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Diffstat (limited to 'compiler/NOTES')
| -rw-r--r-- | compiler/NOTES | 171 |
1 files changed, 171 insertions, 0 deletions
diff --git a/compiler/NOTES b/compiler/NOTES new file mode 100644 index 0000000000..8c62750008 --- /dev/null +++ b/compiler/NOTES @@ -0,0 +1,171 @@ + +------------------------- +*** unexpected failure for jtod_circint(opt) + + + New back end thoughts + +----------------------------------------------------------------------------- +Codegen notes + +* jumps to ImpossibleBranch should be removed. + +* Profiling: + - when updating a closure with an indirection to a function, + we should make a permanent indirection. + + - check that we're bumping the scc count appropriately + +* check perf & binary sizes against the HEAD + +----------------------------------------------------------------------------- +C backend notes + +* use STGCALL macros for foreign calls (doesn't look like volatile regs + are handled properly at the mo). + +----------------------------------------------------------------------------- +Cmm parser notes + +* switches + +* need to cater for unexported procedures/info tables? + +* We should be able to get rid of entry labels, use info labels only. + - we need a %ENTRY_LBL(info_lbl) macro, so that instead of + JMP_(foo_entry) we can write jump %ENTRY_LBL(foo_info). + +----------------------------------------------------------------------------- + +* Move arg-descr from LFInfo to ClosureInfo? + But: only needed for functions + +* Move all of CgClosure.link_caf into NewCaf, and newDynCaf + +* If the case binder is dead, and the constr is nullary, + do we need to assign to Node? + + +------------------------- +* Relation between separate type sigs and pattern type sigs +f :: forall a. a->a +f :: b->b = e -- No: monomorphic + +f :: forall a. a->a +f :: forall a. a->a -- OK + +f :: forall a. [a] -> [a] +f :: forall b. b->b = e ??? + + +------------------------------- +NB: all floats are let-binds, but some non-rec lets + may be unlifted (with RHS ok-for-speculation) + + +simplArg: [use strictness] + [used for non-top-lvl non-rec RHS or function arg] + if strict-type || demanded + simplStrictExpr + else + simplExpr ---> (floats,expr) + float all the floats if exposes constr app, return expr + +simpl (applied lambda) ==> simplNonRecBind +simpl (Let (NonRec ...) ..) ==> simplNonRecBind + +simpl (Let (Rec ...) ..) ==> simplRecBind + +simplRecBind: + simplify binders (but not its IdInfo) + simplify the pairs one at a time + using simplRecPair + +simplNonRecBind: [was simplBeta] + [used for non-top-lvl non-rec bindings] + - check for PreInlineUnconditionally + - simplify binder, including its IdInfo + - simplArg + - if strict-type + addCaseBind [which makes a let if ok-for-spec] + else + completeLazyBind + +simplLazyBind: [binder already simplified, but not its IdInfo] + [used for both rec and top-lvl non-rec] + [must not be strict/unboxed; case not allowed] + - check for PreInlineUnconditionally + - substituteIdInfo and add result to in-scope + [so that rules are available in rec rhs] + - simplExpr --> (floats,expr) + - float: lifted floats only + if exposes constructor or pap (even if non-triv args) + or if top level + - completeLazyBind + + +completeLazyBind: [given a simplified RHS] + [used for both rec and non-rec bindings, top level and not] + - try discarding dead + - try PostInlineUnconditionally + - let-bind coerce arg and repeat + - try rhs tylam (float) + - try eta expand (float) [not if any float is unlifted && (non-spec || top_lvl || rec)] + - let-bind constructor args [not if any float is ..as above..] + + - add unfolding [this is the only place we add an unfolding] + add arity + + + +Right hand sides and arguments +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +In many ways we want to treat + (a) the right hand side of a let(rec), and + (b) a function argument +in the same way. But not always! In particular, we would +like to leave these arguments exactly as they are, so they +will match a RULE more easily. + + f (g x, h x) + g (+ x) + +It's harder to make the rule match if we ANF-ise the constructor, +or eta-expand the PAP: + + f (let { a = g x; b = h x } in (a,b)) + g (\y. + x y) + +On the other hand if we see the let-defns + + p = (g x, h x) + q = + x + +then we *do* want to ANF-ise and eta-expand, so that p and q +can be safely inlined. + +Even floating lets out is a bit dubious. For let RHS's we float lets +out if that exposes a value, so that the value can be inlined more vigorously. +For example + + r = let x = e in (x,x) + +Here, if we float the let out we'll expose a nice constructor. We did experiments +that showed this to be a generally good thing. But it was a bad thing to float +lets out unconditionally, because that meant they got allocated more often. + +For function arguments, there's less reason to expose a constructor (it won't +get inlined). Just possibly it might make a rule match, but I'm pretty skeptical. +So for the moment we don't float lets out of function arguments either. + + +Eta expansion +~~~~~~~~~~~~~~ +For eta expansion, we want to catch things like + + case e of (a,b) -> \x -> case a of (p,q) -> \y -> r + +If the \x was on the RHS of a let, we'd eta expand to bring the two +lambdas together. And in general that's a good thing to do. Perhaps +we should eta expand wherever we find a (value) lambda? Then the eta +expansion at a let RHS can concentrate solely on the PAP case. |