byte-optimize-log

byte-optimize-log is a variable defined in `bytecomp.el'.
Its value is

nil

Documentation:
If non-nil, the byte-compiler will log its optimizations.
If this is 'source, then only source-level optimizations will be logged.
If it is 'byte, then only byte-level optimizations will be logged.
The information is logged to `byte-compile-log-buffer'.

You can customize this variable.