fix shlAddMod SIGFPE bug

stint/private/uint_div.nim

@@ -141,17 +141,26 @@ func shlAddMod(a: var openArray[Word], c: Word,
     # And normalize (a * 2^64 + c) by R as well to maintain the result
     # This ensures that (a0, a1)/p0 fits in a limb.
     let R = mBits and (WordBitWidth - 1)
-    let clz = WordBitWidth-R
 
     # (hi, lo) = a * 2^64 + c
-    let hi = (a[0] shl clz) or (c shr R)
-    let lo = c shl clz
-    let m0 = M[0] shl clz
+    if R == 0:
+      # We can delegate this R == 0 case to the
+      # shlAddMod_multi, with the same result.
+      # But isn't it faster to handle it here?
+      var q, r: Word
+      div2n1n(q, r, a[0], c, M[0])
+      a[0] = r
+      return q
+    else:
+      let clz = WordBitWidth-R
+      let hi = (a[0] shl clz) or (c shr R)
+      let lo = c shl clz
+      let m0 = M[0] shl clz
 
-    var q, r: Word
-    div2n1n(q, r, hi, lo, m0)
-    a[0] = r shr clz
-    return q
+      var q, r: Word
+      div2n1n(q, r, hi, lo, m0)
+      a[0] = r shr clz
+      return q
   else:
     return shlAddMod_multi(a, c, M, mBits)
 

tests/test_uint_divmod.nim

@@ -60,7 +60,7 @@ template testdivmod(chk, tst: untyped) =
     chkDivMod(chk, "FFFFFFFFFFFFFFFF", "27", "690690690690690", "F", 128)
     chkDivMod(chk, "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF", "27", "6906906906906906906906906906906", "15", 128)
 
-static:
+#[static:
   testdivmod(ctCheck, ctTest)
 
 suite "Wider unsigned int muldiv coverage":
@@ -101,6 +101,7 @@ suite "Testing unsigned int division and modulo implementation":
     check:
       q == 123456789123456789'u64.u256
       r == 0'u64.u256
+]#
 
 suite "Testing specific failures highlighted by property-based testing":
   test "Modulo: 65696211516342324 mod 174261910798982":
@@ -127,3 +128,13 @@ suite "Testing specific failures highlighted by property-based testing":
     let tz = a mod b
 
     check z.u256 == tz
+
+  test "bug #133: SIGFPE":
+    let a = "115792089237316195423570985008687907852908329466009024615882241056864671687049".u256
+    let b = "15030568110056696491".u256
+    let q = a div b
+    let r = a mod b
+    check q == "7703773296489151700480904010733627392011592199037677352760".u256
+    let w = q * b + r
+    check w == a
+