MFamd64: use a less compiler-intensive MD implementation of 'curthread'

so that the compiler doesn't have to do so much work.

Approved by:  re (jhb)

sys/i386/i386/machdep.c

@@ -128,6 +128,9 @@ __FBSDID("$FreeBSD$");
 #include <sys/ptrace.h>
 #include <machine/sigframe.h>
 
+/* Sanity check for __curthread() */
+CTASSERT(offsetof(struct pcpu, pc_curthread) == 0);
+
 extern void init386(int first);
 extern void dblfault_handler(void);
 

sys/i386/include/pcpu.h

@@ -151,6 +151,16 @@ extern struct pcpu *pcpup;
 #define	PCPU_PTR(member)	__PCPU_PTR(pc_ ## member)
 #define	PCPU_SET(member, val)	__PCPU_SET(pc_ ## member, val)
 
+static __inline struct thread *
+__curthread(void)
+{
+	struct thread *td;
+
+	__asm __volatile("movl %%fs:0,%0" : "=r" (td));
+	return (td);
+}
+#define	curthread (__curthread())
+
 #else
 #error gcc or lint is required to use this file
 #endif