Promote cns argument to int64_t to ensure correctness

Some operations such as cns(-1) would end up emitting
the wrong thing, because the -1 literal would be interpreted as
int32_t and only four bytes would get copied across by memcpy.
The result is that cns(-1) would in fact emit the constant
4294967295 (= 2^32-1) which is bound to lead to wrong code.

A former workaround involved using cns(-1LL). I removed this
use case to show that this is now kosher (and added a test to
show that nothing broke - use TRACE=hhir:2 to check).

hphp/runtime/vm/jit/extradata.h

@@ -160,9 +160,25 @@ struct ConstData : IRExtraData {
                   "Constant data was larger than supported");
     static_assert(std::is_pod<T>::value,
                   "Constant data wasn't a pod?");
-    std::memcpy(&m_dataBits, &data, sizeof data);
+    const auto toCopy = promoteIfNeeded(data);
+    std::memcpy(&m_dataBits, &toCopy, sizeof(toCopy));
   }
 
+  template<typename T>
+  struct needsPromotion {
+    static constexpr bool value = std::is_integral<T>::value  ||
+                                  std::is_same<T,bool>::value ||
+                                  std::is_enum<T>::value;
+  };
+
+  template<class T>
+  typename std::enable_if<needsPromotion<T>::value, int64_t>::type
+  promoteIfNeeded(T t) { return t; }
+
+  template<class T>
+  typename std::enable_if<!needsPromotion<T>::value, T>::type
+  promoteIfNeeded(T t) { return t; }
+
   template<class T>
   T as() const {
     T ret;

hphp/runtime/vm/jit/hhbctranslator.cpp

@@ -3251,7 +3251,7 @@ void HhbcTranslator::emitMod() {
   SSATmp *res = m_tb->cond(
     curFunc(),
     [&] (Block* taken) {
-      SSATmp* negone = gen(OpEq, tr, cns(-1LL));
+      SSATmp* negone = gen(OpEq, tr, cns(-1));
       gen(JmpNZero, taken, negone);
     },
     [&] {

hphp/test/quick/mod_min_negone.php

@@ -0,0 +1,15 @@
+<?php
+
+function int_min() {
+  return ~PHP_INT_MAX;
+}
+
+function int_negone() {
+  return -1;
+}
+
+function main() {
+  var_dump(int_min() % -1);
+  var_dump(int_min() % int_negone());
+}
+main();

hphp/test/quick/mod_min_negone.php.expect

@@ -0,0 +1,2 @@
+int(0)
+int(0)