LLVM generate llvm.expect for conditional branches

This patch adds likeliness annotations to heap and and stack checks and
modifies the llvm codegen to recognize those to help it generate better
code.

So with this patch

```
...
if ((Sp + 8) - 24 < SpLim) (likely: False) goto c23c; else goto c23d;
...
```

roughly generates:

```
  %ln23k = icmp ult i64 %ln23j, %SpLim_Arg
  %ln23m = call ccc i1 (i1, i1) @llvm.expect.i1( i1 %ln23k, i1 0 )
  br i1 %ln23m, label %c23c, label %c23d
```

Note the call to `llvm.expect` which denotes the expected result for
the comparison.

Test Plan: Look at assembler code with and without this patch. If the
heap-checks moved out of the way we are happy.

Reviewers: austin, simonmar, bgamari

Reviewed By: bgamari

Subscribers: michalt, thomie

Differential Revision: https://phabricator.haskell.org/D2688

GHC Trac Issues: #8321

4 files changed, 73 insertions, 24 deletions

diff --git a/compiler/cmm/CmmContFlowOpt.hs b/compiler/cmm/CmmContFlowOpt.hs
index 455422b47b..ed953ac5a8 100644
--- a/compiler/cmm/CmmContFlowOpt.hs
+++ b/compiler/cmm/CmmContFlowOpt.hs
@@ -282,19 +282,24 @@ blockConcat splitting_procs g@CmmGraph { g_entry = entry_id }
           -- This helps the native codegen a little bit, and probably has no
           -- effect on LLVM.  It's convenient to do it here, where we have the
           -- information about predecessors.
-          --
-          -- NB., only do this if the branch does not have a
-          -- likeliness annotation.
           swapcond_last
-            | CmmCondBranch cond t f Nothing <- shortcut_last
+            | CmmCondBranch cond t f l <- shortcut_last
+            , likelyFalse l
             , numPreds f > 1
             , hasOnePredecessor t
             , Just cond' <- maybeInvertCmmExpr cond
-            = CmmCondBranch cond' f t Nothing
+            = CmmCondBranch cond' f t (invertLikeliness l)
 
             | otherwise
             = shortcut_last
 
+          likelyFalse (Just False) = True
+          likelyFalse Nothing      = True
+          likelyFalse _            = False
+
+          invertLikeliness (Just b)     = Just (not b)
+          invertLikeliness Nothing      = Nothing
+
           -- Number of predecessors for a block
           numPreds bid = mapLookup bid backEdges `orElse` 0
 
diff --git a/compiler/codeGen/StgCmmHeap.hs b/compiler/codeGen/StgCmmHeap.hs
index ebff4402d0..aa8855660b 100644
--- a/compiler/codeGen/StgCmmHeap.hs
+++ b/compiler/codeGen/StgCmmHeap.hs
@@ -636,7 +636,8 @@ do_checks mb_stk_hwm checkYield mb_alloc_lit do_gc = do
 
   case mb_stk_hwm of
     Nothing -> return ()
-    Just stk_hwm -> tickyStackCheck >> (emit =<< mkCmmIfGoto (sp_oflo stk_hwm) gc_id)
+    Just stk_hwm -> tickyStackCheck
+      >> (emit =<< mkCmmIfGoto' (sp_oflo stk_hwm) gc_id (Just False) )
 
   -- Emit new label that might potentially be a header
   -- of a self-recursive tail call.
@@ -651,14 +652,14 @@ do_checks mb_stk_hwm checkYield mb_alloc_lit do_gc = do
     then do
      tickyHeapCheck
      emitAssign hpReg bump_hp
-     emit =<< mkCmmIfThen hp_oflo (alloc_n <*> mkBranch gc_id)
+     emit =<< mkCmmIfThen' hp_oflo (alloc_n <*> mkBranch gc_id) (Just False)
     else do
       when (checkYield && not (gopt Opt_OmitYields dflags)) $ do
          -- Yielding if HpLim == 0
          let yielding = CmmMachOp (mo_wordEq dflags)
                                   [CmmReg (CmmGlobal HpLim),
                                    CmmLit (zeroCLit dflags)]
-         emit =<< mkCmmIfGoto yielding gc_id
+         emit =<< mkCmmIfGoto' yielding gc_id (Just False)
 
   tscope <- getTickScope
   emitOutOfLine gc_id
diff --git a/compiler/codeGen/StgCmmMonad.hs b/compiler/codeGen/StgCmmMonad.hs
index 836bf30f29..2184e12a8c 100644
--- a/compiler/codeGen/StgCmmMonad.hs
+++ b/compiler/codeGen/StgCmmMonad.hs
@@ -26,6 +26,8 @@ module StgCmmMonad (
         getCodeR, getCode, getCodeScoped, getHeapUsage,
 
         mkCmmIfThenElse, mkCmmIfThen, mkCmmIfGoto,
+        mkCmmIfThenElse', mkCmmIfThen', mkCmmIfGoto',
+
         mkCall, mkCmmCall,
 
         forkClosureBody, forkLneBody, forkAlts, codeOnly,
@@ -833,30 +835,50 @@ getCmm code
 
