From ac73c1b13a4fef725f643e6985cdcb6f6fc1bef5 Mon Sep 17 00:00:00 2001
From: Sylvain Henry <sylvain@haskus.fr>
Date: Tue, 6 Aug 2019 19:35:16 +0200
Subject: [PATCH] Faster exactLog2

Make `exactLog2` faster (use `countLeadingZeros` and Int32 bit-ops).

On my Core i7-9700k Criterion reports ~50% speedup (from 16 to 8ns).
---
 compiler/utils/Util.hs | 22 ++++++++--------------
 1 file changed, 8 insertions(+), 14 deletions(-)

diff --git a/compiler/utils/Util.hs b/compiler/utils/Util.hs
index dba5a4495f..ecbef752f1 100644
--- a/compiler/utils/Util.hs
+++ b/compiler/utils/Util.hs
@@ -1123,22 +1123,16 @@ toArgs str
 -----------------------------------------------------------------------------
 -- Integers
 
--- This algorithm for determining the $\log_2$ of exact powers of 2 comes
--- from GCC.  It requires bit manipulation primitives, and we use GHC
--- extensions.  Tough.
-
+-- | Determine the $\log_2$ of exact powers of 2
 exactLog2 :: Integer -> Maybe Integer
 exactLog2 x
-  = if (x <= 0 || x >= 2147483648) then
-       Nothing
-    else
-       if (x .&. (-x)) /= x then
-          Nothing
-       else
-          Just (pow2 x)
-  where
-    pow2 x | x == 1 = 0
-           | otherwise = 1 + pow2 (x `shiftR` 1)
+   | x <= 0                               = Nothing
+   | x > fromIntegral (maxBound :: Int32) = Nothing
+   | x' .&. (-x') /= x'                   = Nothing
+   | otherwise                            = Just (fromIntegral c)
+      where
+         x' = fromIntegral x :: Int32
+         c = countTrailingZeros x'
 
 {-
 -- -----------------------------------------------------------------------------
-- 
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