PmCheck: Formulate as translation between Clause Trees

We used to check `GrdVec`s arising from multiple clauses and guards in
isolation. That resulted in a split between `pmCheck` and
`pmCheckGuards`, the implementations of which were similar, but subtly
different in detail. Also the throttling mechanism described in
`Note [Countering exponential blowup]` ultimately got quite complicated
because it had to cater for both checking functions.

This patch realises that pattern match checking doesn't just consider
single guarded RHSs, but that it's always a whole set of clauses, each
of which can have multiple guarded RHSs in turn. We do so by
translating a list of `Match`es to a `GrdTree`:

```haskell
data GrdTree
  = Rhs !RhsInfo
  | Guard !PmGrd !GrdTree      -- captures lef-to-right  match semantics
  | Sequence !GrdTree !GrdTree -- captures top-to-bottom match semantics
  | Empty                      -- For -XEmptyCase, neutral element of Sequence
```

Then we have a function `checkGrdTree` that matches a given `GrdTree`
against an incoming set of values, represented by `Deltas`:

```haskell
checkGrdTree :: GrdTree -> Deltas -> CheckResult
...
```

Throttling is isolated to the `Sequence` case and becomes as easy as one
would expect: When the union of uncovered values becomes too big, just
return the original incoming `Deltas` instead (which is always a
superset of the union, thus a sound approximation).

The returned `CheckResult` contains two things:

1. The set of values that were not covered by any of the clauses, for
   exhaustivity warnings.
2. The `AnnotatedTree` that enriches the syntactic structure of the
   input program with divergence and inaccessibility information.

This is `AnnotatedTree`:

```haskell
data AnnotatedTree
  = AccessibleRhs !RhsInfo
  | InaccessibleRhs !RhsInfo
  | MayDiverge !AnnotatedTree
  | SequenceAnn !AnnotatedTree !AnnotatedTree
  | EmptyAnn
```

Crucially, `MayDiverge` asserts that the tree may force diverging
values, so not all of its wrapped clauses can be redundant.

While the set of uncovered values can be used to generate the missing
equations for warning messages, redundant and proper inaccessible
equations can be extracted from `AnnotatedTree` by
`redundantAndInaccessibleRhss`.

For this to work properly, the interface to the Oracle had to change.
There's only `addPmCts` now, which takes a bag of `PmCt`s. There's a
whole bunch of `PmCt` variants to replace the different oracle functions
from before.

The new `AnnotatedTree` structure allows for more accurate warning
reporting (as evidenced by a number of changes spread throughout GHC's
code base), thus we fix #17465.

Fixes #17646 on the go.

Metric Decrease:
    T11822
    T9233
    PmSeriesS
    haddock.compiler

1 files changed, 3 insertions, 1 deletions

diff --git a/compiler/nativeGen/RegAlloc/Graph/SpillClean.hs b/compiler/nativeGen/RegAlloc/Graph/SpillClean.hs
index 6d7b377d68..79dbf63a66 100644
--- a/compiler/nativeGen/RegAlloc/Graph/SpillClean.hs
+++ b/compiler/nativeGen/RegAlloc/Graph/SpillClean.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 
 -- | Clean out unneeded spill\/reload instructions.
 --
@@ -393,9 +394,11 @@ cleanBackward' liveSlotsOnEntry reloadedBy noReloads acc (li : instrs)
 
                 cleanBackward liveSlotsOnEntry noReloads' (li : acc) instrs
 
+#if __GLASGOW_HASKELL__ <= 810
         -- some other instruction
         | otherwise
         = cleanBackward liveSlotsOnEntry noReloads (li : acc) instrs
+#endif
 
 
 -- | Combine the associations from all the inward control flow edges.
@@ -611,4 +614,3 @@ closeAssoc a assoc
 intersectAssoc :: Assoc a -> Assoc a -> Assoc a
 intersectAssoc a b
         = intersectUFM_C (intersectUniqSets) a b
-