+
+
+data JumpDest = DestBlockId BlockId | DestImm Imm
+
+
+canShortcut :: Instr -> Maybe JumpDest
+canShortcut (JXX ALWAYS id) = Just (DestBlockId id)
+canShortcut (JMP (OpImm imm)) = Just (DestImm imm)
+canShortcut _ = Nothing
+
+
+-- This helper shortcuts a sequence of branches.
+-- The blockset helps avoid following cycles.
+shortcutJump :: (BlockId -> Maybe JumpDest) -> Instr -> Instr
+shortcutJump fn insn = shortcutJump' fn emptyBlockSet insn
+ where shortcutJump' fn seen insn@(JXX cc id) =
+ if elemBlockSet id seen then insn
+ else case fn id of
+ Nothing -> insn
+ Just (DestBlockId id') -> shortcutJump' fn seen' (JXX cc id')
+ Just (DestImm imm) -> shortcutJump' fn seen' (JXX_GBL cc imm)
+ where seen' = extendBlockSet seen id
+ shortcutJump' _ _ other = other
+
+-- Here because it knows about JumpDest
+shortcutStatic :: (BlockId -> Maybe JumpDest) -> CmmStatic -> CmmStatic
+shortcutStatic fn (CmmStaticLit (CmmLabel lab))
+ | Just uq <- maybeAsmTemp lab
+ = CmmStaticLit (CmmLabel (shortBlockId fn emptyUniqSet (BlockId uq)))
+shortcutStatic fn (CmmStaticLit (CmmLabelDiffOff lbl1 lbl2 off))
+ | Just uq <- maybeAsmTemp lbl1
+ = CmmStaticLit (CmmLabelDiffOff (shortBlockId fn emptyUniqSet (BlockId uq)) lbl2 off)
+ -- slightly dodgy, we're ignoring the second label, but this
+ -- works with the way we use CmmLabelDiffOff for jump tables now.
+
+shortcutStatic _ other_static
+ = other_static
+
+shortBlockId
+ :: (BlockId -> Maybe JumpDest)
+ -> UniqSet Unique
+ -> BlockId
+ -> CLabel
+
+shortBlockId fn seen blockid@(BlockId uq) =
+ case (elementOfUniqSet uq seen, fn blockid) of
+ (True, _) -> mkAsmTempLabel uq
+ (_, Nothing) -> mkAsmTempLabel uq
+ (_, Just (DestBlockId blockid')) -> shortBlockId fn (addOneToUniqSet seen uq) blockid'
+ (_, Just (DestImm (ImmCLbl lbl))) -> lbl
+ (_, _other) -> panic "shortBlockId"