[utf8-validator] eliminate unnecessary comparison from must_be_2_3_continuation (#2113)

* [utf8-validator] eliminate unnecessary comparison from must_be_2_3_continuation

* Fix comment in

src/arm64.cpp

@@ -108,10 +108,10 @@ simdjson_unused simdjson_inline simd8<bool> must_be_continuation(const simd8<uin
     return is_second_byte ^ is_third_byte ^ is_fourth_byte;
 }
 
-simdjson_inline simd8<bool> must_be_2_3_continuation(const simd8<uint8_t> prev2, const simd8<uint8_t> prev3) {
-    simd8<bool> is_third_byte  = prev2 >= uint8_t(0xe0u);
-    simd8<bool> is_fourth_byte = prev3 >= uint8_t(0xf0u);
-    return is_third_byte ^ is_fourth_byte;
+simdjson_inline simd8<uint8_t> must_be_2_3_continuation(const simd8<uint8_t> prev2, const simd8<uint8_t> prev3) {
+    simd8<uint8_t> is_third_byte  = prev2.saturating_sub(0xe0u-0x80); // Only 111_____ will be >= 0x80
+    simd8<uint8_t> is_fourth_byte = prev3.saturating_sub(0xf0u-0x80); // Only 1111____ will be >= 0x80
+    return is_third_byte | is_fourth_byte;
 }
 
 } // unnamed namespace

src/generic/dom_parser_implementation.h

@@ -11,7 +11,7 @@ namespace simdjson {
 namespace SIMDJSON_IMPLEMENTATION {
 namespace {
 
-simdjson_inline simd8<bool> must_be_2_3_continuation(const simd8<uint8_t> prev2, const simd8<uint8_t> prev3);
+simdjson_inline simd8<uint8_t> must_be_2_3_continuation(const simd8<uint8_t> prev2, const simd8<uint8_t> prev3);
 simdjson_inline bool is_ascii(const simd8x64<uint8_t>& input);
 
 } // unnamed namespace

src/generic/stage1/utf8_lookup4_algorithm.h

@@ -107,7 +107,7 @@ using namespace simd;
       const simd8<uint8_t> prev_input, const simd8<uint8_t> sc) {
     simd8<uint8_t> prev2 = input.prev<2>(prev_input);
     simd8<uint8_t> prev3 = input.prev<3>(prev_input);
-    simd8<uint8_t> must23 = simd8<uint8_t>(must_be_2_3_continuation(prev2, prev3));
+    simd8<uint8_t> must23 = must_be_2_3_continuation(prev2, prev3);
     simd8<uint8_t> must23_80 = must23 & uint8_t(0x80);
     return must23_80 ^ sc;
   }

src/haswell.cpp

@@ -105,11 +105,10 @@ simdjson_unused simdjson_inline simd8<bool> must_be_continuation(const simd8<uin
   return simd8<int8_t>(is_second_byte | is_third_byte | is_fourth_byte) > int8_t(0);
 }
 
-simdjson_inline simd8<bool> must_be_2_3_continuation(const simd8<uint8_t> prev2, const simd8<uint8_t> prev3) {
-  simd8<uint8_t> is_third_byte  = prev2.saturating_sub(0xe0u-1); // Only 111_____ will be > 0
-  simd8<uint8_t> is_fourth_byte = prev3.saturating_sub(0xf0u-1); // Only 1111____ will be > 0
-  // Caller requires a bool (all 1's). All values resulting from the subtraction will be <= 64, so signed comparison is fine.
-  return simd8<int8_t>(is_third_byte | is_fourth_byte) > int8_t(0);
+simdjson_inline simd8<uint8_t> must_be_2_3_continuation(const simd8<uint8_t> prev2, const simd8<uint8_t> prev3) {
+  simd8<uint8_t> is_third_byte  = prev2.saturating_sub(0xe0u-0x80); // Only 111_____ will be >= 0x80
+  simd8<uint8_t> is_fourth_byte = prev3.saturating_sub(0xf0u-0x80); // Only 1111____ will be >= 0x80
+  return is_third_byte | is_fourth_byte;
 }
 
