[commit: ghc] master: Add missing parentheses in eqBigNatWord# (c55f61c)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/c55f61ca3f008cc70911d962297fb51f84056661/ghc

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commit c55f61ca3f008cc70911d962297fb51f84056661
Author: Reid Barton <rwbarton at gmail.com>
Date:   Sat Jul 25 23:01:49 2015 -0400

    Add missing parentheses in eqBigNatWord#
    
    Summary:
    I'm pretty sure that parentheses were intended here. But oddly, they
    make very little difference.
    
    The presumably intended expression
        (sizeofBigNat# bn ==# 1#) `andI#` (bigNatToWord bn `eqWord#` w#)
    is 1# exactly when bn consists of a single limb equal to w#, clearly.
    
    In the original expression
        sizeofBigNat# bn ==# 1# `andI#` (bigNatToWord bn `eqWord#` w#)
    the right-hand side of ==# is always 0# or 1#. So it is 1# when bn
    consists of a single limb equal to w#. It is also 1# when bn has
    zero limbs and the word past the end of bn does not happen to be
    equal to w#. So in practice the difference is that nullBigNat was
    eqBigNatWord# to almost everything, but eqBigNatWord# is never
    supposed to be called on nullBigNat anyways.
    
    Note that even the corrected version might perform an out-of-bounds
    memory access if passed nullBigNat, because `andI#` is not guaranteed
    to short-circuit, though in fact GHC does convert isZeroBigNat to a
    series of branches in my local build.
    
    Test Plan: validate
    
    Reviewers: hvr, bgamari, goldfire, austin
    
    Reviewed By: hvr, bgamari
    
    Subscribers: thomie
    
    Differential Revision: https://phabricator.haskell.org/D1095


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c55f61ca3f008cc70911d962297fb51f84056661
 libraries/integer-gmp/src/GHC/Integer/Type.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libraries/integer-gmp/src/GHC/Integer/Type.hs b/libraries/integer-gmp/src/GHC/Integer/Type.hs
index 88d1923..d941c4c 100644
--- a/libraries/integer-gmp/src/GHC/Integer/Type.hs
+++ b/libraries/integer-gmp/src/GHC/Integer/Type.hs
@@ -835,7 +835,7 @@ eqBigNatWord bn w# = isTrue# (eqBigNatWord# bn w#)
 
 eqBigNatWord# :: BigNat -> GmpLimb# -> Int#
 eqBigNatWord# bn w#
-    = sizeofBigNat# bn ==# 1# `andI#` (bigNatToWord bn `eqWord#` w#)
+    = (sizeofBigNat# bn ==# 1#) `andI#` (bigNatToWord bn `eqWord#` w#)
 
 
 -- | Same as @'indexBigNat#' bn 0\#@