In terms of normal haskell user experience, would it perhaps be a good
ideas to have information like "this inlinable function transitively calls
an inlinable function that uses a SIMD primop" in the *.hi files?
that way we can, before code gen time, give a warning that compilation of
client module might fail without -fllvm? (whether it fails or not depends
on how much inlining actually happens )
On Tue, Sep 24, 2013 at 7:25 AM, Simon Marlow
Yes - it's really just an aesthetic issue, but I think it would be nice if we know that there's only one pass where these errors show up, so we don't have to worry about emitting a helpful "sorry" rather than a panic in other places too.
Cheers, Simon
On 23/09/2013 21:52, Geoffrey Mainland wrote:
The NCG checks for this, but maybe it would be better to add a check in cmm/CmmCallConv.hs? What do you think?
Geoff
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.
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