Statically estimate execution frequency of CoreAlts (#20378)wip/exec-freq

This patch implements #20378. See `Note [Estimating CoreAlt frequencies]` in the
new module GHC.Core.Opt.ExecFreq for details.

These were the changes:

1. Introduce `newtype Freq = Freq Float` as a type that captures relative
   execution frequency and use it as an additional field in `CoreAlt`.
   The default when we have no estimate available is `NoFreq`, e.g., NaN.
   Otherwise, all `Freq`s of a `Case` should add up to 1.
   Then fix up a whole bunch of use sites.
2. Introduce a new enum `Comparison` for the different kinds of comparison
   operators (LessThan, GreaterOrEqual, NotEqual, ...). Then make `Compare`
   primops also declare what kind of `Comparison` they do. Then introduce a
   function `isComparisonApp_maybe` in GHC.Core.Utils that we can use for our
   estimates (see below).
3. Write a static analysis pass `estimateAltFreqs`, that annotates `CoreAlt`s
   with their relative execution `Freq`. These `Freq`s are determined by
   combining the estimates of different branch heuristics, one of which uses
   the new `isComparisonApp_maybe`.

The main function `estimateAltFreqs` is currently dead, but that is bound to
change in follow-up MRs.

Fixes #20378.

1 files changed, 2 insertions, 2 deletions

diff --git a/utils/genprimopcode/Main.hs b/utils/genprimopcode/Main.hs
index 06a4922aa3..fa4605fb69 100644
--- a/utils/genprimopcode/Main.hs
+++ b/utils/genprimopcode/Main.hs
@@ -830,10 +830,10 @@ mkPOI_LHS_text i
 mkPOI_RHS_text :: Entry -> String
 mkPOI_RHS_text i
    = case cat i of
-        Compare
+        Compare cmp
            -> case ty i of
                  TyF t1 (TyF _ _)
-                    -> "mkCompare " ++ sl_name i ++ ppType t1
+                    -> "mkCompare " ++ sl_name i ++ cmp ++ " " ++ ppType t1
                  _ -> error "Type error in comparison op"
         GenPrimOp
            -> let (argTys, resTy) = flatTys (ty i)