Jacek Caban : ntdll: Reimplement _allmul using 32-bit arithmetic.

[Alexandre Julliard]

Module: wine
Branch: master
Commit: 04117dc8d44dde789d64f717c5d293a81316ad3d
URL:    https://source.winehq.org/git/wine.git/?a=commit;h=04117dc8d44dde789d64f717c5d293a81316ad3d

Author: Jacek Caban <jacek at codeweavers.com>
Date:   Mon Jun  8 18:12:37 2020 +0200

ntdll: Reimplement _allmul using 32-bit arithmetic.

Based on compiler-rt.

Signed-off-by: Jacek Caban <jacek at codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard at winehq.org>

---

 dlls/ntdll/large_int.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/dlls/ntdll/large_int.c b/dlls/ntdll/large_int.c
index 7114b8c2d9..cf8a3df093 100644
--- a/dlls/ntdll/large_int.c
+++ b/dlls/ntdll/large_int.c
@@ -759,7 +759,29 @@ LONGLONG WINAPI _alldiv( LONGLONG a, LONGLONG b )
  */
 LONGLONG WINAPI _allmul( LONGLONG a, LONGLONG b )
 {
-    return a * b;
+    LARGE_INTEGER x = { .QuadPart = a };
+    LARGE_INTEGER y = { .QuadPart = b };
+    LARGE_INTEGER r;
+    unsigned int t;
+
+    const int bits_in_word_2 = 16;
+    const unsigned int lower_mask = ~0u >> bits_in_word_2;
+
+    r.u.LowPart = (x.u.LowPart & lower_mask) * (y.u.LowPart & lower_mask);
+    t = r.u.LowPart >> bits_in_word_2;
+    r.u.LowPart &= lower_mask;
+    t += (x.u.LowPart >> bits_in_word_2) * (y.u.LowPart & lower_mask);
+    r.u.LowPart += (t & lower_mask) << bits_in_word_2;
+    r.u.HighPart = t >> bits_in_word_2;
+    t = r.u.LowPart >> bits_in_word_2;
+    r.u.LowPart &= lower_mask;
+    t += (y.u.LowPart >> bits_in_word_2) * (x.u.LowPart & lower_mask);
+    r.u.LowPart += (t & lower_mask) << bits_in_word_2;
+    r.u.HighPart += t >> bits_in_word_2;
+    r.u.HighPart += (x.u.LowPart >> bits_in_word_2) * (y.u.LowPart >> bits_in_word_2);
+
+    r.u.HighPart += x.u.HighPart * y.u.LowPart + x.u.LowPart * y.u.HighPart;
+    return r.QuadPart;
 }