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Diffstat (limited to 'compiler/coreSyn/CoreSyn.hs')
| -rw-r--r-- | compiler/coreSyn/CoreSyn.hs | 22 |
1 files changed, 22 insertions, 0 deletions
diff --git a/compiler/coreSyn/CoreSyn.hs b/compiler/coreSyn/CoreSyn.hs index 95b05392ae..725e8da826 100644 --- a/compiler/coreSyn/CoreSyn.hs +++ b/compiler/coreSyn/CoreSyn.hs @@ -608,6 +608,8 @@ Join points must follow these invariants: same number of arguments, counting both types and values; we call this the "join arity" (to distinguish from regular arity, which only counts values). + See Note [Join points are less general than the paper] + 2. For join arity n, the right-hand side must begin with at least n lambdas. No ticks, no casts, just lambdas! C.f. CoreUtils.joinRhsArity. @@ -657,6 +659,26 @@ Core Lint will check these invariants, anticipating that any binder whose OccInfo is marked AlwaysTailCalled will become a join point as soon as the simplifier (or simpleOptPgm) runs. +Note [Join points are less general than the paper] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +In the paper "Compiling without continuations", this expression is +perfectly valid: + + join { j = \_ -> e } + in (case blah of ) + ( True -> j void# ) arg + ( False -> blah ) + +assuming 'j' has arity 1. Here the call to 'j' does not look like a +tail call, but actually everything is fine. See Section 3, "Managing \Delta" +in the paper. + +In GHC, however, we adopt a slightly more restrictive subset, in which +join point calls must be tail calls. I think we /could/ loosen it up, but +in fact the simplifier ensures that we always get tail calls, and it makes +the back end a bit easier I think. Generally, just less to think about; +nothing deeper than that. + Note [The type of a join point] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ A join point has the same type it would have as a function. That is, if it takes |