auto merge of #6260 : alexcrichton/rust/issue-3466-no-swap, r=pcwalton

There may be a more efficient implementation of `core::util::swap_ptr`. The issue mentioned using `move_val_init`, but I couldn't figure out what that did, so I just used `copy_memory` a few times instead.

I'm not exactly the best at reading LLVM generated by rust, but this does appear to be optimized away just as expected (when possible).

Author: bors

doc/rust.md

@@ -1946,35 +1946,6 @@ fn avg(v: &[float]) -> float {
 }
 ~~~~
 
-#### Swap expressions
-
-A _swap expression_ consists of an [lvalue](#lvalues-rvalues-and-temporaries) followed by a bi-directional arrow (`<->`) and another [lvalue](#lvalues-rvalues-and-temporaries).
-
-Evaluating a swap expression causes, as a side effect, the values held in the left-hand-side and right-hand-side [lvalues](#lvalues-rvalues-and-temporaries) to be exchanged indivisibly.
-
-Evaluating a swap expression neither changes reference counts,
-nor deeply copies any owned structure pointed to by the moved [rvalue](#lvalues-rvalues-and-temporaries).
-Instead, the swap expression represents an indivisible *exchange of ownership*,
-between the right-hand-side and the left-hand-side of the expression.
-No allocation or destruction is entailed.
-
-An example of three different swap expressions:
-
-~~~~~~~~
-# let mut x = &mut [0];
-# let mut a = &mut [0];
-# let i = 0;
-# struct S1 { z: int };
-# struct S2 { c: int };
-# let mut y = S1{z: 0};
-# let mut b = S2{c: 0};
-
-x <-> a;
-x[i] <-> a[i];
-y.z <-> b.c;
-~~~~~~~~
-
-
 #### Assignment expressions
 
 An _assignment expression_ consists of an [lvalue](#lvalues-rvalues-and-temporaries) expression followed by an
@@ -2015,7 +1986,7 @@ as
 == !=
 &&
 ||
-= <->
+=
 ~~~~
 
 Operators at the same precedence level are evaluated left-to-right.

