Fix documentation and fix "check" bignum backend (#18604)

5 files changed, 19 insertions, 19 deletions

diff --git a/libraries/ghc-bignum/src/GHC/Num/BigNat/Check.hs b/libraries/ghc-bignum/src/GHC/Num/BigNat/Check.hs
index f19eceb3e7..8431a5564d 100644
--- a/libraries/ghc-bignum/src/GHC/Num/BigNat/Check.hs
+++ b/libraries/ghc-bignum/src/GHC/Num/BigNat/Check.hs
@@ -129,8 +129,8 @@ mwaCompareOpBool mwa f g s =
       0# -> case unexpectedValue of
                !_ -> (# s, ra #)
                -- see Note [ghc-bignum exceptions] in GHC.Num.Primitives
-      _  -> case (ra ==# 1#) of -- don't compare MWAs if overflow signaled!
-         1# -> (# s, ra #)
+      _  -> case ra of -- don't compare MWAs if underflow signaled!
+         0# -> (# s, ra #) -- underflow
          _  -> case mwaTrimZeroes# mwa s of { s ->
                case mwaTrimZeroes# mwb s of { s ->
                case mwaCompare mwa mwb s of
diff --git a/libraries/ghc-bignum/src/GHC/Num/BigNat/FFI.hs b/libraries/ghc-bignum/src/GHC/Num/BigNat/FFI.hs
index 3ef2f7046c..cfd96be7a0 100644
--- a/libraries/ghc-bignum/src/GHC/Num/BigNat/FFI.hs
+++ b/libraries/ghc-bignum/src/GHC/Num/BigNat/FFI.hs
@@ -113,7 +113,7 @@ foreign import ccall unsafe ghc_bignat_mul_word
 -- The potential 0 most-significant Words will be removed by the caller if it is
 -- not already done by the backend.
 --
--- Return True to indicate overflow.
+-- Return False# to indicate underflow.
 bignat_sub
    :: MutableWordArray# RealWorld
    -> WordArray#
@@ -136,7 +136,7 @@ foreign import ccall unsafe ghc_bignat_sub
 -- The potential 0 most-significant Words will be removed by the caller if it is
 -- not already done by the backend.
 --
--- Return True to indicate overflow.
+-- Return False# to indicate underflow.
 bignat_sub_word
    :: MutableWordArray# RealWorld
    -> WordArray#
diff --git a/libraries/ghc-bignum/src/GHC/Num/BigNat/GMP.hs b/libraries/ghc-bignum/src/GHC/Num/BigNat/GMP.hs
index d316c65e82..35d3983313 100644
--- a/libraries/ghc-bignum/src/GHC/Num/BigNat/GMP.hs
+++ b/libraries/ghc-bignum/src/GHC/Num/BigNat/GMP.hs
@@ -92,8 +92,8 @@ bignat_sub
 {-# INLINE bignat_sub #-}
 bignat_sub mwa wa wb s =
    case ioWord# (c_mpn_sub mwa wa (wordArraySize# wa) wb (wordArraySize# wb)) s of
-      (# s', 1## #) -> (# s', 0# #) -- overflow
-      (# s', _   #) -> (# s', 1# #) -- no overflow
+      (# s', 1## #) -> (# s', 0# #) -- underflow
+      (# s', _   #) -> (# s', 1# #) -- no underflow
 
 bignat_sub_word
    :: MutableWordArray# RealWorld
@@ -104,8 +104,8 @@ bignat_sub_word
 {-# INLINE bignat_sub_word #-}
 bignat_sub_word mwa wa b s =
    case ioWord# (c_mpn_sub_1 mwa wa (wordArraySize# wa) b) s of
-      (# s', 1## #) -> (# s', 0# #) -- overflow
-      (# s', _   #) -> (# s', 1# #) -- no overflow
+      (# s', 1## #) -> (# s', 0# #) -- underflow
+      (# s', _   #) -> (# s', 1# #) -- no underflow
 
 bignat_mul
    :: MutableWordArray# RealWorld
diff --git a/libraries/ghc-bignum/src/GHC/Num/BigNat/Native.hs b/libraries/ghc-bignum/src/GHC/Num/BigNat/Native.hs
index 592a434337..5c1baa4398 100644
--- a/libraries/ghc-bignum/src/GHC/Num/BigNat/Native.hs
+++ b/libraries/ghc-bignum/src/GHC/Num/BigNat/Native.hs
@@ -131,13 +131,13 @@ bignat_sub_word mwa wa b = go b 0#
 
          | 0## <- carry
          = case mwaArrayCopy# mwa i wa i (sz -# i) s of
-            s' -> (# s', 1# #) -- no overflow
+            s' -> (# s', 1# #) -- no underflow
 
          | True
          = case subWordC# (indexWordArray# wa i) carry of
             (# 0##, 0# #)
                | isTrue# (i ==# sz) -> case mwaShrink# mwa 1# s of
-                                          s' -> (# s', 1# #) -- no overflow
+                                          s' -> (# s', 1# #) -- no underflow
 
