The way to compute the elapsed time in GCC?

[QiangHuang <qianghuang87@xxxxxxxxx>]

    I know that the two pairs of functions
timevar_push()/timevar_pop() and  timevar_start()/timevar_stop() are
used to statistic the elapsed time of a certain subprocess in GCC.

   I wonder that if someone could tell me the exact difference between
the two pairs.

   Tanks in advance ~ ^_^