X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Fcmm%2FCmm.hs;h=a6c3ec4b83000a6b24bad6ad547fbe1dc01b8aa6;hb=8226f00b071ea156753324dc9f2967dabc3cc571;hp=d30f96317e1ca9ca64303e6071412f4020866792;hpb=49c98d143c382a1341e1046f5ca00819a25691ba;p=ghc-hetmet.git

diff --git a/compiler/cmm/Cmm.hs b/compiler/cmm/Cmm.hs
index d30f963..a6c3ec4 100644
--- a/compiler/cmm/Cmm.hs
+++ b/compiler/cmm/Cmm.hs
@@ -133,6 +133,42 @@ data CmmStmt
   | CmmJump CmmExpr [LocalReg]    -- Jump to another function, with these 
 				  -- parameters.
 
+{-
+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 +200,7 @@ 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 +249,7 @@ 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