Added 'return' to C--, and made arguments to 'jump' into CmmExpr

[ghc-hetmet.git] / compiler / cmm / Cmm.hs

diff --git a/compiler/cmm/Cmm.hs b/compiler/cmm/Cmm.hs
index d30f963..c2f8d48 100644 (file)
--- a/compiler/cmm/Cmm.hs
+++ b/compiler/cmm/Cmm.hs
@@ -130,8 +130,47 @@ data CmmStmt
        --      one  -> second block etc
        -- Undefined outside range, and when there's a Nothing
 
-  | CmmJump CmmExpr [LocalReg]    -- Jump to another function, with these 
-                                 -- parameters.
+  | CmmJump CmmExpr               -- Jump to another function,
+    [(CmmExpr, MachHint)]         -- with these parameters.
+
+  | CmmReturn                     -- Return from a function,
+    [(CmmExpr, MachHint)]         -- with these return values.
+
+{-
+Discussion
+~~~~~~~~~~
+
+One possible problem with the above type is that the only way to do a
+non-local conditional jump is to encode it as a branch to a block that
+contains a single jump.  This leads to inefficient code in the back end.
+
+One possible way to fix this would be:
+
+data CmmStat = 
+  ...
+  | CmmJump CmmBranchDest
+  | CmmCondJump CmmExpr CmmBranchDest
+  ...
+
+data CmmBranchDest
+  = Local BlockId
+  | NonLocal CmmExpr [LocalReg]
+
+In favour:
+
++ one fewer constructors in CmmStmt
++ allows both cond branch and switch to jump to non-local destinations
+
+Against:
+
+- not strictly necessary: can already encode as branch+jump
+- not always possible to implement any better in the back end
+- could do the optimisation in the back end (but then plat-specific?)
+- C-- doesn't have it
+- back-end optimisation might be more general (jump shortcutting)
+
+So we'll stick with the way it is, and add the optimisation to the NCG.
+-}
 
 -----------------------------------------------------------------------------
 --             CmmCallTarget
@@ -164,6 +203,7 @@ data CmmExpr
        --        ** is shorthand only, meaning **
        -- CmmMachOp (MO_S_Add rep (CmmReg reg) (CmmLit (CmmInt i rep)))
        --      where rep = cmmRegRep reg
+  deriving Eq
 
 cmmExprRep :: CmmExpr -> MachRep
 cmmExprRep (CmmLit lit)      = cmmLitRep lit
@@ -212,6 +252,7 @@ data CmmLit
         -- It is also used inside the NCG during when generating
         -- position-independent code. 
   | CmmLabelDiffOff CLabel CLabel Int   -- label1 - label2 + offset
+  deriving Eq
 
 cmmLitRep :: CmmLit -> MachRep
 cmmLitRep (CmmInt _ rep)    = rep