About Profile Guided Optimization (PGO)

Profile-guided optimization (PGO) provides dynamic program control flow information to the compiler. The compiler uses this information to make better decisions for many optimizations such as long/short trip count loops, inlining functions and more.

PGO also deals with unbalanced if statements, taking into consideration the number of instructions and the probability of a branch being taken.

Pros
Using PGO can lead to better:

predication

speculation

function splitting

loop optimizations

pipelining vs. unroll vs. nothing

basic block ordering

better decisions of which functions to inline

better register allocation

option statement optimization

function ordering

Cons

different paradigm for builds - three steps
requires you to schedule time in final stages of development when code is more stable
requires a representative workload-not for specialized case