Hey guys,
The following (evil, yes) program compiles and works under ghc -Onot
using -fasm or -fvia-C, but fails to generated correct code under all
GHCs back to ghc-5.04.2 if I turn on -O. (It also works under ghci).
When working it runs and produces the following (correct) result:
paprika$ ./a.out
7777.0
(69,243,8,0)
7777.0
However, if I turn on -O it fails to generate acceptable asm or C.
Am I right in thinking that all uses of unsafeCoerce# should never
cause type-incorrect C or asm code to be generated (no matter what evil
happens at runtime)?
Now, -dcore-lint is fine, but our shiny new 6.4 -dcmm-lint spots the
error :)
Cmm lint error:
in proc s2D4_ret
in basic block c2F7
in MachOp application:
7777.0 :: F32 & 255
Here's the result -O -fvia-C:
paprika$ ghc-6.4 -no-recomp -fglasgow-exts -O -fvia-C t.hs
/tmp/ghc5870.hc: In function `s2D4_ret':
/tmp/ghc5870.hc:253: error: invalid operands to binary &
/tmp/ghc5870.hc: In function `Main_lvl2_entry':
/tmp/ghc5870.hc:412: error: invalid operands to binary &
where relevant lines are:
253: _s2D8 = ((StgFloat)7777.0) & 0xff;
and
472: _s2Dg = ((StgFloat)7777.0) & 0xff;
There appears to be a missing (W_) cast (?).
And -O -fasm:
paprika$ ghc-6.4 -no-recomp -fglasgow-exts -O -fasm t.hs
/tmp/ghc7685.s: Assembler messages:
/tmp/ghc7685.s:309: Error: bad register name `%st(-7)'
/tmp/ghc7685.s:316: Error: bad register name `%fake0'
/tmp/ghc7685.s:317: Error: bad register name `%fake0'
/tmp/ghc7685.s:472: Error: bad register name `%st(-7)'
the relevant lines are:
309: ffree %st(7) ; flds .Ln2GX ; fstp %st(-7)
316: movl 8(%ebp),%fake0
317: orl %eax,%fake0
472: ffree %st(7) ; flds .Ln2HH ; fstp %st(-7)
We get a similar result with ghc-6.2.2 (but of course don't have a
-dcore-lint pass to spot the type error ;)
paprika$ ghc-6.2.2 -no-recomp -keep-tmp-files -fglasgow-exts -O
-fasm t.hs
/tmp/ghc4107.s: Assembler messages:
/tmp/ghc4107.s:278: Error: bad register name `%st(-7)'
/tmp/ghc4107.s:675: Error: bad register name `%fake0'
/tmp/ghc4107.s:676: Error: bad register name `%fake0'
paprika$ ghc-6.2.2 -no-recomp -keep-tmp-files -fglasgow-exts -O
-fvia-C t.hs
/tmp/ghc7876.hc: In function `Main_zdwg_entry':
/tmp/ghc7876.hc:221: error: invalid operands to binary &
And even back to ghc-5.04.2:
zywiec$ ghc-5.04.2 -O -fglasgow-exts -fvia-C t.hs
/tmp/ghc2973.hc: In function `Main_a_entry':
/tmp/ghc2973.hc:27: invalid operands to binary &
zywiec$ ghc-5.04.2 -O -fglasgow-exts -fasm t.hs
/tmp/ghc2977.s: Assembler messages:
/tmp/ghc2977.s:79: Error: bad register name `%st(-7)'
/tmp/ghc2977.s:774: Error: bad register name `%fake0'
/tmp/ghc2977.s:775: Error: bad register name `%fake0'
/tmp/ghc2977.s:776: Error: bad register name `%fake0'
/tmp/ghc2977.s:777: Error: bad register name `%fake0'
-- Don
And here's the nefarious code:
import GHC.Word
import GHC.Float
import GHC.Base
import Data.Bits
main = let f = 7777.0
f' = g f
f''= g' f'
in do putStrLn (show f)
putStrLn (show f')
putStrLn (show f'')
g :: Float -> (Word8,Word8,Word8,Word8)
g (F# f) =
let w = W32# (unsafeCoerce# f)
w0 = fromIntegral (w `shiftR` 24)
w1 = fromIntegral ((w `shiftR` 16) .&. 0xff)
w2 = fromIntegral ((w `shiftR` 8) .&. 0xff)
w3 = fromIntegral (w .&. 0xff)
in (w0,w1,w2,w3)
g' :: (Word8,Word8,Word8,Word8) -> Float
g' (w0,w1,w2,w3) =
let (W32# w) = (fromIntegral w0 `shiftL` 24) .|.
(fromIntegral w1 `shiftL` 16) .|.
(fromIntegral w2 `shiftL` 8) .|.
(fromIntegral w3)
in F# (unsafeCoerce# w)
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