Make indexError work better

The problem here is described at some length in
Note [Boxity for bottoming functions] and
Note [Reboxed crud for bottoming calls] in GHC.Core.Opt.DmdAnal.

This patch adds a SPECIALISE pragma for indexError, which
makes it much less vulnerable to the problem described in
these Notes.

(This came up in another line of work, where a small change made
indexError do reboxing (in nofib/spectral/simple/table_sort)
that didn't happen before my change.  I've opened #22404
to document the fagility.

2 files changed, 24 insertions, 3 deletions

diff --git a/compiler/GHC/Core/Opt/DmdAnal.hs b/compiler/GHC/Core/Opt/DmdAnal.hs
index c6c5d11926..51c0ab702b 100644
--- a/compiler/GHC/Core/Opt/DmdAnal.hs
+++ b/compiler/GHC/Core/Opt/DmdAnal.hs
@@ -1520,6 +1520,9 @@ $wtheresCrud = \ ww ww1 ->
       ...
 ```
 This is currently a bug that we willingly accept and it's documented in #21128.
+
+See also Note [indexError] in base:GHC.Ix, which describes how we use
+SPECIALISE to mitigate this problem for indexError.
 -}
 
 {- *********************************************************************
diff --git a/libraries/base/GHC/Ix.hs b/libraries/base/GHC/Ix.hs
index be02b568a2..abbe62c2f1 100644
--- a/libraries/base/GHC/Ix.hs
+++ b/libraries/base/GHC/Ix.hs
@@ -140,12 +140,30 @@ Note [Out-of-bounds error messages]
 The default method for 'index' generates hoplelessIndexError, because
 Ix doesn't have Show as a superclass.  For particular base types we
 can do better, so we override the default method for index.
--}
 
--- Abstract these errors from the relevant index functions so that
--- the guts of the function will be small enough to inline.
+Note [indexError]
+~~~~~~~~~~~~~~~~~
+We abstract the guts of constructing an out-of-bounds error into `indexError`.
+We give it a NOINLINE pragma, because we don't want to duplicate this
+cold-path code.
+
+We give it a SPECIALISE pragma because we really want it to take
+its arguments unboxed, to avoid reboxing code in the caller, and
+perhaps even some reboxing code in the hot path of a caller.
+See Note [Boxity for bottoming functions] in GHC.Core.Opt.DmdAnal.
+
+The SPECIALISE pragma means that at least the Int-indexed case
+of indexError /will/ unbox its arguments.
 
+The [2] phase is because if we don't give an activation we'll get
+the one from the inline pragama (i.e. never) which is a bit silly.
+See Note [Activation pragmas for SPECIALISE] in GHC.HsToCore.Binds.
+-}
+
+-- indexError: see Note [indexError]
 {-# NOINLINE indexError #-}
+{-# SPECIALISE [2] indexError :: (Int,Int) -> Int -> String -> b #-}
+
 indexError :: Show a => (a,a) -> a -> String -> b
 indexError rng i tp
   = errorWithoutStackTrace (showString "Ix{" . showString tp . showString "}.index: Index " .