Build failure on s390x and mips64el

[LocutusOfBorg]

Hello, I'm not sure if really a ghc bug or not... any idea?
https://buildd.debian.org/status/fetch.php?pkg=haskell-juicypixels&arch=s390x&ver=3.2.9.5-3&stamp=1540808360&raw=0

[ 8 of 30] Compiling Codec.Picture.Png.Metadata ( src/Codec/Picture/Png/Metadata.hs, dist-ghc/build/Codec/Picture/Png/Metadata.o )
[ 9 of 30] Compiling Codec.Picture.ColorQuant ( src/Codec/Picture/ColorQuant.hs, dist-ghc/build/Codec/Picture/ColorQuant.o )

src/Codec/Picture/ColorQuant.hs:123:26: warning: [-Woverflowed-literals]
    Literal 256 is out of the GHC.Word.Word8 range 0..255
    |
123 |       (PixelRGB8 (r .&. (256 - dr)) (g .&. (256 - dg)) (b .&. (256 - db)))

    |                          ^^^

src/Codec/Picture/ColorQuant.hs:123:45: warning: [-Woverflowed-literals]
    Literal 256 is out of the GHC.Word.Word8 range 0..255
    |
123 |       (PixelRGB8 (r .&. (256 - dr)) (g .&. (256 - dg)) (b .&. (256 - db)))

    |                                             ^^^

src/Codec/Picture/ColorQuant.hs:123:64: warning: [-Woverflowed-literals]
    Literal 256 is out of the GHC.Word.Word8 range 0..255
    |
123 |       (PixelRGB8 (r .&. (256 - dr)) (g .&. (256 - dg)) (b .&. (256 - db)))

    |                                                                ^^^
ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for s390x-ibm-linux):
	pprStatics: float
  F32
  F32
  I32
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable
        pprPanic, called at compiler/cmm/PprC.hs:525:5 in ghc:PprC

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

make: *** [/usr/share/cdbs/1/class/hlibrary.mk:147: build-ghc-stamp] Error 1
dpkg-buildpackage: error: debian/rules binary-arch subprocess returned exit status 2

[LocutusOfBorg]

the ghc bug is reported here:
https://ghc.haskell.org/trac/ghc/ticket/15853#ticket

[Twinside]

I should fix those warnings at least :/

[LocutusOfBorg]

looks like compiling with "-O0" works... not sure how good it is...

[Twinside]

I think -O0 on Juicy.Pixels produce really, really slow code