[RFA 1/2] Fix decoding of ARM VFP instructions

[Tom Tromey <tom@tromey.com>]

-Wduplicated-cond pointed out that arm_record_vfp_data_proc_insn
checks "opc1 == 0x0b" twice.  I filed this a while ago as
PR tdep/20362.

Based on the ARM instruction manual at
https://www.scss.tcd.ie/~waldroj/3d1/arm_arm.pdf, I think the
instruction decoding in this function has two bugs.

First, opc1 is computed as:

  opc1 = bits (arm_insn_r->arm_insn, 20, 23);
[...]
  opc1 = opc1 & 0x04;

This means that tests like:

  else if (opc1 == 0x01)

can never be true.

In the ARM manual, "opc1" corresponds to these bits:

    name   bit
    r      20
    q      21
    D      22
    p      23

... where the D bit is not used for VFP instruction decoding.

So, I believe this code should use ~0x04 instead.

Second, VDIV is recognized by the bits "pqrs" being equal to "1000".
This tranlates to opc1 == 0x08 -- not 0x0b.  Note that pqrs==1001 is
an undefined encoding, which is probably why opc2 is not checked here;
this code doesn't seem to really deal with undefined encodings in
general, so I've left that as is.

I don't have an ARM machine or any reasonable way to test this.

ChangeLog
2018-04-21  Tom Tromey  <tom@tromey.com>

	PR tdep/20362:
	* arm-tdep.c (arm_record_vfp_data_proc_insn): Properly mask off D
	bit.  Use correct value for VDIV.
---
 gdb/ChangeLog  | 6 ++++++
 gdb/arm-tdep.c | 5 +++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
index f64df4c574..98bbb0244c 100644
--- a/gdb/arm-tdep.c
+++ b/gdb/arm-tdep.c
@@ -11420,7 +11420,8 @@ arm_record_vfp_data_proc_insn (insn_decode_record *arm_insn_r)
   opc3 = bits (arm_insn_r->arm_insn, 6, 7);
   dp_op_sz = bit (arm_insn_r->arm_insn, 8);
   bit_d = bit (arm_insn_r->arm_insn, 22);
-  opc1 = opc1 & 0x04;
+  /* Mask off the "D" bit.  */
+  opc1 = opc1 & ~0x04;
 
   /* Handle VMLA, VMLS.  */
   if (opc1 == 0x00)
@@ -11485,7 +11486,7 @@ arm_record_vfp_data_proc_insn (insn_decode_record *arm_insn_r)
         }
     }
   /* Handle VDIV.  */
-  else if (opc1 == 0x0b)
+  else if (opc1 == 0x08)
     {
       if (dp_op_sz)
         curr_insn_type = INSN_T1;
-- 
2.13.6