Add a function rdtsc32() to read lower 32 bits from TSC and discard upper

32 bits.  Some times compiler inserts unnecessary instructions to preserve
unused upper 32 bits even when it is casted to a 32-bit value.  It reduces
such compiler mistakes where every cycle counts.

1 files changed, 9 insertions, 0 deletions

diff --git a/sys/amd64/include/cpufunc.h b/sys/amd64/include/cpufunc.h
index b06e490..9b61d37 100644
--- a/sys/amd64/include/cpufunc.h
+++ b/sys/amd64/include/cpufunc.h
@@ -322,6 +322,15 @@ rdtsc(void)
 	return (low | ((uint64_t)high << 32));
 }
 
+static __inline uint32_t
+rdtsc32(void)
+{
+	uint32_t rv;
+
+	__asm __volatile("rdtsc" : "=a" (rv) : : "edx");
+	return (rv);
+}
+
 static __inline void
 wbinvd(void)
 {