Better calling conventions for strict functions (bang patterns)?

[Simon Peyton Jones]

It’s absolutely the case that bang patterns etc tell the caller what to do, but the function CANNOT ASSUME that its argument is evaluated.  Reason: higher order functions.

I think that the way to allow functions that can assume their arg is evaluated is through types: see Type are calling conventions<http://research.microsoft.com/~simonpj/papers/strict-core/tacc-hs09.pdf>.  But it’d be a fairly big deal to implement.

Simon


From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Ryan Newton
Sent: 23 October 2015 14:54
To: ghc-devs at haskell.org; Ömer Sinan Ağacan; Ryan Scott; Chao-Hong Chen; Johan Tibell
Subject: Better calling conventions for strict functions (bang patterns)?

Hi all,

With module-level Strict and StrictData pragmas coming soon, one obvious question is what kind of the code quality GHC can achieve for strict programs.

When it came up in discussion in our research group we realized we didn't actually know whether the bang patterns, `f !x`, on function arguments were enforced by caller or callee.

Here's a Gist that shows the compilation of a trivial function:


foo :: Maybe Int -> Int


foo !x =


  case x of


   Just y -> y


   https://gist.github.com/rrnewton/1ac722189c65f26fe9ac<https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fgist.github.com%2frrnewton%2f1ac722189c65f26fe9ac&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7cb006dcdbfe834ebb6c1e08d2dbb16c03%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=qxrT8r1VSP97xQUF2qqkLlxEtSGi9VOzfmORl25W%2fWY%3d>

If that function is compiled to *assume* its input is in WHNF, it should be just as efficient as the isomorphic MLton/OCaml code, right?  It only needs to branch on the tag, do a field dereference, and return.

But as you can see from the STG and CMM generated, foo does indeed enter the thunk, adding an extra indirect jump.  Here's the body:


      c3aY:


          if ((Sp + -8) < SpLim) goto c3aZ; else goto c3b0;


      c3aZ:


          // nop


          R1 = PicBaseReg + foo_closure;


          call (I64[BaseReg - 8])(R2, R1) args: 8, res: 0, upd: 8;


      c3b0:


          I64[Sp - 8] = PicBaseReg + block_c3aO_info;


          R1 = R2;


          Sp = Sp - 8;


          if (R1 & 7 != 0) goto c3aO; else goto c3aP;


      c3aP:


          call (I64[R1])(R1) returns to c3aO, args: 8, res: 8, upd: 8;


      c3aO:


          if (R1 & 7 >= 2) goto c3aW; else goto c3aX;


      c3aW:


          R1 = P64[R1 + 6] & (-8);


          Sp = Sp + 8;


          call (I64[R1])(R1) args: 8, res: 0, upd: 8;


      c3aX:


          R1 = PicBaseReg + lvl_r39S_closure;


          Sp = Sp + 8;


          call (I64[R1])(R1) args: 8, res: 0, upd: 8;



The call inside c3aP is entering "x" as a thunk, which also incurs all of the stack limit check code.  I believe that IF the input could be assumed to be in WHNF, everything above the label "c3aO" could be omitted.

So... if GHC is going to be a fabulous pure and imperative language, and a fabulous lazy and strict compiler/runtime.. is there some work we can do here to improve this situation? Would the following make sense:

  *   Put together a benchmark suite of all-strict programs with Strict/StrictData (compare a few benchmark's generated code to MLton, if time allows)
  *   Modify GHC to change calling conventions for bang patterns -- caller enforces WHNF rather than callee.  Existing strictness/demand/cardinality analysis would stay the same.
Unless there's something I'm really missing here, the result should be that you can have a whole chain of strict function calls, each of which knows its arguments and the arguments it passes to its callees are all in WHNF, without ever generating thunk-entry sequences.

Thanks for your time,
  -Ryan

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