Clarify integer value ranges

This commit clarifies integer value range for vairables used in
several variance functions, also change to use proper type
conversion to reflect the value ranges.

Change-Id: Ic3234b83a912ce1ad12d1b254f3378763e15cc5c

vpx_dsp/x86/halfpix_variance_sse2.c

diff --git a/vpx_dsp/x86/halfpix_variance_sse2.c b/vpx_dsp/x86/halfpix_variance_sse2.c
index 5782155..4a8fb6d 100644
--- a/vpx_dsp/x86/halfpix_variance_sse2.c
+++ b/vpx_dsp/x86/halfpix_variance_sse2.c
@@ -8,6 +8,8 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include <assert.h>
+
 #include "./vpx_config.h"
 #include "./vpx_dsp_rtcd.h"
 #include "vpx/vpx_integer.h"
@@ -40,7 +42,9 @@
                                     &xsum0, &xxsum0);
 
   *sse = xxsum0;
-  return (xxsum0 - (((uint32_t)xsum0 * xsum0) >> 8));
+  assert(xsum0 <= 255 * 16 * 16);
+  assert(xsum0 >= -255 * 16 * 16);
+  return (xxsum0 - ((uint32_t)((int64_t)xsum0 * xsum0) >> 8));
 }
 
 uint32_t vpx_variance_halfpixvar16x16_v_sse2(const unsigned char *src,
@@ -54,7 +58,9 @@
                                    &xsum0, &xxsum0);
 
   *sse = xxsum0;
-  return (xxsum0 - (((uint32_t)xsum0 * xsum0) >> 8));
+  assert(xsum0 <= 255 * 16 * 16);
+  assert(xsum0 >= -255 * 16 * 16);
+  return (xxsum0 - ((uint32_t)((int64_t)xsum0 * xsum0) >> 8));
 }
 
 
@@ -70,5 +76,7 @@
                                          &xsum0, &xxsum0);
 
   *sse = xxsum0;
-  return (xxsum0 - (((uint32_t)xsum0 * xsum0) >> 8));
+  assert(xsum0 <= 255 * 16 * 16);
+  assert(xsum0 >= -255 * 16 * 16);
+  return (xxsum0 - ((uint32_t)((int64_t)xsum0 * xsum0) >> 8));
 }