Here is part of the 1957 paper, "The FORTRAN Automatic Coding System" by Backus, et al., this snippet on the FREQUENCY statment and its use in a compile-time Monte Carlo simulation of the run-time to optimise the code generated. Quoting ...
The fundamental unit of program is the basic block; a basic block is a stretch of program which has a single entry point and a single exit point. The purpose of section 4 is to prepare for section 5 a table of predecessors (PRED table) which enumerates the basic blocks and lists for every basic block each of the basic blocks which can be its immediate predecessor in flow, together with the absolute frequency of each such basic block link. This table is obtained by an actual "execution" of the program in Monte-Carlo fashion, in which the outcome of conditional transfers arising out of IF-type statements and computed GO TO'S is determined by a random number generator suitably weighted according to whatever FREQUENCY statements have been provided.
Bests. - (Bob)