Commit 9026c77a authored by Simon Marlow's avatar Simon Marlow

comments

parent b724cd41
......@@ -188,6 +188,7 @@ slowCall fun stg_args
" with pat " ++ unpackFS rts_fun)
return r
-- Note [avoid intermediate PAPs]
let n_args = length stg_args
if n_args > arity && optLevel dflags >= 2
then do
......@@ -224,6 +225,32 @@ slowCall fun stg_args
return r
-- Note [avoid intermediate PAPs]
--
-- A slow call which needs multiple generic apply patterns will be
-- almost guaranteed to create one or more intermediate PAPs when
-- applied to a function that takes the correct number of arguments.
-- We try to avoid this situation by generating code to test whether
-- we are calling a function with the correct number of arguments
-- first, i.e.:
--
-- if (TAG(f) != 0} { // f is not a thunk
-- if (f->info.arity == n) {
-- ... make a fast call to f ...
-- }
-- }
-- ... otherwise make the slow call ...
--
-- We *only* do this when the call requires multiple generic apply
-- functions, which requires pushing extra stack frames and probably
-- results in intermediate PAPs. (I say probably, because it might be
-- that we're over-applying a function, but that seems even less
-- likely).
--
-- This very rarely applies, but if it does happen in an inner loop it
-- can have a severe impact on performance (#6084).
--------------
direct_call :: String
-> Convention -- e.g. NativeNodeCall or NativeDirectCall
......
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