Use assume to inform the optimiser about refcount invariants

The reference count can never be 0, unless we're about to drop the data
completely. Using the `assume` intrinsic allows us to inform LLVM about
that invariant, meaning it can avoid unnecessary drops.

Author: Aatch

src/liballoc/rc.rs

@@ -160,6 +160,7 @@ use core::option::Option::{Some, None};
 use core::ptr::{self, PtrExt};
 use core::result::Result;
 use core::result::Result::{Ok, Err};
+use core::intrinsics::assume;
 
 use heap::deallocate;
 
@@ -905,10 +906,20 @@ trait RcBoxPtr<T> {
     fn strong(&self) -> uint { self.inner().strong.get() }
 
     #[inline]
-    fn inc_strong(&self) { self.inner().strong.set(self.strong() + 1); }
+    fn inc_strong(&self) {
+        let strong = self.strong();
+        // The reference count is always at least one unless we're about to drop the type
+        unsafe { assume(strong > 0); }
+        self.inner().strong.set(strong + 1);
+    }
 
     #[inline]
-    fn dec_strong(&self) { self.inner().strong.set(self.strong() - 1); }
+    fn dec_strong(&self) {
+        let strong = self.strong();
+        // The reference count is always at least one unless we're about to drop the type
+        unsafe { assume(strong > 0); }
+        self.inner().strong.set(strong - 1);
+    }
 
     #[inline]
     fn weak(&self) -> uint { self.inner().weak.get() }