addsi3_mips16 and frame pointer with LRA

[Matthew Fortune <Matthew dot Fortune at imgtec dot com>]

Hi Richard/Vladimir,

I believe I finally understand one of the issues with LRA and mips16 but I can't see how to solve it. Take the following instruction:

(insn 5 18 6 2 (set (reg:SI 4 $4)
        (plus:SI (reg/f:SI 78 $frame)
            (const_int 16 [0x10]))) test.c:6 13 {*addsi3_mips16}
     (nil))

$frame will be eliminated to either $sp or the hard frame pointer, which for mips16 is $17. The problem here is that there is no single alternative that accepts either $sp or $17 because the supported immediate range is different for $sp(ks) and $17(d). The "ks" alternative is disregarded (presumably because there is no way to reload into $sp if that ended up being necessary) and instead the "d" alternative is chosen. If the frame pointer is needed then this works well because $17 is used and fits the "d" constraint however when the frame pointer is omitted $sp has to be reloaded into a "d" register even though there is another alternative which it would directly match.

The fragment of the reload dump:

            1 Matching alt: reject+=2
            1 Non-pseudo reload: reject+=2
          alt=4,overall=10,losers=1,rld_nregs=1
            1 Matching alt: reject+=2
            1 Non-pseudo reload: reject+=2
          alt=5,overall=10,losers=1,rld_nregs=1
            1 Non-pseudo reload: reject+=2
            1 Non-pseudo reload: reject+=2
          alt=7,overall=8,losers=1,rld_nregs=1
            1 Non-pseudo reload: reject+=2
            2 Non-pseudo reload: reject+=2
            2 Non input pseudo reload: reject++
            alt=8,overall=17,losers=2 -- refuse
         Choosing alt 7 in insn 5:  (0) d  (1) d  (2) O {*addsi3_mips16}
      Creating newreg=198 from oldreg=78, assigning class M16_REGS to r198

And the end result is:

(insn 20 18 5 2 (set (reg/f:SI 2 $2 [198])
        (reg/f:SI 29 $sp)) test.c:6 290 {*movsi_mips16}
     (nil))
(insn 5 20 6 2 (set (reg:SI 4 $4)
        (plus:SI (reg/f:SI 2 $2 [198])
            (const_int 16 [0x10]))) test.c:6 13 {*addsi3_mips16}
     (nil))

The only way I can currently see to get any direct usage of $sp in an add instruction would be to artificially reduce the permitted immediate range by 1 bit so that there is a single alternative that allows either "ks" or "d" with a 15bit immediate. I don't really want to do that though. I initially allowed $frame to be treated as per $sp but that led to an ICE when $frame was eliminated to $17 and the immediate was out of range.

Have I missed anything that would allow me to support the full immediate range in all cases?

Regards,
Matthew

Matthew Fortune
Leading Software Design Engineer, MIPS processor IP
Imagination Technologies Limited
t: +44 (0)113 242 9814
www.imgtec.com