Reland "Remove V8_HOST_CAN_READ_UNALIGNED and its uses."

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 32dd94f39b64457b6cbe853647222c22d49ba8e5..8203b36a97f1dee7ef9c14a2cf413ae2c370c7e4 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -9172,22 +9172,33 @@ class String: public Name {
   static inline int NonAsciiStart(const char* chars, int length) {
     const char* start = chars;
     const char* limit = chars + length;
-#ifdef V8_HOST_CAN_READ_UNALIGNED
-    DCHECK(unibrow::Utf8::kMaxOneByteChar == 0x7F);
-    const uintptr_t non_one_byte_mask = kUintptrAllBitsSet / 0xFF * 0x80;
-    while (chars + sizeof(uintptr_t) <= limit) {
-      if (*reinterpret_cast<const uintptr_t*>(chars) & non_one_byte_mask) {
-        return static_cast<int>(chars - start);
+
+    if (length >= kIntptrSize) {
+      // Check unaligned bytes.
+      while (!IsAligned(reinterpret_cast<intptr_t>(chars), sizeof(uintptr_t))) {
+        if (static_cast<uint8_t>(*chars) > unibrow::Utf8::kMaxOneByteChar) {
+          return static_cast<int>(chars - start);
+        }
+        ++chars;
+      }
+      // Check aligned words.
+      DCHECK(unibrow::Utf8::kMaxOneByteChar == 0x7F);
+      const uintptr_t non_one_byte_mask = kUintptrAllBitsSet / 0xFF * 0x80;
+      while (chars + sizeof(uintptr_t) <= limit) {
+        if (*reinterpret_cast<const uintptr_t*>(chars) & non_one_byte_mask) {
+          return static_cast<int>(chars - start);
+        }
+        chars += sizeof(uintptr_t);
       }
-      chars += sizeof(uintptr_t);
     }
-#endif
+    // Check remaining unaligned bytes.
     while (chars < limit) {
       if (static_cast<uint8_t>(*chars) > unibrow::Utf8::kMaxOneByteChar) {
         return static_cast<int>(chars - start);
       }
       ++chars;
     }
+
     return static_cast<int>(chars - start);
   }