Add workaround to turn off nursery size heuristic

SpiderMonkey 98 introduced a size heuristic for the nursery GC region
(https://phabricator.services.mozilla.com/D136637). As this heuristic
uses a wall-clock time duration, it results in a severe performance
regression on slower systems for our use case.

This commit adds a workaround to turn off that heuristic, by telling
SpiderMonkey that a "page load" (something which doesn't have a meaning
in the context of pyrogenesis) is in progress, as that heuristic is
disabled for page loads.

Co-Authored by: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Fixes #7714

(cherry picked from commit 11dd480b67)
Signed-off-by: Itms <itms@wildfiregames.com>

source/scriptinterface/ScriptContext.cpp

@@ -27,6 +27,8 @@
 #include "scriptinterface/ScriptEngine.h"
 #include "scriptinterface/ScriptInterface.h"
 
+#include "js/friend/PerformanceHint.h"
+
 void GCSliceCallbackHook(JSContext* UNUSED(cx), JS::GCProgress progress, const JS::GCDescription& UNUSED(desc))
 {
 	/**
@@ -129,6 +131,10 @@ ScriptContext::ScriptContext(int contextSize, int heapGrowthBytesGCTrigger):
 
 	JS::ContextOptionsRef(m_cx).setStrictMode(true);
 
+	// Workaround to turn off nursery size heuristic.
+	// See https://gitea.wildfiregames.com/0ad/0ad/issues/7714 for details.
+	js::gc::SetPerformanceHint(m_cx, js::gc::PerformanceHint::InPageLoad);
+
 	ScriptEngine::GetSingleton().RegisterContext(m_cx);
 
 	JS::SetJobQueue(m_cx, m_JobQueue.get());
@@ -139,6 +145,9 @@ ScriptContext::~ScriptContext()
 {
 	ENSURE(ScriptEngine::IsInitialised() && "The ScriptEngine must be active (initialized and not yet shut down) when destroying a ScriptContext!");
 
+	// Switch back to normal performance mode to avoid assertion in debug mode.
+	js::gc::SetPerformanceHint(m_cx, js::gc::PerformanceHint::Normal);
+
 	JS_DestroyContext(m_cx);
 	ScriptEngine::GetSingleton().UnRegisterContext(m_cx);
 }