Refactor the story around switches (#10137)

This re-implements the code generation for case expressions at the Stg →
Cmm level, both for data type cases as well as for integral literal
cases. (Cases on float are still treated as before).

The goal is to allow for fancier strategies in implementing them, for a
cleaner separation of the strategy from the gritty details of Cmm, and
to run this later than the Common Block Optimization, allowing for one
way to attack #10124. The new module CmmSwitch contains a number of
notes explaining this changes. For example, it creates larger
consecutive jump tables than the previous code, if possible.

nofib shows little significant overall improvement of runtime. The
rather large wobbling comes from changes in the code block order
(see #8082, not much we can do about it). But the decrease in code size
alone makes this worthwhile.

```
        Program           Size    Allocs   Runtime   Elapsed  TotalMem
            Min          -1.8%      0.0%     -6.1%     -6.1%     -2.9%
            Max          -0.7%     +0.0%     +5.6%     +5.7%     +7.8%
 Geometric Mean          -1.4%     -0.0%     -0.3%     -0.3%     +0.0%
```

Compilation time increases slightly:
```
        -1 s.d.                -----            -2.0%
        +1 s.d.                -----            +2.5%
        Average                -----            +0.3%
```

The test case T783 regresses a lot, but it is the only one exhibiting
any regression. The cause is the changed order of branches in an
if-then-else tree, which makes the hoople data flow analysis traverse
the blocks in a suboptimal order. Reverting that gets rid of this
regression, but has a consistent, if only very small (+0.2%), negative
effect on runtime. So I conclude that this test is an extreme outlier
and no reason to change the code.

Differential Revision: https://phabricator.haskell.org/D720

1 files changed, 3 insertions, 5 deletions

diff --git a/compiler/cmm/CmmCommonBlockElim.hs b/compiler/cmm/CmmCommonBlockElim.hs
index 95910d16d5..09124106d5 100644
--- a/compiler/cmm/CmmCommonBlockElim.hs
+++ b/compiler/cmm/CmmCommonBlockElim.hs
@@ -8,6 +8,7 @@ where
 import BlockId
 import Cmm
 import CmmUtils
+import CmmSwitch (eqSwitchTargetWith)
 import CmmContFlowOpt
 import Prelude hiding (iterate, succ, unzip, zip)
 
@@ -203,13 +204,10 @@ eqLastWith eqBid (CmmCondBranch c1 t1 f1) (CmmCondBranch c2 t2 f2) =
   c1 == c2 && eqBid t1 t2 && eqBid f1 f2
 eqLastWith eqBid (CmmCall t1 c1 g1 a1 r1 u1) (CmmCall t2 c2 g2 a2 r2 u2) =
   t1 == t2 && eqMaybeWith eqBid c1 c2 && a1 == a2 && r1 == r2 && u1 == u2 && g1 == g2
-eqLastWith eqBid (CmmSwitch e1 bs1) (CmmSwitch e2 bs2) =
-  e1 == e2 && eqListWith (eqMaybeWith eqBid) bs1 bs2
+eqLastWith eqBid (CmmSwitch e1 ids1) (CmmSwitch e2 ids2) =
+  e1 == e2 && eqSwitchTargetWith eqBid ids1 ids2
 eqLastWith _ _ _ = False
 
-eqListWith :: (a -> b -> Bool) -> [a] -> [b] -> Bool
-eqListWith eltEq es es' = all (uncurry eltEq) (List.zip es es')
-
 eqMaybeWith :: (a -> b -> Bool) -> Maybe a -> Maybe b -> Bool
 eqMaybeWith eltEq (Just e) (Just e') = eltEq e e'
 eqMaybeWith _ Nothing Nothing = True