doc/tutorial-ffi.md

@@ -151,6 +151,7 @@ wrapping `malloc` and `free`:
 ~~~~
 use core::libc::{c_void, size_t, malloc, free};
 use core::unstable::intrinsics;
+use core::util;
 
 // a wrapper around the handle returned by the foreign code
 pub struct Unique<T> {
@@ -184,7 +185,8 @@ impl<T: Owned> Drop for Unique<T> {
     fn finalize(&self) {
         unsafe {
             let mut x = intrinsics::init(); // dummy value to swap in
-            x <-> *self.ptr; // moving the object out is needed to call the destructor
+            // moving the object out is needed to call the destructor
+            util::replace_ptr(self.ptr, x);
             free(self.ptr as *c_void)
         }
     }

src/libcore/cell.rs

@@ -12,6 +12,7 @@
 
 use cast::transmute_mut;
 use prelude::*;
+use util::replace;
 
 /*
 A dynamic, mutable location.
@@ -48,9 +49,7 @@ pub impl<T> Cell<T> {
             fail!(~"attempt to take an empty cell");
         }
 
-        let mut value = None;
-        value <-> self.value;
-        value.unwrap()
+        replace(&mut self.value, None).unwrap()
     }
 
     /// Returns the value, failing if the cell is full.

src/libcore/comm.rs

@@ -21,6 +21,7 @@ use uint;
 use unstable;
 use vec;
 use unstable::Exclusive;
+use util::replace;
 
 use pipes::{recv, try_recv, wait_many, peek, PacketHeader};
 
@@ -149,9 +150,8 @@ impl<T: Owned> GenericChan<T> for Chan<T> {
     #[inline(always)]
     fn send(&self, x: T) {
         unsafe {
-            let mut endp = None;
             let mut self_endp = transmute_mut(&self.endp);
-            endp <-> *self_endp;
+            let endp = replace(self_endp, None);
             *self_endp = Some(streamp::client::data(endp.unwrap(), x))
         }
     }
@@ -161,9 +161,8 @@ impl<T: Owned> GenericSmartChan<T> for Chan<T> {
     #[inline(always)]
     fn try_send(&self, x: T) -> bool {
         unsafe {
-            let mut endp = None;
             let mut self_endp = transmute_mut(&self.endp);
-            endp <-> *self_endp;
+            let endp = replace(self_endp, None);
             match streamp::client::try_data(endp.unwrap(), x) {
                 Some(next) => {
                     *self_endp = Some(next);
@@ -179,9 +178,8 @@ impl<T: Owned> GenericPort<T> for Port<T> {
     #[inline(always)]
     fn recv(&self) -> T {
         unsafe {
-            let mut endp = None;
             let mut self_endp = transmute_mut(&self.endp);
-            endp <-> *self_endp;
+            let endp = replace(self_endp, None);
             let streamp::data(x, endp) = recv(endp.unwrap());
             *self_endp = Some(endp);
             x
@@ -191,9 +189,8 @@ impl<T: Owned> GenericPort<T> for Port<T> {
     #[inline(always)]
     fn try_recv(&self) -> Option<T> {
         unsafe {
-            let mut endp = None;
             let mut self_endp = transmute_mut(&self.endp);
-            endp <-> *self_endp;
+            let endp = replace(self_endp, None);
             match try_recv(endp.unwrap()) {
                 Some(streamp::data(x, endp)) => {
                     *self_endp = Some(endp);
@@ -209,14 +206,13 @@ impl<T: Owned> Peekable<T> for Port<T> {
     #[inline(always)]
     fn peek(&self) -> bool {
         unsafe {
-            let mut endp = None;
             let mut self_endp = transmute_mut(&self.endp);
-            endp <-> *self_endp;
+            let mut endp = replace(self_endp, None);
             let peek = match endp {
                 Some(ref mut endp) => peek(endp),
                 None => fail!(~"peeking empty stream")
             };
-            *self_endp <-> endp;
+            *self_endp = endp;
             peek
         }
     }
@@ -267,8 +263,7 @@ impl<T:Owned> GenericPort<T> for PortSet<T> {
             let mut result = None;
             // we have to swap the ports array so we aren't borrowing
             // aliasable mutable memory.
-            let mut ports = ~[];
-            ports <-> *self_ports;
+            let mut ports = replace(self_ports, ~[]);
             while result.is_none() && ports.len() > 0 {
                 let i = wait_many(ports);
                 match ports[i].try_recv() {
@@ -281,7 +276,7 @@ impl<T:Owned> GenericPort<T> for PortSet<T> {
                     }
                 }
             }
-            ports <-> *self_ports;
+            *self_ports = ports;
             result
         }
     }
@@ -320,8 +315,7 @@ impl<T: Owned> GenericChan<T> for SharedChan<T> {
     fn send(&self, x: T) {
         let mut xx = Some(x);
         do self.ch.with_imm |chan| {
-            let mut x = None;
-            x <-> xx;
+            let x = replace(&mut xx, None);
             chan.send(x.unwrap())
         }
     }
@@ -331,8 +325,7 @@ impl<T: Owned> GenericSmartChan<T> for SharedChan<T> {
     fn try_send(&self, x: T) -> bool {
         let mut xx = Some(x);
         do self.ch.with_imm |chan| {
-            let mut x = None;
-            x <-> xx;
+            let x = replace(&mut xx, None);
             chan.try_send(x.unwrap())
         }
     }

src/libcore/hashmap.rs

@@ -176,16 +176,13 @@ priv impl<K:Hash + Eq,V> HashMap<K, V> {
     /// Expands the capacity of the array and re-insert each of the
