From cb43dfcd86ff1904bd70bf355f1658eb66489842 Mon Sep 17 00:00:00 2001
From: simonmar <unknown>
Date: Tue, 7 Jan 2003 14:31:20 +0000
Subject: [PATCH] [project @ 2003-01-07 14:31:19 by simonmar] Update comments

---
 ghc/compiler/codeGen/CgBindery.lhs |   14 +++-----------
 ghc/compiler/codeGen/CgMonad.lhs   |   10 +++++++++-
 2 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/ghc/compiler/codeGen/CgBindery.lhs b/ghc/compiler/codeGen/CgBindery.lhs
index 8e1a9fd..edfe45e 100644
--- a/ghc/compiler/codeGen/CgBindery.lhs
+++ b/ghc/compiler/codeGen/CgBindery.lhs
@@ -435,17 +435,9 @@ There are four kinds of things on the stack:
 	- free slots (recorded in the stack free list)
 	- non-pointer data slots (recorded in the stack free list)
 
-We build up a bitmap of non-pointer slots by looking down the
-environment for all the non-pointer variables, and merging this with
-the slots recorded in the stack free list.
-
-There's a bit of a hack here to do with update frames: since nothing
-is recorded in either the environment or the stack free list for an
-update frame, the code below defaults to assuming the slots taken up
-by an update frame contain pointers.  Furthermore, update frames are
-always in slots 0-2 at the bottom of the stack.  The bitmap will
-therefore end at slot 3, which is what we want (the update frame info
-pointer has its own bitmap to describe the update frame).
+We build up a bitmap of non-pointer slots by searching the environment
+for all the pointer variables, and subtracting these from a bitmap
+with initially all bits set (up to the size of the stack frame).
 
 \begin{code}
 buildLivenessMask 
diff --git a/ghc/compiler/codeGen/CgMonad.lhs b/ghc/compiler/codeGen/CgMonad.lhs
index 2a7e3ea..a14b77a 100644
--- a/ghc/compiler/codeGen/CgMonad.lhs
+++ b/ghc/compiler/codeGen/CgMonad.lhs
@@ -1,7 +1,7 @@
 %
 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
 %
-% $Id: CgMonad.lhs,v 1.36 2002/12/11 15:36:26 simonmar Exp $
+% $Id: CgMonad.lhs,v 1.37 2003/01/07 14:31:20 simonmar Exp $
 %
 \section[CgMonad]{The code generation monad}
 
@@ -206,6 +206,14 @@ type StackUsage =
 	 Int,		   -- realSp:  Virtual offset of real stack pointer
 	 Int)		   -- hwSp:    Highest value ever taken by virtSp
 
+-- ToDo (SDM, 7 Jan 2003): I'm not sure that the distinction between
+-- Free and NonPointer in the free list is needed any more.  It used
+-- to be needed because we constructed bitmaps from the free list, but
+-- now we construct bitmaps by finding all the live pointer bindings
+-- instead.  Non-pointer stack slots (i.e. saved cost centres) can
+-- just be removed from the free list instead of being recorded as a
+-- NonPointer.
+
 type HeapUsage =
 	(HeapOffset,	-- virtHp: Virtual offset of highest-allocated word
 	 HeapOffset)	-- realHp: Virtual offset of real heap ptr
-- 
1.7.10.4