X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2FNOTES;h=14725ddbfbcc57b9e7d99e0440e7eda860302916;hb=2c6d73e2ca9a545c4295c6f532cd3612e7fd3d8d;hp=67b4c623b9135c8ba91eca3a4ce5295df393ce8f;hpb=0004357ccaa3149cb112f5f5df1af60e65baad79;p=ghc-hetmet.git

diff --git a/ghc/compiler/NOTES b/ghc/compiler/NOTES
index 67b4c62..14725dd 100644
--- a/ghc/compiler/NOTES
+++ b/ghc/compiler/NOTES
@@ -54,3 +54,57 @@ completeLazyBind: 	[given a simplified RHS]
 
   - 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.