add weak memcpy, memmove and memset

src/arm.rs

@@ -1,60 +1,54 @@
-extern "C" {
-    fn memcpy(dest: *mut u8, src: *const u8, n: usize) -> *mut u8;
-    fn memmove(dest: *mut u8, src: *const u8, n: usize) -> *mut u8;
-    fn memset(dest: *mut u8, c: i32, n: usize) -> *mut u8;
-}
-
 // FIXME: The `*4` and `*8` variants should be defined as aliases.
 
 #[no_mangle]
 pub unsafe extern "aapcs" fn __aeabi_memcpy(dest: *mut u8, src: *const u8, n: usize) {
-    memcpy(dest, src, n);
+    ::libc::memcpy(dest, src, n);
 }
 #[no_mangle]
 pub unsafe extern "aapcs" fn __aeabi_memcpy4(dest: *mut u8, src: *const u8, n: usize) {
-    memcpy(dest, src, n);
+    ::libc::memcpy(dest, src, n);
 }
 #[no_mangle]
 pub unsafe extern "aapcs" fn __aeabi_memcpy8(dest: *mut u8, src: *const u8, n: usize) {
-    memcpy(dest, src, n);
+    ::libc::memcpy(dest, src, n);
 }
 
 #[no_mangle]
 pub unsafe extern "aapcs" fn __aeabi_memmove(dest: *mut u8, src: *const u8, n: usize) {
-    memmove(dest, src, n);
+    ::libc::memmove(dest, src, n);
 }
 #[no_mangle]
 pub unsafe extern "aapcs" fn __aeabi_memmove4(dest: *mut u8, src: *const u8, n: usize) {
-    memmove(dest, src, n);
+    ::libc::memmove(dest, src, n);
 }
 #[no_mangle]
 pub unsafe extern "aapcs" fn __aeabi_memmove8(dest: *mut u8, src: *const u8, n: usize) {
-    memmove(dest, src, n);
+    ::libc::memmove(dest, src, n);
 }
 
 // Note the different argument order
 #[no_mangle]
 pub unsafe extern "aapcs" fn __aeabi_memset(dest: *mut u8, n: usize, c: i32) {
-    memset(dest, c, n);
+    ::libc::memset(dest, c, n);
 }
 #[no_mangle]
 pub unsafe extern "aapcs" fn __aeabi_memset4(dest: *mut u8, n: usize, c: i32) {
-    memset(dest, c, n);
+    ::libc::memset(dest, c, n);
 }
 #[no_mangle]
 pub unsafe extern "aapcs" fn __aeabi_memset8(dest: *mut u8, n: usize, c: i32) {
-    memset(dest, c, n);
+    ::libc::memset(dest, c, n);
 }
 
 #[no_mangle]
 pub unsafe extern "aapcs" fn __aeabi_memclr(dest: *mut u8, n: usize) {
-    memset(dest, 0, n);
+    ::libc::memset(dest, 0, n);
 }
 #[no_mangle]
 pub unsafe extern "aapcs" fn __aeabi_memclr4(dest: *mut u8, n: usize) {
-    memset(dest, 0, n);
+    ::libc::memset(dest, 0, n);
 }
 #[no_mangle]
 pub unsafe extern "aapcs" fn __aeabi_memclr8(dest: *mut u8, n: usize) {
-    memset(dest, 0, n);
+    ::libc::memset(dest, 0, n);
 }

src/lib.rs

@@ -2,9 +2,12 @@
 #![cfg_attr(not(test), no_std)]
 #![feature(asm)]
 #![feature(core_intrinsics)]
+#![feature(linkage)]
 #![feature(naked_functions)]
+#![no_builtins]
 // TODO(rust-lang/rust#35021) uncomment when that PR lands
 // #![feature(rustc_builtins)]
+// #![rustc_builtins]
 
 #[cfg(test)]
 extern crate core;
@@ -14,6 +17,11 @@ use core::mem;
 #[cfg(target_arch = "arm")]
 pub mod arm;
 
+// NOTE LLVM's weak linkage only appears to work with the ELF backend
+#[cfg(not(any(target_os = "macos",
+              target_os = "windows")))]
+pub mod libc;
+
 #[cfg(test)]
 mod test;
 

src/libc.rs

@@ -0,0 +1,44 @@
+// NOTE Implementations copy pasted from rlibc (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
+}