[LLVMdev] AArch64: GHC compilation issue.

Carter Schonwald carter.schonwald at gmail.com
Thu Jan 9 13:23:26 PST 2014

Hey Karel,
the GHC unregisterized via C code gen requires using GCC. Unless Clang
starts support some of the more eclectic GCC C extensions, you wont' be
able to use Clang/LLVM for doing an unregisterized build.

GCC 4.8 and newer seems to have aarch64 support.


On Wed, Jan 8, 2014 at 4:51 PM, Karel Gardas <karel.gardas at centrum.cz>wrote:

> Hi Tim,
> On 01/ 8/14 10:24 PM, Tim Northover wrote:
>> Hi Karel,
>>  I've observed the same issue with LLVM 3.4 as distributed by Ubuntu 13.10
>>> and with LLVM HEAD compiled on January 6. I'm able to provide the
>>> byte-code
>>> file which results in this issue, but would first like to know if this
>>> is a
>>> known issue in AArch64 target support or if I shall submit it somewhere
>>> or
>>> even if I did some mistake in invoking llc or providing wrong set of
>>> options
>>> to it.
>> I've certainly not seen similar. Usually "Cannot select" errors are
>> fixed very quickly since they're so easy to narrow down and fix. I
>> think this is a real bug and the .ll file would be very useful.
> the bziped2 file is here for you https://app.box.com/s/
> 0i43c5j1bq40yheocmn9
>  It compiles to amd64 code by default so I added just -march=aarch64
>>> and was in impression that it should be enough to target AArch64
>>> platform...
>>> If that's wrong, please let me know.
>> That's on the edge of OK. There are numerous tricky ABI issues that
>> could be caused by simply swapping a -march argument to LLVM . Clang,
>> for example, wouldn't be able to compile correct C or C++ code with
>> just that switch. But GHC presumably has less stringent ABI
>> compatibility requirements, so it might get away with it.
> I hope so.
>  On the other hand, I think x86 (and possibly ARM) has specific code to
>> handle the GHC calling conventions, which I'm fairly sure AArch64
>> doesn't. That *might* just be a matter of performance, or it might be
>> crucial. I kept meaning to get GHC working but never really got around
>> to it.
> Ah, I've not mentioned it before, but I'm attempting to port GHC to
> AArch64 and the first step is to get so called unregisterised build
> working. Unregisterised build does not require this specific GHC calling
> convention. Once this is working I'll come here hopefully with some
> patch or with ask for idea/help with modification of AArch64 target
> support to make GHC calling convention a reality. But certainly I'm not
> there yet...
> Hmm, but since you mentioned it. Has anything changed in the LLVM so
> it's more easy to write such calling convention? IIRC, the calling
> convention is here just to tell LLVM to omit usage of some registers as
> they are pinned to the haskell virtual registers... I hope I'm not too
> misleading here...And in the past creating such calling convention where
> the required regs were removed was the only way probably...
> Thanks!
> Karel
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