fiber: leave guest thread stacks uninitialized, drop the unused rewind stack

Each fiber zero-filled two 2 MiB std::arrays, the stack and a rewind
stack nothing ever switched to, so every guest thread held 4 MiB of RAM
however little stack it used: ~750 MiB for title 1's ~190 threads. The
stack is now allocated without being touched, so the host backs only the
pages a thread uses.

(cherry picked from commit f5e7d0f2eed854a0f821f8a04253f04cf77db575)

1 files changed, 7 insertions, 6 deletions

diff --git a/src/common/fiber.cpp b/src/common/fiber.cpp
index 0e7f28c5b9..613ac97e22 100644
--- a/src/common/fiber.cpp
+++ b/src/common/fiber.cpp
@@ -25,8 +25,10 @@ struct Fiber::FiberImpl {
     FiberImpl() {}
 
     u32 canary_1 = CANARY_VALUE;
-    std::array<u8, DEFAULT_STACK_SIZE> stack{};
-    std::array<u8, DEFAULT_STACK_SIZE> rewind_stack{};
+    // Left uninitialized: the host gives a stack's pages memory only as they are used. Zero-filled
+    // std::arrays (and a second stack nothing switched to) made every fiber, one per guest thread,
+    // hold 4 MiB: ~750 MiB for title 1's ~190 threads.
+    std::unique_ptr<u8[]> stack;
     u32 canary_2 = CANARY_VALUE;
 
     boost::context::detail::fcontext_t context{};
@@ -38,7 +40,6 @@ struct Fiber::FiberImpl {
     std::shared_ptr<Fiber> previous_fiber;
 
     u8* stack_limit = nullptr;
-    u8* rewind_stack_limit = nullptr;
     bool is_thread_fiber = false;
     bool released = false;
 };
@@ -49,10 +50,10 @@ void Fiber::SetRewindPoint(std::function<void()>&& rewind_func) {
 
 Fiber::Fiber(std::function<void()>&& entry_point_func) : impl{std::make_unique<FiberImpl>()} {
     impl->entry_point = std::move(entry_point_func);
-    impl->stack_limit = impl->stack.data();
-    impl->rewind_stack_limit = impl->rewind_stack.data();
+    impl->stack.reset(new u8[DEFAULT_STACK_SIZE]);
+    impl->stack_limit = impl->stack.get();
     u8* stack_base = impl->stack_limit + DEFAULT_STACK_SIZE;
-    impl->context = boost::context::detail::make_fcontext(stack_base, impl->stack.size(), [](boost::context::detail::transfer_t transfer) -> void {
+    impl->context = boost::context::detail::make_fcontext(stack_base, DEFAULT_STACK_SIZE, [](boost::context::detail::transfer_t transfer) -> void {
         auto* fiber = static_cast<Fiber*>(transfer.data);
         ASSERT(fiber && fiber->impl && fiber->impl->previous_fiber && fiber->impl->previous_fiber->impl);
         ASSERT(fiber->impl->canary_1 == CANARY_VALUE);