Yes, I think we can probably do a better job of compiling case
expressions. The current compilation strategy optimises for code size,
but at the expense of performance in the fast path. We can tweak this
tradeoff, perhaps under the control of a flag.
Ideally the sequence should start by assuming that the closure is
already evaluated, e.g.
loop:
tag = R2 & 7;
if (tag == 1) then // code for []
else if (tag == 2) then // code for (:)
else evaluate; jump back to loop
The nice thing is that now that we don't need proc points, "loop" is
just a label that we can directly jump to. Unfortunately this only
works when using the NCG, not with LLVM, because LLVM requires proc
points, so we might need to fall back to a different strategy for LLVM.
Similar topics came up here:
https://ghc.haskell.org/trac/ghc/ticket/8905 and I think there was
another ticket but I can't find it now.
Cheers
Simon
On 23/10/2015 19:00, Ryan Newton wrote:
1. Small tweaks: The CMM code above seems to be /betting/ than the
thunk is unevaluated, because it does the stack check and stack
write /before/ the predicate test that checks if the thunk is
evaluated (if(R1 & 7!= 0) gotoc3aO; elsegotoc3aP;). With a
bang-pattern function, couldn't it make the opposite bet? That
is, branch on whether the thunk is evaluated first, and then the
wasted computation is only a single correctly predicted branch
(and a read of a tag that we need to read anyway).
Oh, a small further addition would be needed for this tweak. In the
generated code above "Sp = Sp + 8;" happens /late/, but I think it could
happen right after the call to the thunk. In general, does it seem
feasible to separate the slowpath from fastpath as in the following
tweak of the example CMM?
* // Skip to the chase if it's already evaluated:*
* start:*
* if (R2 & 7 != 0) goto fastpath; else goto slowpath;*
*
*
* slowpath: // Formerly 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;*
* call (I64[R1])(R1) returns to fastpath, args: 8, res: 8, upd: 8;*
* // Sp bump moved to here so it's separate from "fastpath"*
* Sp = Sp + 8;*
*
*
* fastpath: // Formerly c3aO*
* if (R1 & 7 >= 2) goto c3aW; else goto c3aX;*
* c3aW:*
* R1 = P64[R1 + 6] & (-8);*
* call (I64[R1])(R1) args: 8, res: 0, upd: 8;*
* c3aX:*
* R1 = PicBaseReg + lvl_r39S_closure;*
* call (I64[R1])(R1) args: 8, res: 0, upd: 8;*
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