Re: Loop invariant code corrupting xstormy16 insn

[Zdenek Dvorak <rakdver at kam dot mff dot cuni dot cz>]

Hello,

>   I think that I have found a small bug in the loop invariant code.
>   The problem is exhibited when building newlib for the xstormy16-elf
>   toolchain although it may happen with other targets as well.  The
>   problem occurs when an insn contains an r-value use of a register
>   that is going to hoisted as well as a clobber of that register, like
>   this:
> 
>     (parallel [
>               (set (pc)
>                    (if_then_else (lt:SI (reg:SI 45)    <---
>                                         (reg:SI 26))
>                                  (label_ref:HI 129)
>                                  (pc)))
>               (clobber (reg:SI 45))                    <-- must be the same
>               (clobber (scratch:BI))
>               ])
> 
>   The important point here is that the register referenced in the
>   first clobber must match the register used as the first argument of
>   the condition operation.  (This comes from the "ineqbranchsi"
>   pattern in the xstormy16 machine description).
> 
>   The problem is that the loop-invariant optimization was deciding
>   that (reg:SI 45) could be hoisted out of the loop, whereas really it
>   should be left alone.  I have been looking at the loop invariant
>   code trying to figure out what needs to be changed, but so far I
>   have not been able to puzzle it out.
> 
>   I think that the real problem might be the clobbers in the
>   ineqbranchsi pattern, but I do not know how it could be changed to
>   fix this.
> 
>   Any ideas ?

invariant motion needs to verify that reg:45 is dead at the beginning of the
loop. We cannot just avoid moving insns that contain clobbers, as then we would
not move insns like

(insn 23 22 25 3 xxx.c:6 (parallel [
            (set (reg:SI 64 [ ivtmp.33 ])
                (plus:SI (reg:SI 64 [ ivtmp.33 ])
                    (const_int 1 [0x1])))
            (clobber (reg:CC 17 flags))
        ]) 148 {*addsi_1} (nil)
    (expr_list:REG_UNUSED (reg:CC 17 flags)
        (nil)))

that only clobber flags.

Zdenek