recursion-ninja pushed to branch wip/fix-25664 at Glasgow Haskell Compiler / GHC

Commits:

4 changed files:

Changes:

  • compiler/GHC/Cmm/Opt.hs
    ... ... @@ -395,26 +395,39 @@ cmmMachOpFoldM platform mop [x, (CmmLit (CmmInt 1 rep))]
    395 395 
         one  = CmmLit (CmmInt 1 (wordWidth platform))
    396 396
    397 397 
     -- Now look for multiplication/division by powers of 2 (integers).
    398 
    -
    399 
    -cmmMachOpFoldM platform mop [x, (CmmLit (CmmInt n _))]
    398 
    +--
    399 
    +-- Naively this is as simple a matter as left/right bit shifts,
    400 
    +-- but the Cmm representation if integral values quickly complicated the matter.
    401 
    +--
    402 
    +-- We must carefully narrow the value to be within the range of values for the
    403 
    +-- type's logical bit-width. However, Cmm only represents values as *signed*
    404 
    +-- integers internally yet the logical type may be unsigned. If we are dealing
    405 
    +-- with a negative integer type at width @_w@, the only negative number that
    406 
    +-- wraps around to be a positive power of 2 after calling narrowU is -2^(_w - 1)
    407 
    +-- which wraps round to 2^(_w - 1), and multiplying by -2^(_w - 1) is indeed
    408 
    +-- the same as a left shift by (w - 1), so this is OK.
    409 
    +--
    410 
    +-- ToDo: See #25664 (comment 605821) describing a change to the Cmm literal representation.
    411 
    +-- When/If this is completed, this code must be refactored to account for the explicit width sizes.
    412 
    +cmmMachOpFoldM platform mop [x, (CmmLit (CmmInt n _w))]
    400 413 
       = case mop of
    401 414 
             MO_Mul rep
    402 
    -           | Just p <- exactLog2 n ->
    415 
    +           | Just p <- exactLog2 (narrowU rep n) ->
    403 416 
                      Just $! (cmmMachOpFold platform (MO_Shl rep) [x, CmmLit (CmmInt p $ wordWidth platform)])
    404 417 
             MO_U_Quot rep
    405 
    -           | Just p <- exactLog2 n ->
    418 
    +           | Just p <- exactLog2 (narrowU rep n) ->
    406 419 
                      Just $! (cmmMachOpFold platform (MO_U_Shr rep) [x, CmmLit (CmmInt p $ wordWidth platform)])
    407 420 
             MO_U_Rem rep
    408 
    -           | Just _ <- exactLog2 n ->
    421 
    +           | Just _ <- exactLog2 (narrowU rep n)  ->
    409 422 
                      Just $! (cmmMachOpFold platform (MO_And rep) [x, CmmLit (CmmInt (n - 1) rep)])
    410 423 
             MO_S_Quot rep
    411 
    -           | Just p <- exactLog2 n,
    424 
    +           | Just p <- exactLog2 (narrowS rep n),
    412 425 
                  CmmReg _ <- x ->   -- We duplicate x in signedQuotRemHelper, hence require
    413 426 
                                     -- it is a reg.  FIXME: remove this restriction.
    414 427 
                     Just $! (cmmMachOpFold platform (MO_S_Shr rep)
    415 428 
                       [signedQuotRemHelper rep p, CmmLit (CmmInt p $ wordWidth platform)])
    416 429 
             MO_S_Rem rep
    417 
    -           | Just p <- exactLog2 n,
    430 
    +           | Just p <- exactLog2 (narrowS rep n),
    418 431 
                  CmmReg _ <- x ->   -- We duplicate x in signedQuotRemHelper, hence require
    419 432 
                                     -- it is a reg.  FIXME: remove this restriction.
    420 433 
                     -- We replace (x `rem` 2^p) by (x - (x `quot` 2^p) * 2^p).

  • testsuite/tests/cmm/opt/T25664.hs
    1 
    +{-# OPTIONS_GHC -O -fno-full-laziness #-}
    2 
    +{-# LANGUAGE MagicHash #-}
    3 
    +
    4 
    +import GHC.Exts
    5 
    +import GHC.Int
    6 
    +
    7 
    +mb8 :: Int8 -> Int8
    8 
    +{-# OPAQUE mb8 #-}
    9 
    +mb8 (I8# i) = I8# (i `quotInt8#` (noinline intToInt8# 128#))
    10 
    +
    11 
    +mb16 :: Int16 -> Int16
    12 
    +{-# OPAQUE mb16 #-}
    13 
    +mb16 (I16# i) = I16# (i `quotInt16#` (noinline intToInt16# 32768#))
    14 
    +
    15 
    +main :: IO ()
    16 
    +main = print (mb8 minBound) >> print (mb16 minBound)
    17 
    +

  • testsuite/tests/cmm/opt/T25664.stdout
    1 
    +1
    2 
    +1

  • testsuite/tests/cmm/opt/all.T
    ... ... @@ -12,3 +12,6 @@ test('T25771', [cmm_src, only_ways(['optasm']),
    12 12 
                     grep_errmsg(r'(12\.345|0\.6640625)',[1]),
    13 13 
                     ],
    14 14 
          compile, ['-ddump-cmm'])
    15 
    +
    16 
    +# Cmm should correctly account for word size when performing MUL/DIV/REM by a power of 2 optimization.
    17 
    +test('T25664', normal, compile_and_run, [''])
    \ No newline at end of file

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