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 |
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