Create explicit variable for initial hash value

Summary:
Create an inline constexpr variable JenkinsHashInit that represents the
inital value of a string hash. Use this value at all existing callsites
that initialize a hash value.

Reviewed By: jpporto

Differential Revision: D43285003

fbshipit-source-id: cf83e1814067e11289082d8acb9f024a28e74f2d

include/hermes/Support/HashString.h

@@ -30,7 +30,7 @@ template <typename T>
 uint32_t hashString(llvh::ArrayRef<T> str) {
   static_assert(
       sizeof(JenkinsHash) == sizeof(uint32_t), "Jenkins Hash must be 32-bit");
-  hermes::JenkinsHash hash = 0;
+  hermes::JenkinsHash hash = hermes::JenkinsHashInit;
   for (const T c : str) {
     hash = hermes::updateJenkinsHash(hash, c);
   }

include/hermes/Support/JenkinsHash.h

@@ -13,6 +13,7 @@
 namespace hermes {
 
 using JenkinsHash = uint32_t;
+inline constexpr JenkinsHash JenkinsHashInit = 0;
 
 namespace jenkins_details {
 template <typename CharT>

lib/BCGen/HBC/BytecodeDisassembler.cpp

@@ -514,7 +514,7 @@ void BytecodeVisitor::visitInstructionsInBody(
 
 class BytecodeHasher : public BytecodeVisitor {
  protected:
-  uint32_t hash_{0};
+  JenkinsHash hash_{JenkinsHashInit};
   bool useStrings_;
   bool useIntConstants_;
   uint8_t opcode_{0xff};

lib/BCGen/HBC/ConsecutiveStringStorage.cpp

@@ -233,7 +233,7 @@ class StringPacker {
         continue;
       }
       const CharT *chars = entry.chars_.data();
-      JenkinsHash hash = 0;
+      JenkinsHash hash = JenkinsHashInit;
       size_t i = charsSize;
       while (i--) {
         hash = updateJenkinsHash(hash, chars[i]);