implement pointer arithmetic with GEP

Closes #8118, #7136

~~~rust
extern mod extra;

use std::vec;
use std::ptr;

fn bench_from_elem(b: &mut extra::test::BenchHarness) {
    do b.iter {
        let v: ~[u8] = vec::from_elem(1024, 0u8);
    }
}

fn bench_set_memory(b: &mut extra::test::BenchHarness) {
    do b.iter {
        let mut v: ~[u8] = vec::with_capacity(1024);
        unsafe {
            let vp = vec::raw::to_mut_ptr(v);
            ptr::set_memory(vp, 0, 1024);
            vec::raw::set_len(&mut v, 1024);
        }
    }
}

fn bench_vec_repeat(b: &mut extra::test::BenchHarness) {
    do b.iter {
        let v: ~[u8] = ~[0u8, ..1024];
    }
}
~~~

Before:

    test bench_from_elem ... bench: 415 ns/iter (+/- 17)
    test bench_set_memory ... bench: 85 ns/iter (+/- 4)
    test bench_vec_repeat ... bench: 83 ns/iter (+/- 3)

After:

    test bench_from_elem ... bench: 84 ns/iter (+/- 2)
    test bench_set_memory ... bench: 84 ns/iter (+/- 5)
    test bench_vec_repeat ... bench: 84 ns/iter (+/- 3)

4 files changed, 8 insertions, 8 deletions

diff --git a/src/libextra/arena.rs b/src/libextra/arena.rs
index 3c10c3545bf..efe1d47563e 100644
--- a/src/libextra/arena.rs
+++ b/src/libextra/arena.rs
@@ -116,7 +116,7 @@ unsafe fn destroy_chunk(chunk: &Chunk) {
     let fill = chunk.fill;
 
     while idx < fill {
-        let tydesc_data: *uint = transmute(ptr::offset(buf, idx));
+        let tydesc_data: *uint = transmute(ptr::offset(buf, idx as int));
         let (tydesc, is_done) = un_bitpack_tydesc_ptr(*tydesc_data);
         let (size, align) = ((*tydesc).size, (*tydesc).align);
 
@@ -127,7 +127,7 @@ unsafe fn destroy_chunk(chunk: &Chunk) {
         //debug!("freeing object: idx = %u, size = %u, align = %u, done = %b",
         //       start, size, align, is_done);
         if is_done {
-            ((*tydesc).drop_glue)(ptr::offset(buf, start) as *i8);
+            ((*tydesc).drop_glue)(ptr::offset(buf, start as int) as *i8);
         }
 
         // Find where the next tydesc lives
@@ -176,7 +176,7 @@ impl Arena {
             //debug!("idx = %u, size = %u, align = %u, fill = %u",
             //       start, n_bytes, align, head.fill);
 
-            ptr::offset(vec::raw::to_ptr(this.pod_head.data), start)
+            ptr::offset(vec::raw::to_ptr(this.pod_head.data), start as int)
         }
     }
 
@@ -233,7 +233,7 @@ impl Arena {
             //       start, n_bytes, align, head.fill);
 
             let buf = vec::raw::to_ptr(self.head.data);
-            return (ptr::offset(buf, tydesc_start), ptr::offset(buf, start));
+            return (ptr::offset(buf, tydesc_start as int), ptr::offset(buf, start as int));
         }
     }
 
diff --git a/src/libextra/c_vec.rs b/src/libextra/c_vec.rs
index f72d3ee694f..e656360aa5a 100644
--- a/src/libextra/c_vec.rs
+++ b/src/libextra/c_vec.rs
@@ -122,7 +122,7 @@ pub unsafe fn c_vec_with_dtor<T>(base: *mut T, len: uint, dtor: @fn())
 pub fn get<T:Clone>(t: CVec<T>, ofs: uint) -> T {
     assert!(ofs < len(t));
     return unsafe {
-        (*ptr::mut_offset(t.base, ofs)).clone()
+        (*ptr::mut_offset(t.base, ofs as int)).clone()
     };
 }
 
@@ -133,7 +133,7 @@ pub fn get<T:Clone>(t: CVec<T>, ofs: uint) -> T {
  */
 pub fn set<T>(t: CVec<T>, ofs: uint, v: T) {
     assert!(ofs < len(t));
-    unsafe { *ptr::mut_offset(t.base, ofs) = v };
+    unsafe { *ptr::mut_offset(t.base, ofs as int) = v };
 }
 
 /*
diff --git a/src/libextra/ebml.rs b/src/libextra/ebml.rs
index c9b0cbb2753..f66677c21f7 100644
--- a/src/libextra/ebml.rs
+++ b/src/libextra/ebml.rs
@@ -150,7 +150,7 @@ pub mod reader {
 
         unsafe {
             let (ptr, _): (*u8, uint) = transmute(data);
-            let ptr = offset(ptr, start);
+            let ptr = offset(ptr, start as int);
             let ptr: *i32 = transmute(ptr);
             let val = bswap32(*ptr);
             let val: u32 = transmute(val);
diff --git a/src/libextra/par.rs b/src/libextra/par.rs
index f462a0cfae7..4069c68e71c 100644
--- a/src/libextra/par.rs
+++ b/src/libextra/par.rs
@@ -58,7 +58,7 @@ fn map_slices<A:Clone + Send,B:Clone + Send>(
                 let f = do future_spawn() || {
                     unsafe {
                         let len = end - base;
-                        let slice = (ptr::offset(p, base),
+                        let slice = (ptr::offset(p, base as int),
                                      len * sys::size_of::<A>());
                         info!("pre-slice: %?", (base, slice));
                         let slice : &[A] =