Optimize for kernels which are known to have the vfork syscall. Don't confuse the CPUs branch prediction unit by jumping to the return address.

sysdeps/unix/sysv/linux/i386/vfork.S

@@ -1,4 +1,4 @@
-/* Copyright (C) 1999 Free Software Foundation, Inc.
+/* Copyright (C) 1999, 2002 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
    Contributed by Andreas Schwab <schwab@gnu.org>.
 
@@ -20,6 +20,7 @@
 #include <sysdep.h>
 #define _ERRNO_H	1
 #include <bits/errno.h>
+#include <kernel-features.h>
 
 /* Clone the calling process, but without copying the whole address space.
    The calling process is suspended until the new process exits or is
@@ -36,22 +37,32 @@ ENTRY (__vfork)
 	/* Stuff the syscall number in EAX and enter into the kernel.  */
 	movl	$SYS_ify (vfork), %eax
 	int	$0x80
-	cmpl	$-4095, %eax
-	jae	.Lerror		/* Branch forward if it failed.  */
 
-	/* Jump to the return PC.  */
-	jmp	*%ecx
-
-.Lerror:
-	/* Push back the return PC.  */
+	/* Jump to the return PC.  Don't jump directly since this
+	   disturbs the branch target cache.  Instead push the return
+	   address back on the stack.  */
 	pushl	%ecx
 
+	cmpl	$-4095, %eax
+	/* Branch forward if it failed.  */
+# ifdef __ASSUME_VFORK_SYSCALL
+	jae	SYSCALL_ERROR_LABEL
+.Lpseudo_end:
+# else
+	jae	.Lerror
+# endif
+
+	ret
+
+# ifndef __ASSUME_VFORK_SYSCALL
+.Lerror:
 	/* Check if vfork syscall is known at all.  */
 	cmpl	$-ENOSYS, %eax
 	jne	SYSCALL_ERROR_LABEL
-
+# endif
 #endif
 
+#ifndef __ASSUME_VFORK_SYSCALL
 	/* If we don't have vfork, fork is close enough.  */
 
 	movl	$SYS_ify (fork), %eax
@@ -60,7 +71,7 @@ ENTRY (__vfork)
 	jae	SYSCALL_ERROR_LABEL
 .Lpseudo_end:
 	ret
-
+#endif
 PSEUDO_END (__vfork)
 
 weak_alias (__vfork, vfork)