[PATCH 4/4] Don't throw an error in 'info registers' for unavailable MIPS GP registers.

[Maciej W. Rozycki]

On Mon, 17 Apr 2017, John Baldwin wrote:

> >  That needs to be fixed then.  Previously there was no need to handle FIR 
> > specially.  Overall we need to handle configurations without FPU as well.
> 
> That might be true, but that is a larger patch (and it doesn't help with my
> kernel target case where only a subset of GPRs are valid).

 Minimising changes is not our goal though, unlike making them correct.  
And I think we need to tell apart a situation where a register (FIR) is 
invalid according to the OS ABI and where a subset of registers may not 
always be accessible.

> >  I'm somewhat put off by the truncated message in the 32-bit case though 
> > -- unless a better proposition comes up, then how about using `xxxxxxxx'
> > and `xxxxxxxxxxxxxxxx' for the 32-bit and 64-bit case respectively as with 
> > some previous effort?  What do other target backends do?
> 
> I don't disagree with the 32-bit format and am certainly open to other
> options.  Other architectures don't generally use a table and use the full
> "<unavailable>" string, e.g. on a FreeBSD 64-bit x86 kernel (which uses the
> "stock" method to print registers rather than a gdbarch-specific one):
> 
> (kgdb) info registers 
> rax            <unavailable>
> rbx            0xfffff800085cb500       -8795952728832
> rcx            <unavailable>
> rdx            <unavailable>
> rsi            <unavailable>
> rdi            <unavailable>
> rbp            0xfffffe04674819c0       0xfffffe04674819c0
> rsp            0xfffffe0467481968       0xfffffe0467481968
> r8             <unavailable>
> r9             <unavailable>
> r10            <unavailable>
> r11            <unavailable>
> r12            0xffffffff80d43b00       -2133574912
> r13            0xfffff801fb2f3000       -8787583881216
> r14            0x0      0
> r15            0xffffffff80d43b00       -2133574912
> rip            0xffffffff8058bb12       0xffffffff8058bb12 <sched_switch+1218>
> eflags         <unavailable>
> cs             0x20     32
> ss             0x28     40
> ds             <unavailable>
> es             <unavailable>
> fs             <unavailable>
> gs             <unavailable>

 Thanks for checking that.  NB I find output above quite messy, especially 
the lack of column alignment, e.g. `r14' vs `r15'.  It makes it hard to 
read for me.

 I've given your proposal some thought and I'm of mixed minds between two 
possibilites I have come up with for the condensed format, specifically 
`xxxxxxxx' vs `<absent>', which I think are both suitable without being 
messy:

(gdb) info registers
          zero       at       v0       v1       a0       a1       a2       a3
 R0   00000000 fffffff8 77fde8cc 00000000 77ff0000 77fec458 7ffffbd4 7ffffbdc
            t0       t1       t2       t3       t4       t5       t6       t7
 R8   c34e4bff ea5f612e 81010100 2f2f2f2f 7ffffce1 ffffffff f0000000 7ffff4e8
            s0       s1       s2       s3       s4       s5       s6       s7
 R16  78007010 00401360 xxxxxxxx xxxxxxxx xxxxxxxx 004f0000 00000063 0000006c
            t8       t9       k0       k1       gp       sp       s8       ra
 R24  77fdf64c 00401360 7ffff67c 00000000 78007010 7ffffbd0 00000061 77fcc97c
        status       lo       hi badvaddr    cause       pc
      0100a413 008cbcee 00000006 77f83e90 00800024 00401360
          fcsr      fir  restart
      00000000 00f30000 00000000
(gdb)

vs:

(gdb) info registers
          zero       at       v0       v1       a0       a1       a2       a3
 R0   00000000 fffffff8 77fde8cc 00000000 77ff0000 77fec458 7ffffbd4 7ffffbdc
            t0       t1       t2       t3       t4       t5       t6       t7
 R8   c34e4bff ea5f612e 81010100 2f2f2f2f 7ffffce1 ffffffff f0000000 7ffff4e8
            s0       s1       s2       s3       s4       s5       s6       s7
 R16  78007010 00401360 <absent> <absent> <absent> 004f0000 00000063 0000006c
            t8       t9       k0       k1       gp       sp       s8       ra
 R24  77fdf64c 00401360 7ffff67c 00000000 78007010 7ffffbd0 00000061 77fcc97c
        status       lo       hi badvaddr    cause       pc
      0100a413 008cbcee 00000006 77f83e90 00800024 00401360
          fcsr      fir  restart
      00000000 00f30000 00000000
(gdb)

The use of angle brackets with the latter variant is consistent with other 
targets, so I might have just a slight preference for it, but I'll be 
happy to accept input from other people.

 Where space is at disposal full `<unavailable>' could be printed, e.g:

(gdb) info registers s3 s4 s5
s3: <unavailable>
s4: <unavailable>
s5: 0x4f0000
(gdb)

or:

(gdb) info registers
                  zero               at               v0               v1
 R0   0000000000000000 fffffffffffffff0   <unavailable>  ffffffffc0000000
                    a0               a1               a2               a3
 R4     <unavailable>  0000000000000001 000000fffffffbb8 000000fffffffbc8
                    a4               a5               a6               a7
 R8     <unavailable>  0000000000000000 8101010101010100 2f2f2f2f2f2f2f2f
                    t0               t1               t2               t3
 R12  0000000000000000 0000000000000001 ffffffff8122f5d8   <unavailable>
                    s0               s1               s2               s3
 R16  000000fff8006000 0000000120000970 000000fff8006000 0000000000500000
                    s4               s5               s6               s7
 R20  0000000000521ec8 0000000000522608 0000000000000000 0000000000000000
                    t8               t9               k0               k1
 R24  000000000000161a 0000000120000970 0000000000000000 0000000000000000
                    gp               sp               s8               ra
 R28  000000fff8006000 000000fffffffbb0 0000000000000000   <unavailable>
                status               lo               hi         badvaddr
      0000000000109cf3 000000000d35ec75 0000000000000000   <unavailable>
                 cause               pc
      0000000000800024 0000000120000970
                  fcsr              fir          restart
              00000000         00f30000 0000000000000000
(gdb)

 Comments?  Thoughts?

  Maciej