     /// existing buckets.
     fn resize(&mut self, new_capacity: uint) {
-        let old_capacity = self.buckets.len();
         self.resize_at = resize_at(new_capacity);
 
-        let mut old_buckets = vec::from_fn(new_capacity, |_| None);
-        self.buckets <-> old_buckets;
+        let old_buckets = replace(&mut self.buckets,
+                                  vec::from_fn(new_capacity, |_| None));
 
         self.size = 0;
-        for uint::range(0, old_capacity) |i| {
-            let mut bucket = None;
-            bucket <-> old_buckets[i];
+        do vec::consume(old_buckets) |_, bucket| {
             self.insert_opt_bucket(bucket);
         }
     }
@@ -265,13 +262,11 @@ priv impl<K:Hash + Eq,V> HashMap<K, V> {
         };
 
         let len_buckets = self.buckets.len();
-        let mut bucket = None;
-        self.buckets[idx] <-> bucket;
+        let bucket = replace(&mut self.buckets[idx], None);
 
         let value = match bucket {
             None => None,
-            Some(bucket) => {
-                let Bucket{value: value, _} = bucket;
+            Some(Bucket{value, _}) => {
                 Some(value)
             },
         };
@@ -281,8 +276,7 @@ priv impl<K:Hash + Eq,V> HashMap<K, V> {
         let size = self.size - 1;
         idx = self.next_bucket(idx, len_buckets);
         while self.buckets[idx].is_some() {
-            let mut bucket = None;
-            bucket <-> self.buckets[idx];
+            let bucket = replace(&mut self.buckets[idx], None);
             self.insert_opt_bucket(bucket);
             idx = self.next_bucket(idx, len_buckets);
         }
@@ -613,15 +607,13 @@ pub impl<K: Hash + Eq, V> HashMap<K, V> {
     }
 
     fn consume(&mut self, f: &fn(K, V)) {
-        let mut buckets = ~[];
-        self.buckets <-> buckets;
+        let buckets = replace(&mut self.buckets, ~[]);
         self.size = 0;
 
         do vec::consume(buckets) |_, bucket| {
             match bucket {
                 None => {},
-                Some(bucket) => {
-                    let Bucket{key: key, value: value, _} = bucket;
+                Some(Bucket{key, value, _}) => {
                     f(key, value)
                 }
             }

src/libcore/pipes.rs

@@ -93,6 +93,7 @@ use unstable::intrinsics;
 use ptr;
 use task;
 use vec;
+use util::replace;
 
 static SPIN_COUNT: uint = 0;
 
@@ -428,8 +429,7 @@ fn try_recv_<T:Owned>(p: &mut Packet<T>) -> Option<T> {
     // optimistic path
     match p.header.state {
       Full => {
-        let mut payload = None;
-        payload <-> p.payload;
+        let payload = replace(&mut p.payload, None);
         p.header.state = Empty;
         return Some(payload.unwrap())
       },
@@ -480,8 +480,7 @@ fn try_recv_<T:Owned>(p: &mut Packet<T>) -> Option<T> {
             fail!(~"blocking on already blocked packet")
           },
           Full => {
-            let mut payload = None;
-            payload <-> p.payload;
+            let payload = replace(&mut p.payload, None);
             let old_task = swap_task(&mut p.header.blocked_task, ptr::null());
             if !old_task.is_null() {
                 unsafe {
@@ -675,8 +674,7 @@ impl<T:Owned,Tbuffer:Owned> Drop for SendPacketBuffered<T,Tbuffer> {
         unsafe {
             let this: &mut SendPacketBuffered<T,Tbuffer> = transmute(self);
             if this.p != None {
-                let mut p = None;
-                p <-> this.p;
+                let p = replace(&mut this.p, None);
                 sender_terminate(p.unwrap())
             }
         }
@@ -695,9 +693,7 @@ pub fn SendPacketBuffered<T,Tbuffer>(p: *mut Packet<T>)
 
 pub impl<T,Tbuffer> SendPacketBuffered<T,Tbuffer> {
     fn unwrap(&mut self) -> *mut Packet<T> {
-        let mut p = None;
-        p <-> self.p;
-        p.unwrap()
+        replace(&mut self.p, None).unwrap()
     }
 
     fn header(&mut self) -> *mut PacketHeader {
@@ -713,9 +709,7 @@ pub impl<T,Tbuffer> SendPacketBuffered<T,Tbuffer> {
 
     fn reuse_buffer(&mut self) -> BufferResource<Tbuffer> {
         //error!("send reuse_buffer");
-        let mut tmp = None;
-        tmp <-> self.buffer;
-        tmp.unwrap()
+        replace(&mut self.buffer, None).unwrap()
     }
 }
 
@@ -738,8 +732,7 @@ impl<T:Owned,Tbuffer:Owned> Drop for RecvPacketBuffered<T,Tbuffer> {
         unsafe {
             let this: &mut RecvPacketBuffered<T,Tbuffer> = transmute(self);
             if this.p != None {
-                let mut p = None;
-                p <-> this.p;
+                let p = replace(&mut this.p, None);
                 receiver_terminate(p.unwrap())
             }
         }
@@ -748,15 +741,11 @@ impl<T:Owned,Tbuffer:Owned> Drop for RecvPacketBuffered<T,Tbuffer> {
 
 pub impl<T:Owned,Tbuffer:Owned> RecvPacketBuffered<T, Tbuffer> {
     fn unwrap(&mut self) -> *mut Packet<T> {
-        let mut p = None;
-        p <-> self.p;
-        p.unwrap()
+        replace(&mut self.p, None).unwrap()
     }
 
     fn reuse_buffer(&mut self) -> BufferResource<Tbuffer> {
-        let mut tmp = None;
-        tmp <-> self.buffer;
-        tmp.unwrap()
+        replace(&mut self.buffer, None).unwrap()
     }
 }