diff options
Diffstat (limited to 'compiler/cmm/CmmSwitch.hs')
| -rw-r--r-- | compiler/cmm/CmmSwitch.hs | 66 |
1 files changed, 66 insertions, 0 deletions
diff --git a/compiler/cmm/CmmSwitch.hs b/compiler/cmm/CmmSwitch.hs index 3edfe5ce68..ce779465e3 100644 --- a/compiler/cmm/CmmSwitch.hs +++ b/compiler/cmm/CmmSwitch.hs @@ -251,6 +251,68 @@ data SwitchPlan -- findSingleValues -- 5. The thus collected pieces are assembled to a balanced binary tree. +{- + Note [Two alts + default] + ~~~~~~~~~~~~~~~~~~~~~~~~~ + +Discussion and a bit more info at #14644 + +When dealing with a switch of the form: +switch(e) { + case 1: goto l1; + case 3000: goto l2; + default: goto ldef; +} + +If we treat it as a sparse jump table we would generate: + +if (e > 3000) //Check if value is outside of the jump table. + goto ldef; +else { + if (e < 3000) { //Compare to upper value + if(e != 1) //Compare to remaining value + goto ldef; + else + goto l2; + } + else + goto l1; +} + +Instead we special case this to : + +if (e==1) goto l1; +else if (e==3000) goto l2; +else goto l3; + +This means we have: +* Less comparisons for: 1,<3000 +* Unchanged for 3000 +* One more for >3000 + +This improves code in a few ways: +* One comparison less means smaller code which helps with cache. +* It exchanges a taken jump for two jumps no taken in the >range case. + Jumps not taken are cheaper (See Agner guides) making this about as fast. +* For all other cases the first range check is removed making it faster. + +The end result is that the change is not measurably slower for the case +>3000 and faster for the other cases. + +This makes running this kind of match in an inner loop cheaper by 10-20% +depending on the data. +In nofib this improves wheel-sieve1 by 4-9% depending on problem +size. + +We could also add a second conditional jump after the comparison to +keep the range check like this: + cmp 3000, rArgument + jg <default> + je <branch 2> +While this is fairly cheap it made no big difference for the >3000 case +and slowed down all other cases making it not worthwhile. +-} + -- | Does the target support switch out of the box? Then leave this to the -- target! @@ -272,6 +334,10 @@ createSwitchPlan (SwitchTargets _signed (lo,hi) Nothing m) --Checking If |range| = 2 is enough if we have two unique literals , hi - lo == 1 = IfEqual x1 l1 (Unconditionally l2) +-- See Note [Two alts + default] +createSwitchPlan (SwitchTargets _signed _range (Just defLabel) m) + | [(x1, l1), (x2,l2)] <- M.toAscList m + = IfEqual x1 l1 (IfEqual x2 l2 (Unconditionally defLabel)) createSwitchPlan (SwitchTargets signed range mbdef m) = -- pprTrace "createSwitchPlan" (text (show ids) $$ text (show (range,m)) $$ text (show pieces) $$ text (show flatPlan) $$ text (show plan)) $ plan |