Fix overview for int dot to allow saturation of 16-bit intermediates

proposals/relaxed-simd/Overview.md

@@ -261,22 +261,45 @@ When the second operand of the product has the high bit set in a lane, that
 lane's result is implementation defined.
 
 ```python
-def dot_product(signed, elements, a, b, c):
+def i16x8_dot_i8x16_i7x16_s(a, b):
   intermediate = []
   result = []
   for i in range(16):
     if (b[i] & 0x80):
-      lhs = as_signed(a[i]) if signed else a[i]
-      rhs = IMPLEMENTATION_DEFINED_ONE_OF(as_signed(b[i]), b[i])
+      lhs = as_signed(a[i])
+      rhs = IMPLEMENTATION_DEFINED_ONE_OF(as_signed(b[i]), as_unsigned(b[i]))
       intermediate[i] = lhs * rhs
     else:
-      intermediate[i] = (as_signed(a[i]) if signed else a[i]) * b[i]
-  for i in range(0, 16, elements):
-    result[i/elements] = sum(intermediate[i:i+elements])
-    result[i/elements] += c[i/elements] if c else 0
+      intermediate[i] = as_signed(a[i]) * b[i]
+  for i in range(0, 16, 2):
+    result[i/2] = IMPLEMENTATION_DEFINED_ONE_OF(
+      intermediate[i] + intermediate[i+1],
+      saturate(intermediate[i] + intermediate[i+1]))
 
-i16x8_dot_i8x16_i7x16_s(a, b)        = dot_product(signed=True, elements=2, a, b)
-i32x4.dot_i8x16_i7x16_add_s(a, b, c) = dot_product(signed=True, elements=4, a, b, c)
+def i32x4.dot_i8x16_i7x16_add_s(a, b, c):
+  intermediate = []
+  tmp = []
+  result = []
+  for i in range(16):
+    if (b[i] & 0x80):
+      lhs = as_signed(a[i])
+      rhs = IMPLEMENTATION_DEFINED_ONE_OF(as_signed(b[i]), as_unsigned(b[i]))
+      intermediate[i] = lhs * rhs
+    else:
+      intermediate[i] = as_signed(a[i]) * b[i]
+
+  for i in range(0, 16, 2):
+    tmp[i/2] = IMPLEMENTATION_DEFINED_ONE_OF(
+      intermediate[i] + intermediate[i+1],
+      saturate(intermediate[i] + intermediate[i+1]))
+
+  for i in range(0, 8, 2):
+    dst[i/4] = tmp[i] + tmp[i+1]
+
+  for i in range(0, 4):
+    dst[i] += c[i]
+
+saturate(x) = min(INT16_MAX, max(INT16_MIN, x))
 ```
 
 ## Binary format