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, 2 deletions

diff --git a/compiler/cmm/CmmProcPoint.hs b/compiler/cmm/CmmProcPoint.hs
index 2add4741ef..a31048206b 100644
--- a/compiler/cmm/CmmProcPoint.hs
+++ b/compiler/cmm/CmmProcPoint.hs
@@ -18,6 +18,7 @@ import PprCmm ()
 import CmmUtils
 import CmmInfo
 import CmmLive (cmmGlobalLiveness)
+import CmmSwitch
 import Data.List (sortBy)
 import Maybes
 import Control.Monad
@@ -295,7 +296,7 @@ splitAtProcPoints dflags entry_label callPPs procPoints procMap
                     case lastNode block of
                       CmmBranch id          -> add_if_pp id rst
                       CmmCondBranch _ ti fi -> add_if_pp ti (add_if_pp fi rst)
-                      CmmSwitch _ tbl       -> foldr add_if_pp rst (catMaybes tbl)
+                      CmmSwitch _ ids       -> foldr add_if_pp rst $ switchTargetsToList ids
                       _                     -> rst
 
                   -- when jumping to a PP that has an info table, if
@@ -382,7 +383,7 @@ replaceBranches env cmmg
     last :: CmmNode O C -> CmmNode O C
     last (CmmBranch id)          = CmmBranch (lookup id)
     last (CmmCondBranch e ti fi) = CmmCondBranch e (lookup ti) (lookup fi)
-    last (CmmSwitch e tbl)       = CmmSwitch e (map (fmap lookup) tbl)
+    last (CmmSwitch e ids)       = CmmSwitch e (mapSwitchTargets lookup ids)
     last l@(CmmCall {})          = l { cml_cont = Nothing }
             -- NB. remove the continuation of a CmmCall, since this
             -- label will now be in a different CmmProc.  Not only