X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Fcmm%2FCmmExpr.hs;h=a4d07c2c60316f6956af3e51d0241fa7219800ba;hb=f96194794bf099020706c3816d1a5678b40addbb;hp=d09be34b6a943ad261435ad786cfe5b67ac37c00;hpb=d436c70d43fb905c63220040168295e473f4b90a;p=ghc-hetmet.git

diff --git a/compiler/cmm/CmmExpr.hs b/compiler/cmm/CmmExpr.hs
index d09be34..a4d07c2 100644
--- a/compiler/cmm/CmmExpr.hs
+++ b/compiler/cmm/CmmExpr.hs
@@ -95,13 +95,32 @@ data Area
   deriving (Eq, Ord)
 
 data AreaId
-  = Old -- entry parameters, jumps, and returns share one call area at old end of stack
+  = Old            -- See Note [Old Area]
   | Young BlockId
   deriving (Eq, Ord)
 
+{- Note [Old Area] 
+~~~~~~~~~~~~~~~~~~
+There is a single call area 'Old', allocated at the extreme old
+end of the stack frame (ie just younger than the return address)
+which holds:
+  * incoming (overflow) parameters, 
+  * outgoing (overflow) parameter to tail calls,
+  * outgoing (overflow) result values 
+  * the update frame (if any)
+
+Its size is the max of all these requirements.  On entry, the stack
+pointer will point to the youngest incoming parameter, which is not
+necessarily at the young end of the Old area.
+
+End of note -}
+
 type SubArea    = (Area, Int, Int) -- area, offset, width
 type SubAreaSet = FiniteMap Area [SubArea]
+
 type AreaMap    = FiniteMap Area Int
+     -- Byte offset of the oldest byte of the Area, 
+     -- relative to the oldest byte of the Old Area
 
 data CmmLit
   = CmmInt Integer  Width