Kill alwaysLowMem

We're hitting the assert in some jobs (see task) and it's not
a measureable win.

hphp/runtime/vm/class.cpp

@@ -514,8 +514,6 @@ void PreClassRepoProxy::GetPreClassesStmt
 ClassPtr Class::newClass(PreClass* preClass, Class* parent) {
   unsigned classVecLen = (parent != nullptr) ? parent->m_classVecLen+1 : 1;
   void* mem = Util::low_malloc(sizeForNClasses(classVecLen));
-  always_assert(IMPLIES(alwaysLowMem(), ptr_is_low_mem(mem)) &&
-         "All Classes must be allocated at 32-bit addresses");
   try {
     return ClassPtr(new (mem) Class(preClass, parent, classVecLen));
   } catch (...) {

hphp/runtime/vm/class.h

@@ -645,18 +645,6 @@ public:
   Class(PreClass* preClass, Class* parent, unsigned classVecLen);
   void atomicRelease();
 
-  static bool alwaysLowMem() {
-    // jemalloc 3.2.0 and later support allocating objects in low memory
-    // (addresses that fit in 32 bits), and we take advantage of this in
-    // translated code.
-#if defined(JEMALLOC_VERSION) &&                                        \
-  ((JEMALLOC_VERSION_MAJOR == 3 && JEMALLOC_VERSION_MINOR >= 2) ||      \
-   JEMALLOC_VERSION_MAJOR > 3)
-    return use_jemalloc && RuntimeOption::RepoAuthoritative;
-#else
-    return false;
-#endif
-  }
   static size_t sizeForNClasses(unsigned nClasses) {
     return offsetof(Class, m_classVec) + (sizeof(Class*) * nClasses);
   }

hphp/runtime/vm/translator/hopt/codegen.cpp

@@ -1578,11 +1578,7 @@ void CodeGenerator::cgExtendsClass(IRInstruction* inst) {
   // Test if it is the exact same class.  TODO(#2044801): we should be
   // doing this control flow at the IR level.
   if (!(testClass->attrs() & AttrAbstract)) {
-    if (Class::alwaysLowMem()) {
-      a.  cmpl   (r32(rTestClass), r32(rObjClass));
-    } else {
-      a.  cmpq   (rTestClass, rObjClass);
-    }
+    a.    cmpq   (rTestClass, rObjClass);
     a.    jne8   (notExact);
     a.    movb   (1, rdst);
     a.    jmp8   (out);
@@ -1600,11 +1596,7 @@ asm_label(a, notExact);
   a.    jb8    (falseLabel);
 
   // If it's a subclass, rTestClass must be at the appropriate index.
-  if (Class::alwaysLowMem()) {
-    a.  cmpl   (r32(rTestClass), rObjClass[vecOffset]);
-  } else {
-    a.  cmpq   (rTestClass, rObjClass[vecOffset]);
-  }
+  a.    cmpq   (rTestClass, rObjClass[vecOffset]);
   a.    sete   (rdst);
   a.    jmp8   (out);
 

hphp/runtime/vm/translator/translator-x64.cpp

@@ -10509,11 +10509,7 @@ TranslatorX64::emitInstanceCheck(const Tracelet& t,
       // (meaning the class isn't defined yet) but that's ok: if it is null
       // the cmp will always fail.
       int offset = Class::classVecOff() + sizeof(Class*) * (parentVecLen-1);
-      if (Class::alwaysLowMem()) {
-        a.cmp_reg32_disp_reg64(r(cls), offset, r(inCls));
-      } else {
-        a.cmp_reg64_disp_reg64(r(cls), offset, r(inCls));
-      }
+      a.cmp_reg64_disp_reg64(r(cls), offset, r(inCls));
       if (verifying) {
         parentFailJe = a.code.frontier;
         a.jne8(parentFailJe);