inside the GHC code generator

Simon Peyton-Jones simonpj at
Fri Feb 24 04:30:22 EST 2006

| last days i studied GHC code generator, reading -ddumps of all sorts,
| GHC sources, various papers and John's appeals :)
| what i carried from this investigation:
| GHC has great high-level optimization. moreover, GHC now allows to
| program STG almost directly. when i look at STG code i don't see
| anything that can be done better - at least for these simple loops i
| tried to compile. i see just unboxed variables, primitive operations
| and simple loops represented as tail recursion. fine.
| then STG compiled to the simplified C (called abstract C earlier and
| quasi C-- now), what is next can be translated:
| * to real C and then compiled by gcc
| * to assembler by build-in simple C-- compiler
| * to assembler by some external C-- compiler (at least it is
| theoretically possible)

Good stuff Bulat.  There's plenty of interesting stuff to be done here.

However, let me strongly urge you *not* to focus attention primarily on
the gcc route.  Compiling via C has received a lot of attention over the
years, and there are many papers describing cool hacks for doing so.
GHC does not do as well as it could. 

But there are serious obstacles.  That's not gcc's fault -- it wasn't
designed for this.  Accurate GC is one of them, tail calls is another,
and there are plenty more smaller things that bite you only after you've
invested a lot of time.  This way lies madness.

C-- was *designed* for this purpose.  GHC uses C-- as its intermediate
language (just before emitting C).  So a good route is this:

* Write C-- to C-- optimisations

* Then, if you like, translate that code to C.  Already you will be
doing better than GHC does today, because the C-- to C-- optimiser will
let you generate better C

* But you can also emit C--, or native code; both of these paths will
directly benefit from your C-- optimisations.

The point is that none of this relies on Quick C--; you can always use
an alternative back end.

You can almost do this today. GHC uses C-- as an intermediate language.
But alas, GHC's code generator does not take advantage of C--'s native
calls or parameter passing.  Instead, it pushes things on an auxiliary
stack etc.  (Those Sp memory operations you see.)  This isn't necessary.
We are planning to split GHC's code generator into two parts
	A) Generate C-- with native calls, with an implicit C-- stack
	B) Perform CPS conversion, to eliminate all calls in favour of
		using an explicit stack
The current code gen is A followed by B.  But A is a much more suitable
optimisation platform, and gives more flexibility.

Chris Thompson, and undergrad at Cambridge, is doing (B) as his
undergrad project, although it remains to be seen whether he'll have
enough complete to be usable.  

Another shortcoming is that the native code generator in GHC isn't
capable of dealing with backward jumps to labels (because GHC hasn't
needed that so far).  But if you did C-- optimisation, you'd probably
generate such jumps.  It'd be great to beef up the native code gen to
handle that.

Many of the optimisations you describe (perhaps all) are readily
expressible in the C-- intermediate language, and by working at that
level you will be independent of with the back end is gcc, a native code
generator, or Quick C--, or some other C-- compiler.  Much better.


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