[project @ 2002-02-01 17:40:11 by sewardj]

[ghc-hetmet.git] / ghc / docs / comm / the-beast / ncg.html

diff --git a/ghc/docs/comm/the-beast/ncg.html b/ghc/docs/comm/the-beast/ncg.html
index cfef919..2b2d2fc 100644 (file)
--- a/ghc/docs/comm/the-beast/ncg.html
+++ b/ghc/docs/comm/the-beast/ncg.html
@@ -96,6 +96,29 @@
         implement on all targets, and their meaning is intended to be
         unambiguous, and the same on all targets, regardless of word
         size or endianness.
         implement on all targets, and their meaning is intended to be
         unambiguous, and the same on all targets, regardless of word
         size or endianness.

+        <p>
+        <b>A note on <code>MagicId</code>s.</b>
+        Those which are assigned to
+        registers on the current target are left unmodified.  Those
+        which are not are stored in memory as offsets from
+        <code>BaseReg</code> (which is assumed to permanently have the
+        value <code>(&MainCapability.r)</code>), so the constant folder
+        calculates the offsets and inserts suitable loads/stores.  One
+        complication is that not all archs have <code>BaseReg</code>
+        itself in a register, so for those (sparc), we instead
+        generate the address as an offset from the static symbol
+        <code>MainCapability</code>, since the register table lives in
+        there.
+        <p>
+        Finally, <code>BaseReg</code> does occasionally itself get
+        mentioned in Stix expression trees, and in this case what is
+        denoted is precisely <code>(&MainCapability.r)</code>, not, as
+        in all other cases, the value of memory at some offset from
+        the start of the register table.  Since what it denotes is an
+        r-value and not an l-value, assigning <code>BaseReg</code> is
+        meaningless, so the machinery checks to ensure this never
+        happens.  All these details are taken into account by the
+        constant folder.

     <p>
     <li><b>Instruction selection.</b>  This is the only majorly
         target-specific phase.  It turns Stix statements and
     <p>
     <li><b>Instruction selection.</b>  This is the only majorly
         target-specific phase.  It turns Stix statements and