
+
+    gdb_test "break *test_add_rn_pc_start" \
+	"Breakpoint.*at.* file .*$srcfile, line.*" \
+	"break test_add_rn_pc_start"
+
+    gdb_continue_to_breakpoint "continue to test_add_rn_pc_start" \
+	".*add.*r3,.*pc.*"
+
+    set pc_val [get_integer_valueof "\$pc" 0]
+
+    gdb_test "break *test_add_rn_pc_end" \
+	"Breakpoint.*at.* file .*$srcfile, line.*" \
+	"break test_add_rn_pc_end"
+
+    gdb_continue_to_breakpoint "continue to test_add_rn_pc_end" \
+	".*bx lr.*"
+
+    set r3_val [get_integer_valueof "\$r3" 0]
+    # Test the value in r3 is correct.
+    gdb_assert { [expr {$pc_val + 4 + 4} == $r3_val] }
+}
+
 # Get things started.
 
 clean_restart ${testfile}
@@ -410,6 +449,8 @@ test_pop_pc
 
 test_str_pc
 
+test_add_rn_pc
+
 ##########################################
 
 # Done, run program to exit.