             (# l  , c  #) -> case mwaWrite# mwa i l s of
                               s1 -> go (int2Word# c) (i +# 1#) s1
diff --git a/libraries/ghc-bignum/src/GHC/Num/WordArray.hs b/libraries/ghc-bignum/src/GHC/Num/WordArray.hs
index eded6a83ee..dffb7e5797 100644
--- a/libraries/ghc-bignum/src/GHC/Num/WordArray.hs
+++ b/libraries/ghc-bignum/src/GHC/Num/WordArray.hs
@@ -318,7 +318,7 @@ mwaAddInplaceWord# mwa i y   s = case readWordArray# mwa i s of
 -- | Sub Word# inplace (at the specified offset) in the mwa with carry
 -- propagation.
 --
--- Return False# on overflow
+-- Return False# on underflow
 mwaSubInplaceWord#
    :: MutableWordArray# d
    -> Int#
@@ -328,9 +328,9 @@ mwaSubInplaceWord#
 mwaSubInplaceWord# mwa ii iw s1 = case mwaSize# mwa s1 of
    (# is, sz #) ->
       let
-         go _ 0## s = (# s, 1# #) -- no overflow
+         go _ 0## s = (# s, 1# #) -- no underflow
          go i y   s
-            | isTrue# (i >=# sz) = (# s, 0# #) -- overflow
+            | isTrue# (i >=# sz) = (# s, 0# #) -- underflow
             | True = case readWordArray# mwa i s of
                (# s1, x #) -> let !(# l,h #) = subWordC# x y
                   in case mwaWrite# mwa i l s1 of
@@ -368,16 +368,16 @@ mwaTrimCompare k mwa wb s1
 --
 -- We don't trim the resulting array!
 --
--- Return False# on overflow.
+-- Return False# on underflow.
 mwaSubInplaceArray :: MutableWordArray# d -> Int# -> WordArray# -> State# d -> (# State# d, Bool# #)
 mwaSubInplaceArray mwa off wb = go (wordArraySize# wb -# 1#)
    where
       go i s
-         | isTrue# (i <# 0#) = (# s, 1# #) -- no overflow
+         | isTrue# (i <# 0#) = (# s, 1# #) -- no underflow
          | True
          = case mwaSubInplaceWord# mwa (off +# i) (indexWordArray# wb i) s of
             (# s2, 1# #) -> go (i -# 1#) s2
-            (# s2, _  #) -> (# s2, 0# #) -- overflow
+            (# s2, _  #) -> (# s2, 0# #) -- underflow
 
 -- | Add array inplace (a the specified offset) in the mwa with carry propagation.
 --
@@ -398,19 +398,19 @@ mwaAddInplaceArray mwa off wb = go 0# 0##
 --
 -- We don't trim the resulting array!
 --
--- Return False# on overflow.
+-- Return False# on underflow.
 mwaSubInplaceMutableArray :: MutableWordArray# d -> Int# -> MutableWordArray# d -> State# d -> (# State# d, Bool# #)
 mwaSubInplaceMutableArray mwa off mwb s0 =
    case mwaSize# mwb s0 of
       (# s1, szB #) -> go (szB -# 1#) s1
    where
       go i s
-         | isTrue# (i <# 0#) = (# s, 1# #) -- no overflow
+         | isTrue# (i <# 0#) = (# s, 1# #) -- no underflow
          | True
          = case readWordArray# mwb i s of
             (# s1, bi #) -> case mwaSubInplaceWord# mwa (off +# i) bi s1 of
                (# s2, 1# #) -> go (i -# 1#) s2
-               (# s2, _  #) -> (# s2, 0# #) -- overflow
+               (# s2, _  #) -> (# s2, 0# #) -- underflow
 
 -- | Sub an array inplace and then trim zeroes
 --