compiler: Small optimisation of assertMwip/T22739

In #22739 @AndreasK noticed that assertM performed the action to compute
the asserted predicate regardless of whether DEBUG is enabled. This is
inconsistent with the other assertion operations and general convention.
Fix this.

Closes #22739.

1 files changed, 7 insertions, 1 deletions

diff --git a/compiler/GHC/Utils/Panic/Plain.hs b/compiler/GHC/Utils/Panic/Plain.hs
index 10b963bf5d..a12ddb4050 100644
--- a/compiler/GHC/Utils/Panic/Plain.hs
+++ b/compiler/GHC/Utils/Panic/Plain.hs
@@ -29,6 +29,8 @@ import GHC.Utils.Constants
 import GHC.Utils.Exception as Exception
 import GHC.Stack
 import GHC.Prelude.Basic
+
+import Control.Monad (when)
 import System.IO.Unsafe
 
 -- | This type is very similar to 'GHC.Utils.Panic.GhcException', but it omits
@@ -150,4 +152,8 @@ massert cond = withFrozenCallStack (assert cond (pure ()))
 
 assertM :: (HasCallStack, Monad m) => m Bool -> m ()
 {-# INLINE assertM #-}
-assertM mcond = withFrozenCallStack (mcond >>= massert)
+assertM mcond
+  | debugIsOn = withFrozenCallStack $ do
+      res <- mcond
+      when (not res) assertPanic'
+  | otherwise = return ()