X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Fcmm%2FZipCfgCmmRep.hs;h=43e310c80c3f242393496d1d66a344c07481a2b5;hb=74f14cbc3c51135809977b29427c7c6a2af2cc34;hp=05203e54d2a081a8cab3b04aa51d28e099daa6c0;hpb=e6243a818496aad82b6f47511d3bd9bc800f747d;p=ghc-hetmet.git

diff --git a/compiler/cmm/ZipCfgCmmRep.hs b/compiler/cmm/ZipCfgCmmRep.hs
index 05203e5..43e310c 100644
--- a/compiler/cmm/ZipCfgCmmRep.hs
+++ b/compiler/cmm/ZipCfgCmmRep.hs
@@ -64,7 +64,7 @@ data Middle
   | MidStore  CmmExpr CmmExpr    -- Assign to memory location.  Size is
                                  -- given by cmmExprType of the rhs.
 
-  | MidForeignCall               -- A foreign call;
+  | MidForeignCall               -- A foreign call; see Note [Foreign calls]
      ForeignSafety               -- Is it a safe or unsafe call?
      MidCallTarget               -- call target and convention
      CmmFormals                  -- zero or more results
@@ -87,7 +87,8 @@ data Last
         cml_target  :: CmmExpr,  -- never a CmmPrim to a CallishMachOp!
         cml_cont    :: Maybe BlockId,
             -- BlockId of continuation (Nothing for return or tail call)
-        cml_args    :: ByteOff,  -- bytes offset for youngest outgoing arg
+        cml_args    :: ByteOff,  -- byte offset for youngest outgoing arg
+                                 -- (includes update frame, which must be younger)
         cml_ret_off :: Maybe UpdFrameOffset}
           -- stack offset for return (update frames);
           -- The return offset should be Nothing only if we have to create
@@ -141,6 +142,33 @@ data ValueDirection = Arguments | Results
   -- Arguments go with procedure definitions, jumps, and arguments to calls
   -- Results go with returns and with results of calls.
   deriving Eq
+ 
+{- Note [Foreign calls]
+~~~~~~~~~~~~~~~~~~~~~~~
+A MidForeign call is used *all* foreign calls, both *unsafe* and *safe*.
+Unsafe ones are easy: think of them as a "fat machine instruction".
+
+Safe ones are trickier.  A safe foreign call 
+     r = f(x)
+ultimately expands to
+     push "return address"	-- Never used to return to; 
+     	  	  		-- just points an info table
+     save registers into TSO
+     call suspendThread
+     r = f(x)			-- Make the call
+     call resumeThread
+     restore registers
+     pop "return address"
+We cannot "lower" a safe foreign call to this sequence of Cmms, because
+after we've saved Sp all the Cmm optimiser's assumptions are broken.
+Furthermore, currently the smart Cmm constructors know the calling
+conventions for Haskell, the garbage collector, etc, and "lower" them
+so that a LastCall passes no parameters or results.  But the smart 
+constructors do *not* (currently) know the foreign call conventions.
+
+For these reasons use MidForeignCall for all calls. The only annoying thing
+is that a safe foreign call needs an info table.
+-}
 
 ----------------------------------------------------------------------
 ----- Splicing between blocks