mem: Add documentations for REP string insturctions

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/mem/x86_64.rs

@@ -1,5 +1,21 @@
 use super::c_int;
 
+// On most modern Intel and AMD processors, "rep movsq" and "rep stosq" have
+// been enhanced to perform better than an simple qword loop, making them ideal
+// for implementing memcpy/memset. Note that "rep cmps" has received no such
+// enhancement, so it is not used to implement memcmp.
+//
+// On certain recent Intel processors, "rep movsb" and "rep stosb" have been
+// further enhanced to automatically select the best microarchitectural
+// implementation based on length and alignment. See the following features from
+// the "Intel® 64 and IA-32 Architectures Optimization Reference Manual":
+//  - ERMSB - Enhanced REP MOVSB and STOSB (Ivy Bridge and later)
+//  - FSRM - Fast Short REP MOV (Ice Lake and later)
+//  - Fast Zero-Length MOVSB (On no current hardware)
+//  - Fast Short STOSB (On no current hardware)
+// However, to avoid run-time feature detection, we don't use these byte-based
+// instructions for most of the copying, preferring the qword variants.
+
 #[cfg_attr(all(feature = "mem", not(feature = "mangled-names")), no_mangle)]
 pub unsafe extern "C" fn memcpy(dest: *mut u8, src: *const u8, count: usize) -> *mut u8 {
     asm!(