Help the compiler avoid inlining lazy init functions (#443)

Before this change, the compiler generates code that looks like this:

```
  if not initialized:
     goto init
do_work:
  do the actual work
  goto exit
init:
  inilned init()
  goto do_work
exit:
  ret
```

If the initialization code is small, this works fine. But, for (bad)
reasons, is_rdrand_good is particularly huge. Thus, jumping over its
inined code is wasteful because it puts bad pressure on the instruction
cache.

With this change, the generated code looks like this:

```
  if not initialized:
     goto init
do_work:
  do the actual work
  goto exit
init:
  call init()
  goto do_work
exit:
  ret
```

I verified these claims by running:
```
$ cargo asm --rust getrandom_inner --lib --target=x86_64-fortanix-unknown-sgx
```

This is also what other implementations (e.g. OnceCell) do.

While here, I made the analogous change to LazyPtr, and rewrote LazyPtr
to the same form as LazyUsize. I didn't check the generated code for
LazyPtr though.

(Why is `is_rdrand_good` huge? The compiler unrolls the 10 iteration
retry loop, and then it unrolls the 8 iteration self-test loop, so the
result is `rdrand()` is inlined 80 times inside is_rdrand_good. This is
something to address separately as it also affects `getrandom_inner`
itself.)

src/lazy.rs

@@ -36,13 +36,20 @@ impl LazyUsize {
     // init(). Multiple callers can run their init() functions in parallel.
     // init() should always return the same value, if it succeeds.
     pub fn unsync_init(&self, init: impl FnOnce() -> usize) -> usize {
+        #[cold]
+        fn do_init(this: &LazyUsize, init: impl FnOnce() -> usize) -> usize {
+            let val = init();
+            this.0.store(val, Ordering::Relaxed);
+            val
+        }
+
         // Relaxed ordering is fine, as we only have a single atomic variable.
-        let mut val = self.0.load(Ordering::Relaxed);
-        if val == Self::UNINIT {
-            val = init();
-            self.0.store(val, Ordering::Relaxed);
+        let val = self.0.load(Ordering::Relaxed);
+        if val != Self::UNINIT {
+            val
+        } else {
+            do_init(self, init)
         }
-        val
     }
 }
 
@@ -92,19 +99,24 @@ impl LazyPtr {
     // init(). Multiple callers can run their init() functions in parallel.
     // init() should always return the same value, if it succeeds.
     pub fn unsync_init(&self, init: impl Fn() -> *mut c_void) -> *mut c_void {
+        #[cold]
+        fn do_init(this: &LazyPtr, init: impl Fn() -> *mut c_void) -> *mut c_void {
+            let addr = init();
+            this.addr.store(addr, Ordering::Release);
+            addr
+        }
+
         // Despite having only a single atomic variable (self.addr), we still
         // cannot always use Ordering::Relaxed, as we need to make sure a
         // successful call to `init` is "ordered before" any data read through
         // the returned pointer (which occurs when the function is called).
         // Our implementation mirrors that of the one in libstd, meaning that
         // the use of non-Relaxed operations is probably unnecessary.
-        match self.addr.load(Ordering::Acquire) {
-            Self::UNINIT => {
-                let addr = init();
-                self.addr.store(addr, Ordering::Release);
-                addr
-            }
-            addr => addr,
+        let val = self.addr.load(Ordering::Acquire);
+        if val != Self::UNINIT {
+            val
+        } else {
+            do_init(self, init)
         }
     }
 }

src/use_file.rs

@@ -19,6 +19,9 @@ use core::{
 const FILE_PATH: &[u8] = b"/dev/urandom\0";
 const FD_UNINIT: usize = usize::max_value();
 
+// Do not inline this when it is the fallback implementation, but don't mark it
+// `#[cold]` because it is hot when it is actually used.
+#[cfg_attr(any(target_os = "android", target_os = "linux"), inline(never))]
 pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     let fd = get_rng_fd()?;
     sys_fill_exact(dest, |buf| unsafe {
@@ -31,38 +34,44 @@ pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
 // return the same file descriptor. This file descriptor is never closed.
 fn get_rng_fd() -> Result<libc::c_int, Error> {
     static FD: AtomicUsize = AtomicUsize::new(FD_UNINIT);
+
     fn get_fd() -> Option<libc::c_int> {
         match FD.load(Relaxed) {
             FD_UNINIT => None,
             val => Some(val as libc::c_int),
         }
     }
 
-    // Use double-checked locking to avoid acquiring the lock if possible.
-    if let Some(fd) = get_fd() {
-        return Ok(fd);
-    }
+    #[cold]
+    fn get_fd_locked() -> Result<libc::c_int, Error> {
+        // SAFETY: We use the mutex only in this method, and we always unlock it
+        // before returning, making sure we don't violate the pthread_mutex_t API.
+        static MUTEX: Mutex = Mutex::new();
+        unsafe { MUTEX.lock() };
+        let _guard = DropGuard(|| unsafe { MUTEX.unlock() });
 
-    // SAFETY: We use the mutex only in this method, and we always unlock it
-    // before returning, making sure we don't violate the pthread_mutex_t API.
-    static MUTEX: Mutex = Mutex::new();
-    unsafe { MUTEX.lock() };
-    let _guard = DropGuard(|| unsafe { MUTEX.unlock() });
+        if let Some(fd) = get_fd() {
+            return Ok(fd);
+        }
 
-    if let Some(fd) = get_fd() {
-        return Ok(fd);
-    }
+        // On Linux, /dev/urandom might return insecure values.
+        #[cfg(any(target_os = "android", target_os = "linux"))]
+        wait_until_rng_ready()?;
 
-    // On Linux, /dev/urandom might return insecure values.
-    #[cfg(any(target_os = "android", target_os = "linux"))]
-    wait_until_rng_ready()?;
+        let fd = open_readonly(FILE_PATH)?;
+        // The fd always fits in a usize without conflicting with FD_UNINIT.
+        debug_assert!(fd >= 0 && (fd as usize) < FD_UNINIT);
+        FD.store(fd as usize, Relaxed);
 
-    let fd = open_readonly(FILE_PATH)?;
-    // The fd always fits in a usize without conflicting with FD_UNINIT.
-    debug_assert!(fd >= 0 && (fd as usize) < FD_UNINIT);
-    FD.store(fd as usize, Relaxed);
+        Ok(fd)
+    }
 
-    Ok(fd)
+    // Use double-checked locking to avoid acquiring the lock if possible.
+    if let Some(fd) = get_fd() {
+        Ok(fd)
+    } else {
+        get_fd_locked()
+    }
 }
 
 // Polls /dev/random to make sure it is ok to read from /dev/urandom.