[haskell-llvm] [Haskell-cafe] LLVM, type-level?

[Lally Singh]

On Sat, Dec 11, 2010 at 3:23 PM, Henning Thielemann
<lemming at henning-thielemann.de> wrote:
>
> On Sat, 11 Dec 2010, Lally Singh wrote:
>
>> I had tried that as well, but figured it would be best to post about
>> exactly what you had told me :-)
>>
>> Heres' what I have:
>> buildReaderFun :: String -> CodeGenModule (MainFunction)
>> buildReaderFun nm  = do
>>  puts <- newNamedFunction ExternalLinkage "puts" ::
>>   TFunction (Ptr Word8 -> IO Word32)
>>  let callPuts format = (
>>       createNamedFunction ExternalLinkage "main" $
>>       \ argc argv -> do
>>         tmp <- getElementPtr (argv ::Value (Ptr (Ptr Word8)))
>>                (0 :: Int32,  (1 :: Int32, ()))
>>         _ <- call puts tmp
>>         ret (0 :: Int32)) :: CodeGenModule (MainFunction)
>>
>>  withStringNul nm callPuts
>>
>> And the result:
>> /research/phd/libmet/Listener.hs:51:17:
>>   No instance for (GetElementPtr (Ptr Word8) (Int32, ()) Word8)
>>     arising from a use of `getElementPtr'
>>                  at /research/phd/libmet/Listener.hs:(51,17)-(52,47)
>>
>> Is there another instance I should add?
>
> No, these GetElementPtr type errors really point you to a serioues problem.
> GHC proposes to add instances because in the general case this might be a
> solution. But here you try to do something that is simply not supported by
> LLVM, and thus 'llvm' forbids it.
>
> I look into
>  http://hackage.haskell.org/packages/archive/llvm/0.9.0.1/doc/html/LLVM-Core.html#t:GetElementPtr
>  and see that there is no instance for 'optr = Ptr a'. That means in order
> to follow two pointers successively, you have to call getElementPtr twice. I
> assume that with
>
>>         tmp <- getElementPtr (argv ::Value (Ptr (Ptr Word8)))
>>                (0 :: Int32,  (1 :: Int32, ()))
>
> you want to have a pointer to the first (not zeroth) character of the zeroth
> of argv. Is that what you want? I think you have to write
>
>>  tmp0 <- getElementPtr (argv ::Value (Ptr (Ptr Word8))) (0 :: Int32, ())
>>  tmp1 <- load tmp0
>>  tmp  <- getElementPtr tmp1 (1 :: Int32, ())
>
> http://www.llvm.org/docs/GetElementPtr.html#firstptr
> "Why is it okay to index through the first pointer, but subsequent pointers
> won't be dereferenced?"
>

Oh wow, that worked perfectly!!  Thank you!