-- -----------------------------------------------------------------------------
-- Generating the application functions
+-- A SUBTLE POINT about stg_ap functions (can't think of a better
+-- place to put this comment --SDM):
+--
+-- The entry convention to an stg_ap_ function is as follows: all the
+-- arguments are on the stack (we might revisit this at some point,
+-- but it doesn't make any difference on x86), and THERE IS AN EXTRA
+-- EMPTY STACK SLOT at the top of the stack.
+--
+-- Why? Because in several cases, stg_ap_* will need an extra stack
+-- slot, eg. to push a return address in the THUNK case, and this is a
+-- way of pushing the stack check up into the caller which is probably
+-- doing one anyway. Allocating the extra stack slot in the caller is
+-- also probably free, because it will be adjusting Sp after pushing
+-- the args anyway (this might not be true of register-rich machines
+-- when we start passing args to stg_ap_* in regs).
+
mkApplyRetName args
= text "stg_ap_" <> text (map showArg args) <> text "_ret"
where
n_args = length args
+ -- offset of args on the stack, see large comment above.
+ arg_sp_offset = 1
+
-- The SMALLER ARITY cases:
-- if (arity == 1) {
-- Sp[0] = Sp[1];
= text "if (arity == " <> int arity <> text ") {" $$
let
(reg_doc, sp')
- | stack_apply = (empty, 1)
- | otherwise = loadRegArgs 1 these_args
+ | stack_apply = (empty, arg_sp_offset)
+ | otherwise = loadRegArgs arg_sp_offset these_args
in
nest 4 (vcat [
reg_doc,
= text "if (arity == " <> int n_args <> text ") {" $$
let
(reg_doc, sp')
- | stack_apply = (empty, 1)
- | otherwise = loadRegArgs 1 args
+ | stack_apply = (empty, arg_sp_offset)
+ | otherwise = loadRegArgs arg_sp_offset args
in
nest 4 (vcat [
reg_doc,