simd branch ready for review/merge

Geoffrey Mainland mainland at
Tue Sep 24 01:52:11 UTC 2013

The NCG checks for this, but maybe it would be better to add a check in
cmm/CmmCallConv.hs? What do you think?


On 09/23/2013 06:31 AM, Simon Marlow wrote:
> Hey Geoff,
> Sorry for not getting around to reviewing this.  I just skimmed the
> patches and didn't spot anything obviously bogus.
> Regarding the sorry errors, are primops the only place that we need to
> check?  Is it possible that the user might try to call a function that
> takes a SIMD argument and make the NCG fall over that way?
> Cheers,
> Simon
> On 16/09/2013 15:16, Geoffrey Mainland wrote:
>> The SIMD branch, available as wip/simd, is ready for review/merge. It
>> could use some review---Simon and Simon, I'd be especially grateful if
>> you both had a quick look. Some major points:
>> 1) I have added support for AVX 512, although this is necessarily
>> untested. AVX and AVX2 are also both supported.
>> 2) After the recent churn regarding patching LLVM's GHC calling
>> convention, by default only 128-bit wide SIMD vectors are passed in
>> registers, and then only on X86_64. There is a "hidden" flag,
>> -fllvm-pass-vectors-in-regs, that causes GHC to generate LLVM code that
>> assumes all vectors are passed in registers by LLVM. This can be used
>> with a suitably patched version of LLVM, and if we get LLVM 3.4 patched,
>> we can consider turning it on by default for LLVM 3.4+. This would mean
>> that we couldn't mix LLVM <3.3-compiled object files with LLVM
>>> 3.4-compiled object files, but I don't see that as much of a problem.
>> 3) utils/genprimcode has been hacked up to allow us to write vector
>> operations once and have them instantiated at multiple vector types. I'm
>> not thrilled with this solution, but after discussing with Simon PJ,
>> what I've implemented seems to be the minimal reasonable solution to the
>> problem of exploding primop boilerplate. The changes are documented in
>> compiler/prelude/primops.txt.pp.
>> 4) Error handling is sub-optimal. My patch checks to make sure that
>> vector primops can be compiled efficiently based on the current set of
>> dynamic flags. For example, if -mavx is not specified and the user tries
>> to use a primop that adds together two 256-bit wide vectors of
>> double-precision elements, the user will see an error message like:
>> ghc-stage2: sorry! (unimplemented feature or known bug)
>>    (GHC version 7.7.20130916 for x86_64-unknown-linux):
>>      256-bit wide floating point SIMD vector instructions require at
>> least -mavx.
>> This is because the only good place to check for this kind of error is
>> during STG->Cmm translation (in compiler/codeGen/StgCmmPrim.hs), and we
>> don't have much of an error handling infrastructure there in contrast to
>> when we're working in the typechecking/renaming monad. If there is a
>> better way/place to do this, please let me know.
>> Thanks,
>> Geoff
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