[Bug binutils/23033] New: gdb disassemble: ARM Thumb2 0x4503 is bad, not "cmp r3,r0"

[jreiser at BitWagon dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=23033

            Bug ID: 23033
           Summary: gdb disassemble: ARM Thumb2 0x4503 is bad, not "cmp
                    r3,r0"
           Product: binutils
           Version: 2.31 (HEAD)
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: binutils
          Assignee: unassigned at sourceware dot org
          Reporter: jreiser at BitWagon dot com
  Target Milestone: ---

The ARM Thumb2 instruction word 0x4503 is disassembled as
    cmp r3,r0
by GNU gdb (GDB) 8.1.50.20180405-git built from source:
   commit 043e9508be0a4ea7ab56fd5716e4c06d609d13be (HEAD -> master,
origin/master, origin/HEAD)
   Author: Nick Clifton <address@hidden>
   Date:   Thu Apr 5 14:44:05 2018 +0100

However the hardware documentation
   ARM Architecture Reference Manual
   ARMv8, for ARMv8-A architecture profile
   ARM DDI 0487A.f (ID032515)
   Copyright 2013-2015 ARM Limited
section F7.1.37 "CMP (register)" on page F7-2716 (.pdf page 2714)
says that 0x4503 is UNPREDICTABLE because both bit 7 and bit 6
(the 0xc0 bits) are 0, while the instruction 0x45XX as CMP
requires at least one "hi register" numbered >= 8.
The Thumb1 instruction  0x4280|(Rm<<3)|Rn  is CMP for two
"lo registers" each numbered < 8.

===== test case "4503.S"
        .thumb_func
_start: .globl _start
        .short 0x4503
        cmp r3,r0
        cmp r11,r0
=====
$ gcc -c 4503.S
$ build-gdb/gdb/gdb --data-directory=build-gdb/gdb/data-directory 4503.o
GNU gdb (GDB) 8.1.50.20180405-git

(gdb) x/3i 1
   0x1 <_start>:        cmp     r3, r0   <===== should be "bad" 
   0x3 <_start+2>:      cmp     r3, r0
   0x5 <_start+4>:      cmp     r11, r0
(gdb) x/3xh 0
0x0 <_start>:   0x4503  0x4283  0x4583
(gdb) q

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