combining row with removing NAs

N = 50000
x = y = rep(NA, N)
x[sample(N, N/2)] = rnorm(N/2)
y[is.na(x)] = rnorm(N/2)
dat = data.frame(x, y)

f1 = function(dat){dat$z = dat$x; dat$z[is.na(dat$x)] = dat$y[is.na(dat$x)]; dat}
f2 = function(dat){dat$z = ifelse(is.na(dat$x), dat$y, dat$x); dat}
f3 = function(dat){dat$z = with(dat, ifelse(is.na(x), y, x)); dat}
f4 = function(dat){dat$z = na.omit(c(unlist(t(dat)))); dat}
f5 = function(dat){dat$z = rowSums(dat, TRUE); dat}

library(rbenchmark)
benchmark(f1(dat), f2(dat), f3(dat), f4(dat), f5(dat),
  columns = c("test", "replications", "elapsed", "relative"),
    order = "relative", replications = 20)
#      test replications elapsed relative
# 5 f5(dat)           20    0.28    1.000
# 1 f1(dat)           20    0.30    1.071
# 4 f4(dat)           20    0.40    1.429
# 2 f2(dat)           20    0.46    1.643
# 3 f3(dat)           20    0.58    2.071