PmCheck: Only ever check constantly many models against a single patternwip/pmcheck-limit-deltas

Introduces a new flag `-fmax-pmcheck-deltas` to achieve that. Deprecates
the old `-fmax-pmcheck-iter` mechanism in favor of this new flag.

From the user's guide:

Pattern match checking can be exponential in some cases. This limit makes sure
we scale polynomially in the number of patterns, by forgetting refined
information gained from a partially successful match. For example, when
matching `x` against `Just 4`, we split each incoming matching model into two
sub-models: One where `x` is not `Nothing` and one where `x` is `Just y` but
`y` is not `4`. When the number of incoming models exceeds the limit, we
continue checking the next clause with the original, unrefined model.

This also retires the incredibly hard to understand "maximum number of
refinements" mechanism, because the current mechanism is more general
and should catch the same exponential cases like PrelRules at the same
time.

-------------------------
Metric Decrease:
    T11822
-------------------------

1 files changed, 11 insertions, 4 deletions

diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index c2d0322cd9..b2c927fffb 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -974,7 +974,6 @@ data DynFlags = DynFlags {
   debugLevel            :: Int,         -- ^ How much debug information to produce
   simplPhases           :: Int,         -- ^ Number of simplifier phases
   maxSimplIterations    :: Int,         -- ^ Max simplifier iterations
-  maxPmCheckIterations  :: Int,         -- ^ Max no iterations for pm checking
   ruleCheck             :: Maybe String,
   inlineCheck           :: Maybe String, -- ^ A prefix to report inlining decisions about
   strictnessBefore      :: [Int],       -- ^ Additional demand analysis
@@ -998,6 +997,10 @@ data DynFlags = DynFlags {
                                         --   error messages
   maxUncoveredPatterns  :: Int,         -- ^ Maximum number of unmatched patterns to show
                                         --   in non-exhaustiveness warnings
+  maxPmCheckModels      :: Int,         -- ^ Soft limit on the number of models
+                                        --   the pattern match checker checks
+                                        --   a pattern against. A safe guard
+                                        --   against exponential blow-up.
   simplTickFactor       :: Int,         -- ^ Multiplier for simplifier ticks
   specConstrThreshold   :: Maybe Int,   -- ^ Threshold for SpecConstr
   specConstrCount       :: Maybe Int,   -- ^ Max number of specialisations for any one function
@@ -1926,7 +1929,6 @@ defaultDynFlags mySettings (myLlvmTargets, myLlvmPasses) =
         debugLevel              = 0,
         simplPhases             = 2,
         maxSimplIterations      = 4,
-        maxPmCheckIterations    = 2000000,
         ruleCheck               = Nothing,
         inlineCheck             = Nothing,
         binBlobThreshold        = 500000, -- 500K is a good default (see #16190)
@@ -1935,6 +1937,7 @@ defaultDynFlags mySettings (myLlvmTargets, myLlvmPasses) =
         maxRefHoleFits     = Just 6,
         refLevelHoleFits   = Nothing,
         maxUncoveredPatterns    = 4,
+        maxPmCheckModels        = 100,
         simplTickFactor         = 100,
         specConstrThreshold     = Just 2000,
         specConstrCount         = Just 3,
@@ -3599,12 +3602,16 @@ dynamic_flags_deps = [
             "vectors registers are now passed in registers by default."
   , make_ord_flag defFlag "fmax-uncovered-patterns"
       (intSuffix (\n d -> d { maxUncoveredPatterns = n }))
+  , make_ord_flag defFlag "fmax-pmcheck-models"
+      (intSuffix (\n d -> d { maxPmCheckModels = n }))
   , make_ord_flag defFlag "fsimplifier-phases"
       (intSuffix (\n d -> d { simplPhases = n }))
   , make_ord_flag defFlag "fmax-simplifier-iterations"
       (intSuffix (\n d -> d { maxSimplIterations = n }))
-  , make_ord_flag defFlag "fmax-pmcheck-iterations"
-      (intSuffix (\n d -> d{ maxPmCheckIterations = n }))
+  , (Deprecated, defFlag "fmax-pmcheck-iterations"
+      (intSuffixM (\_ d ->
+       do { deprecate $ "use -fmax-pmcheck-models instead"
+          ; return d })))
   , make_ord_flag defFlag "fsimpl-tick-factor"
       (intSuffix (\n d -> d { simplTickFactor = n }))
   , make_ord_flag defFlag "fspec-constr-threshold"