Re: performance counter ~0.4% error finding retired instruction count

[Paul Mackerras <paulus@samba.org>]

I can think of three ways to eliminate the PLT resolver overhead on
execvp:

(1) Do execvp on a non-executable file first to get execvp resolved:

	char tmpnam[16];
	int fd;
	char *args[1];

	strcpy(tmpname, "/tmp/perfXXXXXX");
	fd = mkstemp(tmpname);
	if (fd >= 0) {
		args[1] = NULL;
		execvp(tmpname, args);
		close(fd);
		unlink(tmpname);
	}
	enable_counters();
	execvp(prog, argv);

(2) Look up execvp in glibc and call it directly:

	int (*execptr)(const char *, char *const []);

	execptr = dlsym(RTLD_NEXT, "execvp");
	enable_counters();
	(*execptr)(prog, argv);

(3) Resolve the executable path ourselves and then invoke the execve
system call directly:

	char *execpath;

	execpath = search_path(getenv("PATH"), prog);
	enable_counters();
	syscall(NR_execve, execpath, argv, envp);

(4) Same as (1), but rely on "" being an invalid program name for
execvp:

	execvp("", argv);
	enable_counters();
	execvp(prog, argv);

What do you guys think?  Does any of these appeal more than the
others?  I'm leaning towards (4) myself.

Paul.