cursor: prevent llvm-opt inserting memcpy in r/w

src/libio/cursor.rs

@@ -1,6 +1,8 @@
 #[cfg(feature = "alloc")]
 use alloc::vec::Vec;
 
+use core::arch::asm;
+
 use crate::{Read, Write};
 
 #[derive(Debug, Clone)]
@@ -49,6 +51,9 @@ impl<T: AsRef<[u8]>> Read for Cursor<T> {
         let len = buf.len().min(data.len());
         // ``copy_from_slice`` generates AXI bursts, use a regular loop instead
         for i in 0..len {
+            // HACK: On opt-level=2 or above, LLVM 18 may insert memcpy calls as an optimization pass,
+            // causing AXI bursts. 
+            unsafe { asm!("", options(preserves_flags, nostack, readonly)); }
             buf[i] = data[i];
         }
         self.pos += len;
@@ -64,6 +69,8 @@ impl Write for Cursor<&mut [u8]> {
         let data = &mut self.inner[self.pos..];
         let len = buf.len().min(data.len());
         for i in 0..len {
+            // HACK: On opt-level=2 or above, this is too fast and sends corrupted packets.
+            unsafe { asm!("", options(preserves_flags, nostack, readonly)); }
             data[i] = buf[i];
         }
         self.pos += len;