Emacs version: GNU Emacs 27.0.91 (build 1, x86_64-apple-darwin18.7.0, NS appkit-1671.60 Version 10.14.6 (Build 18G5033)) of 2020-07-24
Built d5acc509415869bce22c49ae311f2960494a0bdc locally
To reproduce, evaluate this
(set-char-table-range composition-function-table 47 '([".\\(?:\\(?:\\*\\*\\|//\\|==\\)\\|[*/=>]\\)" 0 font-shape-gstring]))
And then evaluate
(setq mode-name "test//")
Emacs will become unresponsive and consume 100% CPU.