diff --git a/src/mem/x86_64.rs b/src/mem/x86_64.rs
index 3b372d10..dd98e37c 100644
--- a/src/mem/x86_64.rs
+++ b/src/mem/x86_64.rs
@@ -16,6 +16,7 @@
 // feature is present at compile-time. We don't bother detecting other features.
 // Note that ERMSB does not enhance the backwards (DF=1) "rep movsb".
 
+use core::arch::asm;
 use core::intrinsics;
 use core::mem;
 
@@ -34,16 +35,26 @@ pub unsafe fn copy_forward(dest: *mut u8, src: *const u8, count: usize) {
 
 #[inline(always)]
 #[cfg(not(target_feature = "ermsb"))]
-pub unsafe fn copy_forward(dest: *mut u8, src: *const u8, count: usize) {
-    let qword_count = count >> 3;
-    let byte_count = count & 0b111;
-    // FIXME: Use the Intel syntax once we drop LLVM 9 support on rust-lang/rust.
-    core::arch::asm!(
-        "repe movsq (%rsi), (%rdi)",
-        "mov {byte_count:e}, %ecx",
-        "repe movsb (%rsi), (%rdi)",
-        byte_count = in(reg) byte_count,
+pub unsafe fn copy_forward(mut dest: *mut u8, mut src: *const u8, count: usize) {
+    let (pre_byte_count, qword_count, byte_count) = rep_param(dest, count);
+    // Separating the blocks gives the compiler more freedom to reorder instructions.
+    asm!(
+        "rep movsb",
+        inout("ecx") pre_byte_count => _,
+        inout("rdi") dest => dest,
+        inout("rsi") src => src,
+        options(att_syntax, nostack, preserves_flags)
+    );
+    asm!(
+        "rep movsq",
         inout("rcx") qword_count => _,
+        inout("rdi") dest => dest,
+        inout("rsi") src => src,
+        options(att_syntax, nostack, preserves_flags)
+    );
+    asm!(
+        "rep movsb",
+        inout("ecx") byte_count => _,
         inout("rdi") dest => _,
         inout("rsi") src => _,
         options(att_syntax, nostack, preserves_flags)
@@ -52,22 +63,28 @@ pub unsafe fn copy_forward(dest: *mut u8, src: *const u8, count: usize) {
 
 #[inline(always)]
 pub unsafe fn copy_backward(dest: *mut u8, src: *const u8, count: usize) {
-    let qword_count = count >> 3;
-    let byte_count = count & 0b111;
-    // FIXME: Use the Intel syntax once we drop LLVM 9 support on rust-lang/rust.
-    core::arch::asm!(
+    let (pre_byte_count, qword_count, byte_count) = rep_param(dest, count);
+    // We can't separate this block due to std/cld
+    asm!(
         "std",
-        "repe movsq (%rsi), (%rdi)",
-        "movl {byte_count:e}, %ecx",
-        "addq $7, %rdi",
-        "addq $7, %rsi",
-        "repe movsb (%rsi), (%rdi)",
+        "rep movsb",
+        "sub $7, %rsi",
+        "sub $7, %rdi",
+        "mov {qword_count}, %rcx",
+        "rep movsq",
+        "test {pre_byte_count:e}, {pre_byte_count:e}",
+        "add $7, %rsi",
+        "add $7, %rdi",
+        "mov {pre_byte_count:e}, %ecx",
+        "rep movsb",
         "cld",
-        byte_count = in(reg) byte_count,
-        inout("rcx") qword_count => _,
-        inout("rdi") dest.add(count).wrapping_sub(8) => _,
-        inout("rsi") src.add(count).wrapping_sub(8) => _,
-        options(att_syntax, nostack)
+        pre_byte_count = in(reg) pre_byte_count,
+        qword_count = in(reg) qword_count,
+        inout("ecx") byte_count => _,
+        inout("rdi") dest.add(count - 1) => _,
+        inout("rsi") src.add(count - 1) => _,
+        // We modify flags, but we restore it afterwards
+        options(att_syntax, nostack, preserves_flags)
     );
 }
 
@@ -86,18 +103,29 @@ pub unsafe fn set_bytes(dest: *mut u8, c: u8, count: usize) {
 
 #[inline(always)]
 #[cfg(not(target_feature = "ermsb"))]
-pub unsafe fn set_bytes(dest: *mut u8, c: u8, count: usize) {
-    let qword_count = count >> 3;
-    let byte_count = count & 0b111;
-    // FIXME: Use the Intel syntax once we drop LLVM 9 support on rust-lang/rust.
-    core::arch::asm!(
-        "repe stosq %rax, (%rdi)",
-        "mov {byte_count:e}, %ecx",
-        "repe stosb %al, (%rdi)",
-        byte_count = in(reg) byte_count,
+pub unsafe fn set_bytes(mut dest: *mut u8, c: u8, count: usize) {
+    let c = c as u64 * 0x0101_0101_0101_0101;
+    let (pre_byte_count, qword_count, byte_count) = rep_param(dest, count);
+    // Separating the blocks gives the compiler more freedom to reorder instructions.
+    asm!(
+        "rep stosb",
+        inout("ecx") pre_byte_count => _,
+        inout("rdi") dest => dest,
+        in("rax") c,
+        options(att_syntax, nostack, preserves_flags)
+    );
+    asm!(
+        "rep stosq",
         inout("rcx") qword_count => _,
+        inout("rdi") dest => dest,
+        in("rax") c,
+        options(att_syntax, nostack, preserves_flags)
+    );
+    asm!(
+        "rep stosb",
+        inout("ecx") byte_count => _,
         inout("rdi") dest => _,
-        in("rax") (c as u64) * 0x0101010101010101,
+        in("rax") c,
         options(att_syntax, nostack, preserves_flags)
     );
 }
@@ -156,3 +184,13 @@ pub unsafe fn compare_bytes(a: *const u8, b: *const u8, n: usize) -> i32 {
         c16(a.cast(), b.cast(), n)
     }
 }
+
+/// Determine optimal parameters for a `rep` instruction.
+fn rep_param(dest: *mut u8, mut count: usize) -> (usize, usize, usize) {
+    // Unaligned writes are still slow on modern processors, so align the destination address.
+    let pre_byte_count = ((8 - (dest as usize & 0b111)) & 0b111).min(count);
+    count -= pre_byte_count;
+    let qword_count = count >> 3;
+    let byte_count = count & 0b111;
+    (pre_byte_count, qword_count, byte_count)
+}