diff --git a/library/compiler-builtins/src/mem/x86_64.rs b/library/compiler-builtins/src/mem/x86_64.rs
index a1015f61282a..e9c1c56d539b 100644
--- a/library/compiler-builtins/src/mem/x86_64.rs
+++ b/library/compiler-builtins/src/mem/x86_64.rs
@@ -73,15 +73,21 @@ pub unsafe fn copy_backward(dest: *mut u8, src: *const u8, count: usize) {
     // We can't separate this block due to std/cld
     asm!(
         "std",
+        "test %ecx, %ecx",
+        "jz 1f",
         "rep movsb",
+        "1:",
         "sub $7, %rsi",
         "sub $7, %rdi",
         "mov {qword_count}, %rcx",
         "rep movsq",
+        "test {pre_byte_count:e}, {pre_byte_count:e}",
+        "jz 1f",
         "add $7, %rsi",
         "add $7, %rdi",
         "mov {pre_byte_count:e}, %ecx",
         "rep movsb",
+        "1:",
         "cld",
         pre_byte_count = in(reg) pre_byte_count,
         qword_count = in(reg) qword_count,