Annotate CmmBranch with an optional likely target

Summary:
This allows the code generator to give hints to later code generation
steps about which branch is most likely to be taken.  Right now it
is only taken into account in one place: a special case in
CmmContFlowOpt that swapped branches over to maximise the chance of
fallthrough, which is now disabled when there is a likelihood setting.

Test Plan: validate

Reviewers: austin, simonpj, bgamari, ezyang, tibbe

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1273

4 files changed, 9 insertions, 7 deletions

diff --git a/compiler/codeGen/StgCmmExpr.hs b/compiler/codeGen/StgCmmExpr.hs
index 747f71a630..29db20a765 100644
--- a/compiler/codeGen/StgCmmExpr.hs
+++ b/compiler/codeGen/StgCmmExpr.hs
@@ -876,7 +876,8 @@ emitEnter fun = do
        ; tscope <- getTickScope
        ; emit $
            copyout <*>
-           mkCbranch (cmmIsTagged dflags (CmmReg nodeReg)) lret lcall <*>
+           mkCbranch (cmmIsTagged dflags (CmmReg nodeReg))
+                     lret lcall Nothing <*>
            outOfLine lcall (the_call,tscope) <*>
            mkLabel lret tscope <*>
            copyin
diff --git a/compiler/codeGen/StgCmmLayout.hs b/compiler/codeGen/StgCmmLayout.hs
index c3d8873cfb..593dd6cc18 100644
--- a/compiler/codeGen/StgCmmLayout.hs
+++ b/compiler/codeGen/StgCmmLayout.hs
@@ -226,9 +226,10 @@ slowCall fun stg_args
                                                   (mkIntExpr dflags n_args)
 
              tscope <- getTickScope
-             emit (mkCbranch (cmmIsTagged dflags funv) is_tagged_lbl slow_lbl
+             emit (mkCbranch (cmmIsTagged dflags funv)
+                             is_tagged_lbl slow_lbl (Just True)
                    <*> mkLabel is_tagged_lbl tscope
-                   <*> mkCbranch correct_arity fast_lbl slow_lbl
+                   <*> mkCbranch correct_arity fast_lbl slow_lbl (Just True)
                    <*> mkLabel fast_lbl tscope
                    <*> fast_code
                    <*> mkBranch end_lbl
diff --git a/compiler/codeGen/StgCmmMonad.hs b/compiler/codeGen/StgCmmMonad.hs
index f7a13f9c98..1acf31b327 100644
--- a/compiler/codeGen/StgCmmMonad.hs
+++ b/compiler/codeGen/StgCmmMonad.hs
@@ -831,7 +831,7 @@ mkCmmIfThenElse e tbranch fbranch = do
   endif <- newLabelC
   tid   <- newLabelC
   fid   <- newLabelC
-  return $ catAGraphs [ mkCbranch e tid fid
+  return $ catAGraphs [ mkCbranch e tid fid Nothing
                       , mkLabel tid tscp, tbranch, mkBranch endif
                       , mkLabel fid tscp, fbranch, mkLabel endif tscp ]
 
@@ -839,14 +839,14 @@ mkCmmIfGoto :: CmmExpr -> BlockId -> FCode CmmAGraph
 mkCmmIfGoto e tid = do
   endif <- newLabelC
   tscp  <- getTickScope
-  return $ catAGraphs [ mkCbranch e tid endif, mkLabel endif tscp ]
+  return $ catAGraphs [ mkCbranch e tid endif Nothing, mkLabel endif tscp ]
 
 mkCmmIfThen :: CmmExpr -> CmmAGraph -> FCode CmmAGraph
 mkCmmIfThen e tbranch = do
   endif <- newLabelC
   tid   <- newLabelC
   tscp  <- getTickScope
-  return $ catAGraphs [ mkCbranch e tid endif
+  return $ catAGraphs [ mkCbranch e tid endif Nothing
                       , mkLabel tid tscp, tbranch, mkLabel endif tscp ]
 
 
diff --git a/compiler/codeGen/StgCmmUtils.hs b/compiler/codeGen/StgCmmUtils.hs
index db771c2f9a..a03625262c 100644
--- a/compiler/codeGen/StgCmmUtils.hs
+++ b/compiler/codeGen/StgCmmUtils.hs
@@ -558,7 +558,7 @@ mk_float_switch :: Width -> CmmExpr -> BlockId
               -> FCode CmmAGraph
 mk_float_switch rep scrut deflt _bounds [(lit,blk)]
   = do dflags <- getDynFlags
-       return $ mkCbranch (cond dflags) deflt blk
+       return $ mkCbranch (cond dflags) deflt blk Nothing
   where
     cond dflags = CmmMachOp ne [scrut, CmmLit cmm_lit]
       where