Mark divModInt and friends as INLINE (#19267)

So that we don't get a silly worker `$wdivModInt` and risk inlining
`divModInt#` into `divModInt` or `$wdivModInt`, making both unlikely to
inline at call sites.

Fixes #19267.

There's a spurious metric decrease (was an *increase*) in T12545. That
seems entirely due to shifts in Unique distribution (+5% more
`IntMap.$winsert` calls). The inappropriateness of the acceptance window
is tracked in #19414.

Metric Decrease:
    T12545
Metric Increase:
    T12545

1 files changed, 16 insertions, 0 deletions

diff --git a/libraries/base/GHC/Base.hs b/libraries/base/GHC/Base.hs
index d21226c7c4..e691c558bf 100644
--- a/libraries/base/GHC/Base.hs
+++ b/libraries/base/GHC/Base.hs
@@ -1596,8 +1596,13 @@ getTag x = dataToTag# x
 -- Definitions of the boxed PrimOps; these will be
 -- used in the case of partial applications, etc.
 
+-- See Note [INLINE division wrappers]
 {-# INLINE quotInt #-}
 {-# INLINE remInt #-}
+{-# INLINE divInt #-}
+{-# INLINE modInt #-}
+{-# INLINE quotRemInt #-}
+{-# INLINE divModInt #-}
 
 quotInt, remInt, divInt, modInt :: Int -> Int -> Int
 (I# x) `quotInt`  (I# y) = I# (x `quotInt#` y)
@@ -1625,6 +1630,17 @@ x# `divModInt#` y#
  | otherwise                                =
                                     x# `quotRemInt#` y#
 
+{- Note [INLINE division wrappers]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The Int division functions such as 'quotRemInt' and 'divModInt' have
+been manually worker/wrappered, presumably because they construct
+*nested* products.
+We intend to preserve the exact worker/wrapper split, hence we mark
+the wrappers INLINE (#19267). That makes sure the optimiser doesn't
+accidentally inline the worker into the wrapper, undoing the manual
+split again.
+-}
+
 -- Wrappers for the shift operations.  The uncheckedShift# family are
 -- undefined when the amount being shifted by is greater than the size
 -- in bits of Int#, so these wrappers perform a check and return