Bug 1570259 part 1 - Document and clean up JitContext constructors. r=luke

The next patch will simplify the constructor code a bit more.

Differential Revision: https://phabricator.services.mozilla.com/D41377

--HG--
extra : moz-landing-system : lando

js/src/jit/Ion.cpp

@@ -100,7 +100,7 @@ JitContext::JitContext(CompileRuntime* rt, CompileRealm* realm,
       prev_(CurrentJitContext()),
       realm_(realm),
 #ifdef DEBUG
-      isCompilingWasm_(!realm),
+      isCompilingWasm_(false),
       oom_(false),
 #endif
       assemblerCount_(0) {
@@ -122,9 +122,20 @@ JitContext::JitContext(JSContext* cx, TempAllocator* temp)
 }
 
 JitContext::JitContext(TempAllocator* temp)
-    : JitContext(nullptr, nullptr, temp) {}
+    : cx(nullptr),
+      temp(temp),
+      runtime(nullptr),
+      prev_(CurrentJitContext()),
+      realm_(nullptr),
+#ifdef DEBUG
+      isCompilingWasm_(true),
+      oom_(false),
+#endif
+      assemblerCount_(0) {
+  SetJitContext(this);
+}
 
-JitContext::JitContext() : JitContext(nullptr, nullptr, nullptr) {}
+JitContext::JitContext() : JitContext(nullptr) {}
 
 JitContext::~JitContext() { SetJitContext(prev_); }
 

js/src/jit/Ion.h

@@ -54,10 +54,16 @@ static_assert(sizeof(AbortReasonOr<bool>) <= sizeof(uintptr_t),
 
 class JitContext {
  public:
+  // Constructor for compilations happening on the main thread.
   JitContext(JSContext* cx, TempAllocator* temp);
+
+  // Constructor for off-thread Ion compilations.
   JitContext(CompileRuntime* rt, CompileRealm* realm, TempAllocator* temp);
+
+  // Constructors for Wasm compilation.
   explicit JitContext(TempAllocator* temp);
   JitContext();
+
   ~JitContext();
 
   // Running context when executing on the main thread. Not available during