Branchfree bitmask calculation

include/Sieve.hpp

@@ -255,32 +255,33 @@ class Sieve
     uint64_t m2 = unset_larger[stop % 240];
     const uint64_t* sieve64 = (const uint64_t*) sieve_.data();
 
-    if (start_idx == stop_idx)
-      return popcnt64(sieve64[start_idx] & m1 & m2);
-    else
-    {
-      uint64_t i = start_idx + 1;
-      uint64_t start_bits = sieve64[start_idx] & m1;
-      uint64_t stop_bits = sieve64[stop_idx] & m2;
-      uint64_t cnt = popcnt64(start_bits);
-      cnt += popcnt64(stop_bits);
-      svuint64_t vcnt = svdup_u64(0);
-
-      // Compute this for loop using ARM SVE.
-      // for (i = start_idx + 1; i < stop_idx; i++)
-      //   cnt += popcnt64(sieve64[i]);
-      do {
-        svbool_t pg = svwhilelt_b64(i, stop_idx);
-        svuint64_t vec = svld1_u64(pg, &sieve64[i]);
-        vec = svcnt_u64_z(pg, vec);
-        vcnt = svadd_u64_x(svptrue_b64(), vcnt, vec);
-        i += svcntd();
-      }
-      while (i < stop_idx);
-      cnt += svaddv_u64(svptrue_b64(), vcnt);
+    // Branchfree bitmask calculation:
+    // m1 = (start_idx != stop_idx) ? m1 : m1 & m2;
+    m1 = (m1 * (start_idx != stop_idx)) | ((m1 & m2) * (start_idx == stop_idx));
+    // m2 = (start_idx != stop_idx) ? m2 : 0;
+    m2 *= (start_idx != stop_idx);
 
-      return cnt;
+    uint64_t i = start_idx + 1;
+    uint64_t start_bits = sieve64[start_idx] & m1;
+    uint64_t stop_bits = sieve64[stop_idx] & m2;
+    uint64_t cnt = popcnt64(start_bits);
+    cnt += popcnt64(stop_bits);
+    svuint64_t vcnt = svdup_u64(0);
+
+    // Compute this for loop using ARM SVE.
+    // for (i = start_idx + 1; i < stop_idx; i++)
+    //   cnt += popcnt64(sieve64[i]);
+    do {
+      svbool_t pg = svwhilelt_b64(i, stop_idx);
+      svuint64_t vec = svld1_u64(pg, &sieve64[i]);
+      vec = svcnt_u64_z(pg, vec);
+      vcnt = svadd_u64_x(svptrue_b64(), vcnt, vec);
+      i += svcntd();
     }
+    while (i < stop_idx);
+    cnt += svaddv_u64(svptrue_b64(), vcnt);
+
+    return cnt;
   }
 
 #endif