add weak `memcpy` et al symbols

closes #28

src/lib.rs

@@ -1,10 +1,12 @@
 #![allow(unused_features)]
 #![feature(asm)]
 #![feature(core_intrinsics)]
+#![feature(linkage)]
 #![feature(naked_functions)]
 #![cfg_attr(not(test), no_std)]
 // TODO(rust-lang/rust#35021) uncomment when that PR lands
 // #![feature(rustc_builtins)]
+#![no_builtins]
 
 // We disable #[no_mangle] for tests so that we can verify the test results
 // against the native compiler-rt implementations of the builtins.
@@ -20,6 +22,7 @@ extern crate core;
 pub mod arm;
 
 pub mod udiv;
+pub mod mem;
 pub mod mul;
 pub mod shift;
 

src/mem.rs

@@ -0,0 +1,60 @@
+// NOTE Copied verbatim from the rlibc crate
+// cf. https://crates.io/crates/rlibc
+
+#[linkage = "weak"]
+#[no_mangle]
+pub unsafe extern "C" fn memcpy(dest: *mut u8, src: *const u8, n: usize) -> *mut u8 {
+    let mut i = 0;
+    while i < n {
+        *dest.offset(i as isize) = *src.offset(i as isize);
+        i += 1;
+    }
+    dest
+}
+
+#[linkage = "weak"]
+#[no_mangle]
+pub unsafe extern "C" fn memmove(dest: *mut u8, src: *const u8, n: usize) -> *mut u8 {
+    if src < dest as *const u8 {
+        // copy from end
+        let mut i = n;
+        while i != 0 {
+            i -= 1;
+            *dest.offset(i as isize) = *src.offset(i as isize);
+        }
+    } else {
+        // copy from beginning
+        let mut i = 0;
+        while i < n {
+            *dest.offset(i as isize) = *src.offset(i as isize);
+            i += 1;
+        }
+    }
+    dest
+}
+
+#[linkage = "weak"]
+#[no_mangle]
+pub unsafe extern "C" fn memset(s: *mut u8, c: i32, n: usize) -> *mut u8 {
+    let mut i = 0;
+    while i < n {
+        *s.offset(i as isize) = c as u8;
+        i += 1;
+    }
+    s
+}
+
+#[linkage = "weak"]
+#[no_mangle]
+pub unsafe extern "C" fn memcmp(s1: *const u8, s2: *const u8, n: usize) -> i32 {
+    let mut i = 0;
+    while i < n {
+        let a = *s1.offset(i as isize);
+        let b = *s2.offset(i as isize);
+        if a != b {
+            return a as i32 - b as i32;
+        }
+        i += 1;
+    }
+    0
+}