diff options
Diffstat (limited to 'compiler/cmm/CmmSink.hs')
| -rw-r--r-- | compiler/cmm/CmmSink.hs | 57 |
1 files changed, 34 insertions, 23 deletions
diff --git a/compiler/cmm/CmmSink.hs b/compiler/cmm/CmmSink.hs index 487f0bc244..43444639e1 100644 --- a/compiler/cmm/CmmSink.hs +++ b/compiler/cmm/CmmSink.hs @@ -173,7 +173,7 @@ cmmSink dflags graph = ofBlockList (g_entry graph) $ sink mapEmpty $ blocks liveness = cmmLocalLiveness dflags graph getLive l = mapFindWithDefault Set.empty l liveness - blocks = postorderDfs graph + blocks = revPostorder graph join_pts = findJoinPoints blocks @@ -458,17 +458,7 @@ tryToInline dflags live node assigs = go usages node emptyLRegSet assigs occurs_once = not l_live && l_usages == Just 1 occurs_none = not l_live && l_usages == Nothing - inl_node = case mapExpDeep inl_exp node of - -- See Note [Improving conditionals] - CmmCondBranch (CmmMachOp (MO_Ne w) args) - ti fi l - -> CmmCondBranch (cmmMachOpFold dflags (MO_Eq w) args) - fi ti (inv_likeliness l) - node' -> node' - - inv_likeliness :: Maybe Bool -> Maybe Bool - inv_likeliness Nothing = Nothing - inv_likeliness (Just l) = Just (not l) + inl_node = improveConditional (mapExpDeep inl_exp node) inl_exp :: CmmExpr -> CmmExpr -- inl_exp is where the inlining actually takes place! @@ -479,22 +469,43 @@ tryToInline dflags live node assigs = go usages node emptyLRegSet assigs inl_exp (CmmMachOp op args) = cmmMachOpFold dflags op args inl_exp other = other -{- Note [Improving conditionals] -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -Given - CmmCondBranch ((a >## b) != 1) t f -where a,b, are Floats, the constant folder /cannot/ turn it into - CmmCondBranch (a <=## b) t f -because comparison on floats are not invertible -(see CmmMachOp.maybeInvertComparison). -What we want instead is simply to reverse the true/false branches thus +{- Note [improveConditional] + +cmmMachOpFold tries to simplify conditionals to turn things like + (a == b) != 1 +into + (a != b) +but there's one case it can't handle: when the comparison is over +floating-point values, we can't invert it, because floating-point +comparisions aren't invertible (because NaN). + +But we *can* optimise this conditional by swapping the true and false +branches. Given CmmCondBranch ((a >## b) != 1) t f ---> +we can turn it into CmmCondBranch (a >## b) f t -And we do that right here in tryToInline, just as we do cmmMachOpFold. +So here we catch conditionals that weren't optimised by cmmMachOpFold, +and apply above transformation to eliminate the comparison against 1. + +It's tempting to just turn every != into == and then let cmmMachOpFold +do its thing, but that risks changing a nice fall-through conditional +into one that requires two jumps. (see swapcond_last in +CmmContFlowOpt), so instead we carefully look for just the cases where +we can eliminate a comparison. -} +improveConditional :: CmmNode O x -> CmmNode O x +improveConditional + (CmmCondBranch (CmmMachOp mop [x, CmmLit (CmmInt 1 _)]) t f l) + | neLike mop, isComparisonExpr x + = CmmCondBranch x f t (fmap not l) + where + neLike (MO_Ne _) = True + neLike (MO_U_Lt _) = True -- (x<y) < 1 behaves like (x<y) != 1 + neLike (MO_S_Lt _) = True -- (x<y) < 1 behaves like (x<y) != 1 + neLike _ = False +improveConditional other = other -- Note [dependent assignments] -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |