Fixing volatile and memory attributes for assembly.

For volatile:
You can prevent an asm instruction from being deleted by writing the
keyword volatile after the asm. [...] The volatile keyword indicates
that the instruction has important side-effects. GCC will not
delete a volatile asm if it is reachable.
and
An asm instruction without any output operands will be treated
identically to a volatile asm instruction.

And for memory:
This will cause GCC to not keep memory values cached in
registers across the assembler instruction and not optimize
stores or loads to that memory.

See also:
https://gcc.gnu.org/onlinedocs/gcc-4.7.2/gcc/Extended-Asm.html
http://stackoverflow.com/questions/14449141/the-difference-between-asm-asm-volatile-and-clobbering-memory

1 files changed, 35 insertions, 0 deletions

diff --git a/src/time.rs b/src/time.rs
index ad8f466..7d402f4 100644
--- a/src/time.rs
+++ b/src/time.rs
@@ -1,5 +1,18 @@
+//! Functions to read time stamp counters on x86.
 
 /// Read the time stamp counter.
+///
+/// The RDTSC instruction is not a serializing instruction.
+/// It does not necessarily wait until all previous instructions
+/// have been executed before reading the counter. Similarly,
+/// subsequent instructions may begin execution before the
+/// read operation is performed. If software requires RDTSC to be
+/// executed only after all previous instructions have completed locally,
+/// it can either use RDTSCP or execute the sequence LFENCE;RDTSC.
+///
+/// # Safety
+/// * Causes a GP fault if the TSD flag in register CR4 is set and the CPL
+///   is greater than 0.
 #[allow(unused_mut)]
 pub unsafe fn rdtsc() -> u64 {
     let mut low: u32;
@@ -8,3 +21,25 @@ pub unsafe fn rdtsc() -> u64 {
     asm!("rdtsc" : "={eax}" (low), "={edx}" (high));
     ((high as u64) << 32) | (low as u64)
 }
+
+/// Read the time stamp counter.
+///
+/// The RDTSCP instruction waits until all previous instructions
+/// have been executed before reading the counter.
+/// However, subsequent instructions may begin execution
+/// before the read operation is performed.
+///
+/// Volatile is used here because the function may be used to act as
+/// an instruction barrier.
+///
+/// # Safety
+/// * Causes a GP fault if the TSD flag in register CR4 is set and the
+///   CPL is greater than 0.
+#[allow(unused_mut)]
+pub unsafe fn rdtscp() -> u64 {
+    let mut low: u32;
+    let mut high: u32;
+
+    asm!("rdtscp" : "={eax}" (low), "={edx}" (high) ::: "volatile");
+    ((high as u64) << 32) | (low as u64)
+}
\ No newline at end of file