[Bug binutils/20838] Strange disassembly for 4d in x86-64 mode

[jamesmikedupont at googlemail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20838

--- Comment #2 from James Michael DuPont <jamesmikedupont at googlemail dot 
com> ---
It did not disasm the first instruction as a byte, it subsumed it, the 4d is
lost.

Here are some permutations over the data bytes with different lengths and
values to show what is outputted.

Also the output "rex.WRB add %r8b,(%r8)"  does not compile with gas.



    a.out:     file format elf64-x86-64


    Disassembly of section .text:

    0000000000000000 <.test1>:
           0:   4d                      rex.WRB

    0000000000000003 <.test3>:
           3:   4d 00 00                rex.WRB add %r8b,(%r8)

    0000000000000015 <.test7>:
          15:   4d 00 00                rex.WRB add %r8b,(%r8)
          18:   00 00                   add    %al,(%rax)
          1a:   01                      .byte 0x1

    000000000000001b <.test8>:
          1b:   4d 00 00                rex.WRB add %r8b,(%r8)
          1e:   00 01                   add    %al,(%rcx)

    0000000000000030 <.test12>:
          30:   4d 00 00                rex.WRB add %r8b,(%r8)
          33:   01 00                   add    %eax,(%rax)

    000000000000003b <.test14>:
          3b:   4d 00 00                rex.WRB add %r8b,(%r8)
          3e:   01 00                   add    %eax,(%rax)
          40:   01                      .byte 0x1

    0000000000000041 <.test15>:
          41:   4d 00 00                rex.WRB add %r8b,(%r8)
          44:   01 01                   add    %eax,(%rcx)

    000000000000004c <.test17>:
          4c:   4d 00 00                rex.WRB add %r8b,(%r8)
          4f:   01 01                   add    %eax,(%rcx)
          51:   01                      .byte 0x1

    0000000000000052 <.test18>:
          52:   4d 00 01                rex.WRB add %r8b,(%r9)

    00000000000000a1 <.test33>:
          a1:   4d                      rex.WRB
          a2:   01                      .byte 0x1

    00000000000000a3 <.test34>:
          a3:   4d 01 00                add    %r8,(%r8)

    00000000000000b5 <.test38>:
          b5:   4d 01 00                add    %r8,(%r8)
          b8:   00 00                   add    %al,(%rax)
          ba:   01                      .byte 0x1

    00000000000000bb <.test39>:
          bb:   4d 01 00                add    %r8,(%r8)
          be:   00 01                   add    %al,(%rcx)

    0000000000000141 <.test64>:
         141:   4d                      rex.WRB
         142:   02                      .byte 0x2

    0000000000000143 <.test65>:
         143:   4d 02 00                rex.WRB add (%r8),%r8b

    00000000000001e3 <.test96>:
         1e3:   4d 03 00                add    (%r8),%r8

    0000000000000281 <.test126>:
         281:   4d                      rex.WRB
         282:   04                      .byte 0x4

    0000000000000283 <.test127>:
         283:   4d 04 00                rex.WRB add $0x0,%al

    0000000000000295 <.test131>:
         295:   4d 04 00                rex.WRB add $0x0,%al
         298:   00 00                   add    %al,(%rax)
         29a:   01                      .byte 0x1

    0000000000000321 <.test157>:
         321:   4d                      rex.WRB
         322:   05                      .byte 0x5


    000000000000032f <.test161>:
         32f:   4d 05 00 00 00 00       rex.WRB add $0x0,%rax

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