[Bug binutils/4218] New: objdump on AMD64 fails to decode prefixed 0x90 opcode properly.

[fruffell at cs dot uwaterloo dot ca]

On x86_64 the 0x90 opcode can be prefixed with a rexB prefix allowing it to 
access the extended register r8 (see AMD64 V3, pg. 289 and 378).  However, 
objdump incorrectly displays this as a prefixed nop:

11:  41 90 rexZ nop 

This should read:

11:  41 90 xchg %r8, %rax

If prefixed with 0x66 as well 0x41, then the instruction is accessing the word 
register r8w.  Again, objdump displays:

11:  66    data16
12:  41 90 rexZ nop

instead of:

11:  66 41 90 xchg %r8w, %ax

Other xchg opcodes 0x91-0x97 do not appear to be affected, so this is probably 
a bug related to the processor's special handling of 0x90 (e.g. no zero 
extension of the 64-bit registers for 0x90 to ensure a nop).

-- 
           Summary: objdump on AMD64 fails to decode prefixed 0x90 opcode
                    properly.
           Product: binutils
           Version: 2.17
            Status: NEW
          Severity: normal
          Priority: P2
         Component: binutils
        AssignedTo: unassigned at sources dot redhat dot com
        ReportedBy: fruffell at cs dot uwaterloo dot ca
                CC: bug-binutils at gnu dot org
 GCC build triplet: x86_64-linux-gnu
  GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu


http://sourceware.org/bugzilla/show_bug.cgi?id=4218

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