 
 mkCmmIfThenElse :: CmmExpr -> CmmAGraph -> CmmAGraph -> FCode CmmAGraph
-mkCmmIfThenElse e tbranch fbranch = do
+mkCmmIfThenElse e tbranch fbranch = mkCmmIfThenElse' e tbranch fbranch Nothing
+
+mkCmmIfThenElse' :: CmmExpr -> CmmAGraph -> CmmAGraph
+                 -> Maybe Bool -> FCode CmmAGraph
+mkCmmIfThenElse' e tbranch fbranch likely = do
   tscp  <- getTickScope
   endif <- newLabelC
   tid   <- newLabelC
   fid   <- newLabelC
-  return $ catAGraphs [ mkCbranch e tid fid Nothing
-                      , mkLabel tid tscp, tbranch, mkBranch endif
-                      , mkLabel fid tscp, fbranch, mkLabel endif tscp ]
+
+  let
+    (test, then_, else_, likely') = case likely of
+      Just False | Just e' <- maybeInvertCmmExpr e
+        -- currently NCG doesn't know about likely
+        -- annotations. We manually switch then and
+        -- else branch so the likely false branch
+        -- becomes a fallthrough.
+        -> (e', fbranch, tbranch, Just True)
+      _ -> (e, tbranch, fbranch, likely)
+
+  return $ catAGraphs [ mkCbranch test tid fid likely'
+                      , mkLabel tid tscp, then_, mkBranch endif
+                      , mkLabel fid tscp, else_, mkLabel endif tscp ]
 
 mkCmmIfGoto :: CmmExpr -> BlockId -> FCode CmmAGraph
-mkCmmIfGoto e tid = do
+mkCmmIfGoto e tid = mkCmmIfGoto' e tid Nothing
+
+mkCmmIfGoto' :: CmmExpr -> BlockId -> Maybe Bool -> FCode CmmAGraph
+mkCmmIfGoto' e tid l = do
   endif <- newLabelC
   tscp  <- getTickScope
-  return $ catAGraphs [ mkCbranch e tid endif Nothing, mkLabel endif tscp ]
+  return $ catAGraphs [ mkCbranch e tid endif l, mkLabel endif tscp ]
 
 mkCmmIfThen :: CmmExpr -> CmmAGraph -> FCode CmmAGraph
-mkCmmIfThen e tbranch = do
+mkCmmIfThen e tbranch = mkCmmIfThen' e tbranch Nothing
+
+mkCmmIfThen' :: CmmExpr -> CmmAGraph -> Maybe Bool -> FCode CmmAGraph
+mkCmmIfThen' e tbranch l = do
   endif <- newLabelC
   tid   <- newLabelC
   tscp  <- getTickScope
-  return $ catAGraphs [ mkCbranch e tid endif Nothing
+  return $ catAGraphs [ mkCbranch e tid endif l
                       , mkLabel tid tscp, tbranch, mkLabel endif tscp ]
 
-
 mkCall :: CmmExpr -> (Convention, Convention) -> [CmmFormal] -> [CmmExpr]
        -> UpdFrameOffset -> [CmmExpr] -> FCode CmmAGraph
 mkCall f (callConv, retConv) results actuals updfr_off extra_stack = do
diff --git a/compiler/llvmGen/LlvmCodeGen/CodeGen.hs b/compiler/llvmGen/LlvmCodeGen/CodeGen.hs
index fa47d6ada3..7b610c0a0a 100644
--- a/compiler/llvmGen/LlvmCodeGen/CodeGen.hs
+++ b/compiler/llvmGen/LlvmCodeGen/CodeGen.hs
@@ -118,8 +118,8 @@ stmtToInstrs stmt = case stmt of
     CmmStore addr src    -> genStore addr src
 
     CmmBranch id         -> genBranch id
-    CmmCondBranch arg true false _      -- TODO: likely annotation
-                         -> genCondBranch arg true false
+    CmmCondBranch arg true false likely
+                         -> genCondBranch arg true false likely
     CmmSwitch arg ids    -> genSwitch arg ids
 
     -- Foreign Call
@@ -925,20 +925,41 @@ genBranch id =
 
 
 -- | Conditional branch
-genCondBranch :: CmmExpr -> BlockId -> BlockId -> LlvmM StmtData
-genCondBranch cond idT idF = do
+genCondBranch :: CmmExpr -> BlockId -> BlockId -> Maybe Bool -> LlvmM StmtData
+genCondBranch cond idT idF likely = do
     let labelT = blockIdToLlvm idT
     let labelF = blockIdToLlvm idF
     -- See Note [Literals and branch conditions].
-    (vc, stmts, top) <- exprToVarOpt i1Option cond
+    (vc, stmts1, top1) <- exprToVarOpt i1Option cond
     if getVarType vc == i1
         then do
-            let s1 = BranchIf vc labelT labelF
-            return (stmts `snocOL` s1, top)
+            (vc', (stmts2, top2)) <- case likely of
+              Just b -> genExpectLit (if b then 1 else 0) i1  vc
+              _      -> pure (vc, (nilOL, []))
+            let s1 = BranchIf vc' labelT labelF
+            return (stmts1 `appOL` stmts2 `snocOL` s1, top1 ++ top2)
         else do
             dflags <- getDynFlags
             panic $ "genCondBranch: Cond expr not bool! (" ++ showSDoc dflags (ppr vc) ++ ")"
 
+
+-- | Generate call to llvm.expect.x intrinsic. Assigning result to a new var.
+genExpectLit :: Integer -> LlvmType -> LlvmVar -> LlvmM (LlvmVar, StmtData)
+genExpectLit expLit expTy var = do
+  dflags <- getDynFlags
+
+  let
+    lit = LMLitVar $ LMIntLit expLit expTy
+
+    llvmExpectName
+      | isInt expTy = fsLit $ "llvm.expect." ++ showSDoc dflags (ppr expTy)
+      | otherwise   = panic $ "genExpectedLit: Type not an int!"
+
+  (llvmExpect, stmts, top) <-
+    getInstrinct llvmExpectName expTy [expTy, expTy]
+  (var', call) <- doExpr expTy $ Call StdCall llvmExpect [var, lit] []
+  return (var', (stmts `snocOL` call, top))
+
 {- Note [Literals and branch conditions]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~