cmmMachOpFoldM: unknown unary op: MO_XX_Conv W8 W64

Summary

I thought I'd give a shot at creating all the common instances for new primitives Int8#, etc. and see how they compare performance wise to what we have now. In the process of doing so I got a GHC panic and another assembly error, which I can't really narrow down to an exact place, but a fully reproducible piece of code is attached. I suspect there might really be two different bugs here at play, but considering they happen on the same piece of new functionality I thought they might be related.

This is the error I get:

ghc: panic! (the 'impossible' happened)
  (GHC version 8.10.1:
	cmmMachOpFoldM: unknown unary op: MO_XX_Conv W8 W64

Another problem I get is when I replace divInt8# with this:

divInt8# :: Int8# -> Int8# -> Int8#
x# `divInt8#` y#
  | isTrue# (x# `gtInt8#` zero#) && isTrue# (y# `ltInt8#` zero#) =
    ((x# `subInt8#` one#) `quotInt8#` y#) `subInt8#` one#
  | isTrue# (x# `ltInt8#` zero#) && isTrue# (y# `gtInt8#` zero#) =
    ((x# `plusInt8#` one#) `quotInt8#` y#) `subInt8#` one#
  | otherwise = x# `quotInt8#` y#
  where
    zero# = narrowInt8# 0#
    one# = narrowInt8# 1#

I also get this error message from gcc:

[1 of 2] Compiling MyInt8           ( MyInt8.hs, MyInt8.o )
/tmp/ghc7105_0/ghc_2.s: Assembler messages:

/tmp/ghc7105_0/ghc_2.s:634:0: error:
     Error: invalid instruction suffix for `lea'
    |
634 |         leab -1(%r14),%bl
    | ^

/tmp/ghc7105_0/ghc_2.s:650:0: error:
     Error: invalid instruction suffix for `lea'
    |
650 |         leab 1(%r14),%bl
    | ^

/tmp/ghc7105_0/ghc_2.s:677:0: error:
     Error: invalid instruction suffix for `lea'
    |
677 |         leab 1(%r14),%bl
    | ^
`gcc' failed in phase `Assembler'. (Exit code: 1)

Steps to reproduce

Splitting into two modules seemed to be necessary and it is important to compile with optimization turned on.

$ ghc int8.hs -O1 -fforce-recomp
[1 of 2] Compiling MyInt8           ( MyInt8.hs, MyInt8.o )
[2 of 2] Compiling Main             ( int8.hs, int8.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.8.3 for x86_64-unknown-linux):
	cmmMachOpFoldM: unknown unary op: MO_XX_Conv W8 W64

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

Files to reproduce the bug(s):

• MyInt8.hs

• int8.hs

A full repro is also available in this repo: https://github.com/lehins/bugs/tree/master/haskell/ghc-int8

Expected behavior

Compile into correct binary without errors

Environment

• GHC version used: 8.8.3 and 8.10.1

Optional:

• Operating System: Ubuntu 18.04
• System Architecture: x86_64