Add support for bitreverse primop

    This commit includes the necessary changes in code and
    documentation to support a primop that reverses a word's
    bits. It also includes a test.

4 files changed, 94 insertions, 0 deletions

diff --git a/testsuite/.gitignore b/testsuite/.gitignore
index 7c4453e58a..d9fa58dbdb 100644
--- a/testsuite/.gitignore
+++ b/testsuite/.gitignore
@@ -1277,6 +1277,7 @@ mk/ghcconfig*_test___spaces_ghc*.exe.mk
 /tests/primops/should_run/T6135
 /tests/primops/should_run/T7689
 /tests/primops/should_run/T9430
+/tests/primops/should_run/T16164
 /tests/profiling/should_compile/prof001
 /tests/profiling/should_compile/prof002
 /tests/profiling/should_run/2592
diff --git a/testsuite/tests/primops/should_run/T16164.hs b/testsuite/tests/primops/should_run/T16164.hs
new file mode 100644
index 0000000000..4d4336b8d6
--- /dev/null
+++ b/testsuite/tests/primops/should_run/T16164.hs
@@ -0,0 +1,52 @@
+import Data.Bits        (FiniteBits (..), unsafeShiftL, unsafeShiftR, (.&.),
+                         (.|.))
+import Data.Char        (intToDigit)
+import Data.Word        (Word8, Word16, Word32, Word64, bitReverse8,
+                         bitReverse16, bitReverse32, bitReverse64)
+import Numeric          (showIntAtBase)
+
+-- | Given a word, check:
+--
+-- * if the reverse of its @String@ representation in base 2 matches the
+--   @String@ representation of that word with its bit order reversed.
+--   order reversed, and
+-- * if reversing its bits and then reverse the resulting word's bits again
+--   yields the same word.
+-- Takes the bit reversion function as an argument so different word types
+-- can be used with their own functions.
+test :: (FiniteBits a, Integral a, Show a) => (a -> a) -> a -> Bool
+test bitReverter x =
+    let -- These zeroes are to left-pad the base-2 representation of
+        -- @x@ so that the string has one ASCII character per bit in the
+        -- word type e.g. @Word8@s produce strings with 8 characters.
+        leftPad = countLeadingZeros x
+        -- These zeroes are to left-pad the base-2 representation of
+        -- bit-reversed @x@ so that the string has one ASCII character per bit
+        -- in the word type e.g. @Word8@s produce strings with 8 characters.
+        reverseLeftPad = countTrailingZeros x
+        toBinaryString a = showIntAtBase 2 intToDigit a ""
+        binaryX = replicate leftPad '0' ++ toBinaryString x
+        revX = bitReverter x
+        binaryRevX = replicate reverseLeftPad '0' ++ toBinaryString revX
+        revRevX = bitReverter revX
+    in (x == revRevX) && (reverse binaryX == binaryRevX)
+
+word8s :: [Word8]
+word8s = [29, 31, 61, 102, 129, 129, 153, 213, 241, 246]
+
+word16s :: [Word16]
+word16s = [555, 3298, 4548, 12557, 16464, 16481, 40722, 51736, 55009, 62554]
+
+word32s :: [Word32]
+word32s = [6585, 10944, 21639, 25202, 27228,  836732395, 848624442, 3798715760, 3909052537, 4224371164]
+
+word64s :: [Word64]
+word64s = [2451351, 5096456, 8248539, 13039372, 15656413,  367814400638368418, 15152819454280096771, 15184978641026131315, 16329695467052396714, 17634654963076276082]
+
+main :: IO ()
+main = do
+    let printer f = mapM_ (print . test f)
+    printer bitReverse8 word8s
+    printer bitReverse16 word16s
+    printer bitReverse32 word32s
+    printer bitReverse64 word64s
\ No newline at end of file
diff --git a/testsuite/tests/primops/should_run/T16164.stdout b/testsuite/tests/primops/should_run/T16164.stdout
new file mode 100644
index 0000000000..dbe797f66b
--- /dev/null
+++ b/testsuite/tests/primops/should_run/T16164.stdout
@@ -0,0 +1,40 @@
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
+True
\ No newline at end of file
diff --git a/testsuite/tests/primops/should_run/all.T b/testsuite/tests/primops/should_run/all.T
index 46954e3c58..0d6f869d11 100644
--- a/testsuite/tests/primops/should_run/all.T
+++ b/testsuite/tests/primops/should_run/all.T
@@ -13,6 +13,7 @@ test('T10678',
      compile_and_run, ['-O'])
 test('T11296', normal, compile_and_run, [''])
 test('T13825-compile', normal, compile_and_run, [''])
+test('T16164', normal, compile_and_run, [''])
 test('ArithInt8', omit_ways(['ghci']), compile_and_run, [''])
 test('ArithWord8', omit_ways(['ghci']), compile_and_run, [''])
 test('CmpInt8', normal, compile_and_run, [''])