[Lightning] support for mips32r6

[Bruno Haible]

Hi,

A libffcall bug report [1] made me aware of an incompatible change in
the MIPS ISAs. Namely, for jumping to an address given in a register,
starting with mips32r6, the existing 'jr' instruction does not work
any more: it produces an 'illegal instruction'.

There are many documents of the MIPS ISAs, and before 2014 they
described two instructions JR and JALR:
  JR <rs>           jumps to the address in register <rs>
  JALR <rd>,<rs>    jumps to the address in register <rs> and puts
                    the old PC + 8 into register <rd>
In particular,
  JR <rs>           [SPECIAL, rs, 00000, 00000, hint, 001000]
is equivalent to
  JALR $0,<rs>      [SPECIAL, rs, 00000, 00000, hint, 001001]
(because writing to $0 is a no-op).

In the documents that consider the MIPS32 R6 architecture, such as [2]
from 2014 - look at the descriptions of JR and JALR - you see that
  - in order to get the effect of JR, the bit pattern of JALR with destination
    $0 has to be used,
  - in the JALR instruction, the destination $0 is no longer allowed;
    this is merely a hint for the disassembler.

You can see that the GNU assembler has been adjusted. First, see
the different bit patterns:

$ cat foo.s
        .text
        j       $25
        nop
        jal     $0,$25
        nop
# Assemble this using binutils 2.16.1:
$ mips64-linux-as -32 -o foo.o foo.s
$ mips64-linux-objdump --disassemble foo.o

foo.o:     file format elf32-tradbigmips

Disassembly of section .text:

00000000 <.text>:
   0:   03200008        jr      t9
   4:   00000000        nop
   8:   03200009        jalr    zero,t9
   c:   00000000        nop
        ...

# Now, see how a newer GNU assembler (binutils 2.27) and option -mips32r6
# produce a different result:
$ mips64-linux-as -32 -mips32r6 -o foo.o foo.s
$ mips64-linux-objdump --disassemble foo.o         [binutils 2.16.1]

foo.o:     file format elf32-tradbigmips

Disassembly of section .text:

00000000 <.text>:
   0:   03200009        jalr    zero,t9
   4:   00000000        nop
   8:   03200009        jalr    zero,t9
   c:   00000000        nop
        ...
$ mips64-linux-objdump --disassemble foo.o         [binutils 2.27]

foo.o:     file format elf32-tradbigmips


Disassembly of section .text:

00000000 <.text>:
   0:   03200009        jr      t9
   4:   00000000        nop
   8:   03200009        jr      t9
   c:   00000000        nop
        ...


Note that the JALR $0,<rs> instruction can be used on all architectures;
I verified this by looking at old MIPS ISA documentations from 1995,
and by looking at the source code of QEMU.

Find attached a fix for GNU lightning (untested).

Bruno


[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=925129
[2] 
http://hades.mech.northwestern.edu/images/1/16/MIPS32_Architecture_Volume_II-A_Instruction_Set.pdf

mips32r6-fix.diff
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