Further improve OS support

library/std/src/sys/alloc/mod.rs

@@ -90,5 +90,7 @@ cfg_if::cfg_if! {
         mod xous;
     } else if #[cfg(target_os = "zkvm")] {
         mod zkvm;
+    } else if #[cfg(target_os = "os")] {
+        mod os;
     }
 }

library/std/src/sys/alloc/os.rs

@@ -0,0 +1,32 @@
+use crate::{
+    alloc::{GlobalAlloc, Layout, System},
+    ptr,
+    sync::atomic::{AtomicUsize, Ordering},
+};
+
+#[repr(align(16))]
+struct HeapData([u8; 4 * 1024 * 1024]);
+
+static mut HEAP_DATA: HeapData = HeapData([0; 4 * 1024 * 1024]);
+static HEAP_USED: AtomicUsize = AtomicUsize::new(0);
+
+#[stable(feature = "alloc_system_type", since = "1.28.0")]
+unsafe impl GlobalAlloc for System {
+    #[inline]
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        if layout.align() > 16 {
+            return ptr::null_mut();
+        }
+        let num_blocks =
+            if layout.size() % 16 == 0 { layout.size() / 16 } else { (layout.size() / 16) + 1 };
+        HEAP_USED.fetch_add(num_blocks, Ordering::Relaxed);
+        let ptr = unsafe {
+            ptr::addr_of_mut!(HEAP_DATA.0[HEAP_USED.load(Ordering::Relaxed) - num_blocks])
+                as *mut u8
+        };
+        ptr
+    }
+
+    #[inline]
+    unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
+}

library/std/src/sys/random/mod.rs

@@ -73,6 +73,7 @@ cfg_if::cfg_if! {
     } else if #[cfg(any(
         all(target_family = "wasm", target_os = "unknown"),
         target_os = "xous",
+        target_os = "os",
     ))] {
         // FIXME: finally remove std support for wasm32-unknown-unknown
         // FIXME: add random data generation to xous
@@ -86,6 +87,7 @@ cfg_if::cfg_if! {
     target_os = "android",
     all(target_family = "wasm", target_os = "unknown"),
     target_os = "xous",
+    target_os = "os",
 )))]
 pub fn hashmap_random_keys() -> (u64, u64) {
     let mut buf = [0; 16];

library/std/src/sys/thread_local/mod.rs

@@ -90,6 +90,7 @@ pub(crate) mod guard {
             target_family = "wasm",
             target_os = "uefi",
             target_os = "zkvm",
+            target_os = "os",
         ))] {
             pub(crate) fn enable() {
                 // FIXME: Right now there is no concept of "thread exit" on