compiler/codeGen/StgCmmBind.hs¶
Note [Selectors]¶
We look at the body of the closure to see if it’s a selector—turgid, but nothing deep. We are looking for a closure of {em exactly} the form:
- … = [the_fv] u [] ->
- case the_fv of
- con a_1 … a_n -> a_i
Note [Ap thunks]¶
A more generic AP thunk of the form
x = [ x_1...x_n ] \.. [] -> x_1 ... x_n
A set of these is compiled statically into the RTS, so we just use those. We could extend the idea to thunks where some of the x_i are global ids (and hence not free variables), but this would entail generating a larger thunk. It might be an option for non-optimising compilation, though.
We only generate an Ap thunk if all the free variables are pointers, for semi-obvious reasons.
——– Note [Selectors] ——————
Note [NodeReg clobbered with loopification]¶
Previously we used to pass nodeReg (aka R1) here. With profiling, upon entering a closure, enterFunCCS was called with R1 passed to it. But since R1 may get clobbered inside the body of a closure, and since a self-recursive tail call does not restore R1, a subsequent call to enterFunCCS received a possibly bogus value from R1. The solution is to not pass nodeReg (aka R1) to enterFunCCS. Instead, we pass node, the callee-saved temporary that stores the original value of R1. This way R1 may get modified but loopification will not care.
A function closure pointer may be tagged, so we must take it into account when accessing the free variables.