 } // unnamed namespace

src/icelake.cpp

@@ -107,11 +107,10 @@ simdjson_unused simdjson_inline simd8<bool> must_be_continuation(const simd8<uin
   return simd8<int8_t>(is_second_byte | is_third_byte | is_fourth_byte) > int8_t(0);
 }
 
-simdjson_inline simd8<bool> must_be_2_3_continuation(const simd8<uint8_t> prev2, const simd8<uint8_t> prev3) {
-  simd8<uint8_t> is_third_byte  = prev2.saturating_sub(0xe0u-1); // Only 111_____ will be > 0
-  simd8<uint8_t> is_fourth_byte = prev3.saturating_sub(0xf0u-1); // Only 1111____ will be > 0
-  // Caller requires a bool (all 1's). All values resulting from the subtraction will be <= 64, so signed comparison is fine.
-  return simd8<int8_t>(is_third_byte | is_fourth_byte) > int8_t(0);
+simdjson_inline simd8<uint8_t> must_be_2_3_continuation(const simd8<uint8_t> prev2, const simd8<uint8_t> prev3) {
+  simd8<uint8_t> is_third_byte  = prev2.saturating_sub(0xe0u-0x80); // Only 111_____ will be >= 0x80
+  simd8<uint8_t> is_fourth_byte = prev3.saturating_sub(0xf0u-0x80); // Only 1111____ will be >= 0x80
+  return is_third_byte | is_fourth_byte;
 }
 
 } // unnamed namespace

src/ppc64.cpp

@@ -78,11 +78,10 @@ simdjson_unused simdjson_inline simd8<bool> must_be_continuation(const simd8<uin
   return simd8<int8_t>(is_second_byte | is_third_byte | is_fourth_byte) > int8_t(0);
 }
 
-simdjson_inline simd8<bool> must_be_2_3_continuation(const simd8<uint8_t> prev2, const simd8<uint8_t> prev3) {
-  simd8<uint8_t> is_third_byte  = prev2.saturating_sub(0xe0u-1); // Only 111_____ will be > 0
-  simd8<uint8_t> is_fourth_byte = prev3.saturating_sub(0xf0u-1); // Only 1111____ will be > 0
-  // Caller requires a bool (all 1's). All values resulting from the subtraction will be <= 64, so signed comparison is fine.
-  return simd8<int8_t>(is_third_byte | is_fourth_byte) > int8_t(0);
+simdjson_inline simd8<uint8_t> must_be_2_3_continuation(const simd8<uint8_t> prev2, const simd8<uint8_t> prev3) {
+  simd8<uint8_t> is_third_byte  = prev2.saturating_sub(0xe0u-0x80); // Only 111_____ will be >= 0x80
+  simd8<uint8_t> is_fourth_byte = prev3.saturating_sub(0xf0u-0x80); // Only 1111____ will be >= 0x80
+  return is_third_byte | is_fourth_byte;
 }
 
 } // unnamed namespace

src/westmere.cpp

@@ -109,11 +109,10 @@ simdjson_unused simdjson_inline simd8<bool> must_be_continuation(const simd8<uin
   return simd8<int8_t>(is_second_byte | is_third_byte | is_fourth_byte) > int8_t(0);
 }
 
-simdjson_inline simd8<bool> must_be_2_3_continuation(const simd8<uint8_t> prev2, const simd8<uint8_t> prev3) {
-  simd8<uint8_t> is_third_byte  = prev2.saturating_sub(0xe0u-1); // Only 111_____ will be > 0
-  simd8<uint8_t> is_fourth_byte = prev3.saturating_sub(0xf0u-1); // Only 1111____ will be > 0
-  // Caller requires a bool (all 1's). All values resulting from the subtraction will be <= 64, so signed comparison is fine.
-  return simd8<int8_t>(is_third_byte | is_fourth_byte) > int8_t(0);
+simdjson_inline simd8<uint8_t> must_be_2_3_continuation(const simd8<uint8_t> prev2, const simd8<uint8_t> prev3) {
+  simd8<uint8_t> is_third_byte  = prev2.saturating_sub(0xe0u-0x80); // Only 111_____ will be >= 0x80
+  simd8<uint8_t> is_fourth_byte = prev3.saturating_sub(0xf0u-0x80); // Only 1111____ will be >= 0x80
+  return is_third_byte | is_fourth_byte;
 }
 
 } // unnamed namespace