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[smlnj] View of /sml/trunk/src/compiler/FLINT/opt/flintopt.txt
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View of /sml/trunk/src/compiler/FLINT/opt/flintopt.txt

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Revision 626 - (download) (annotate)
Sat Apr 22 23:24:24 2000 UTC (20 years, 7 months ago) by monnier
File size: 5971 byte(s)
* opt/flintopt.txt: New file.
* main/control.sml (phases): Move loopify+fixfix to before wrap.
* cpsopt/cpsopt.sml (zeroexpand): New function.
* TopLevel/viscomp/control.sml (cpsopt): Add `zeroexpand' to reduce
RA-blowup when compiling ml.grm.sml.
-*- mode: outline; tab-width: 4 -*-

* Sequentially dependent tests

* Ordering issues
** Specialize -> Wrap -> Reify
	Neither Wrap nor Specialize can be done after Reify.
	I don't know whether Specialize works after Wrap, but I wouldn't bet on it
	and in any case, I can't think of a reason why you'd want to do that.
** Lambda Split
	Should take place right after a `fixfix' so as to have good inlining
	annotations.  Also, Split should be done before Reify and Wrap since
	both slightly change the language.
** Loopify
	Loopify does not consider mutually recursive functions, so you want to
	have a `fixfix' before to eliminate apparent mutual recursion.
	Also, to inline small loops, you want to run fixfix afterwards to
	analyze the newly introduced preheaders.

* Current ordering
** lcontract
	Because the input code has lot and lots of called-once functions and
	fcontract is much slower at inlining them.
** fixfix+fcontract
	Do a first pass of inlining and careful contraction.
	The fixfix also reduces mutual recursion, which later helps loopify.
** specialize
	Might as well do it now to get rid of some of those pesky TFN/TAPP.
** loopify+fixfix+(split+)fcontract
	Introduce loop headers and do the best job you can before `wrap'.
	Split needs to be done before going through `wrap', so the earlier
	we can do the various optimizations, the better.
	Even more true for `reify' since we want to do optimizations on a
	really type-safe language rather than on a pseudo-typed language such
	as post-reify-FLINT.
** wrap+fcontract+reify
	Representation analysis, cleanup and "reification" (introduces runtime
	types: turns TFN into FCT, TAPP into APP, ...)
** fcontract+fixfix+fcontract+eta
	We finally got rid of all TFN/TAPP so we can at last inline/uncurry
	polymorphic functions.  We also introduce eta-wrappers for escaping
	functions, which is used by closure conversion.

* CPS
	After CPS conversion, we need to clean up the code a little.
** last_contract
	`contract' would work as well, since it basically does the same, but
	`first_contract' doesn't cut it because we need to be able to drop unused
	arguments.  This is because of functions that always raise an exception and
	hence don't use their continuation argument.  This optimization cannot be
	done in the A-normal form and doesn't seem particularly important on
	benchmarks, but it is crucial to avoid blowing up the register allocator
	when compiling ml.grm.sml (from about 150MB to 400MB).
** minexpand
	The CPS conversion phase does a good bit more work than just convert
	from A-normal to CPS form.  Among other things, it also compiles SWITCHes.
	This can introduce new functions, f.ex. when

		SWITCH v
	     1 => bla
		 3 => bli
		 _ => foo

	is turned into

		def(_) = foo
		if v < 1 then def(0) else
		if v > 2 then def(0) else
		case v
		 1 => bla
		 2 => def(0)
		 3 => bli

	Ideally, new functions like `def' should go through the usual
	optimizations, but there really isn't much to optimize there.  The only
	useful thing we might want to do is inline them.  This is particularly true
	if `def' is a trivial function.  `minexpand' hence goes through one pass of
	`expand' with unrolling and loop-preheaders turned off and with a max
	size-increase of 0 (we only inline if the body is as big as the function
	call).  Again, this is really only useful to avoid blowing up the register
	allocator when compiling ml.grm.sml (from about 150MB down to 80MB).

* Problems
	The presence of types introduces some difficulties.
** LET([...], RAISE ..., lexp)
	Fcontract cannot drop the `lexp' although it is dead.  The problem is that
	fcontract would need to know the type of `lexp' to update the type
	annotation on RAISE, but fcontract does not have this info.
	Interestingly, even using `recover' would not provide fcontract with the
	necessary type.
** Inlining/Uncurrying in the presence of TFN
	Many small inlinable loops are polymorphic (think of map, fold, ...).
	Flintopt currently cannot inline TFNs which is why we need a `fixfix' phase
	after reify.  Sadly, inlining TFNs would not completely solve the problem
	because the inlining heuristic relies on conditional inlining:

		TFN(map, FIX([(map, [f], body)], map), ...)

	Here `map' might be big enough that we don't want to always inline it.
	Instead we only want to inline it when the `f' argument is a known
	function.  But that means that when we see TAPP(map, tic), we can't know
	whether to inline `map' or not until we see the corresponding(s) APP(s).
	Better yet, we don't want to inline at the TAPP site but at the APP site.

	So one way to solve this would be to extend fcontract so that it expands
	the TAPP in its environment (allowing inlining at the APP site) but not in
	the code.

	Another way is to simply always inline at the TAPP site if the body
	of the TFN is something potentially inlinable.  It would introduce
	unnecessary code duplication in some cases, but since TFNs are typically
	only applied a small number of times, it shouldn't be too bad.
	The problem is that there are some very big and never-inlined functions
	that are marked as `potentially inlinable'.  My guess is that this should
	work just fine and might be "easily" doable inside `specialize'.
	Of course, ordering issues would then come up: for the small loop to be
	marked as `maybe-inline', we might need to run loopify+fixfix before
	specialize,
** WCAST
	As noted earlier, we currently work around the above problem by adding
	a `fixfix' phase after reify.  Sadly, this does not always work because
	sometimes reify translates a TFN into:

		FIX(f2[] = FN(...))
		f1 = f2[]
		f = PRIMOP(wcast, f1)

	Here again, we get stuck because the relationship between `f1' and `f' is
	obfuscated by the WCAST.  The knuth-bendix benchmark suffers a more then
	20% slowdown because of this problem.
	Solving the above problem would solve this one as well, but eliminating
	WCAST (which has been planned for some time now) would also help.

root@smlnj-gforge.cs.uchicago.edu
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