diff options
Diffstat (limited to 'compiler/coreSyn/CoreSyn.hs')
| -rw-r--r-- | compiler/coreSyn/CoreSyn.hs | 18 |
1 files changed, 15 insertions, 3 deletions
diff --git a/compiler/coreSyn/CoreSyn.hs b/compiler/coreSyn/CoreSyn.hs index d94761b237..e3ad4715f1 100644 --- a/compiler/coreSyn/CoreSyn.hs +++ b/compiler/coreSyn/CoreSyn.hs @@ -681,9 +681,21 @@ Join points must follow these invariants: 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. - 2a. Moreover, this same constraint applies to any unfolding of the binder. - Reason: if we want to push a continuation into the RHS we must push it - into the unfolding as well. + 2a. Moreover, this same constraint applies to any unfolding of + the binder. Reason: if we want to push a continuation into + the RHS we must push it into the unfolding as well. + + 2b. The Arity (in the IdInfo) of a join point is the number of value + binders in the top n lambdas, where n is the join arity. + + So arity <= join arity; the former counts only value binders + while the latter counts all binders. + e.g. Suppose $j has join arity 1 + let j = \x y. e in case x of { A -> j 1; B -> j 2 } + Then its ordinary arity is also 1, not 2. + + The arity of a join point isn't very important; but short of setting + it to zero, it is helpful to have an invariant. E.g. #17294. 3. If the binding is recursive, then all other bindings in the recursive group must also be join points. |