I want to speed up integer overflow checking for hardening purposes by keeping a sticky overflow flag and only trapping when necessary. I want to keep it super simple while hopefully giving the optimizers room to do their thing.
In the codegen part of clang:
- each function gets an i1 for storing overflow information, initialized to 0
- each integer overflow check ORs its result into the overflow flag
- before each function call, return instruction, or other side-effecting operation, execude ud2 if overflow is set