According to Frank so LTO works

[Defragster]

https://forum.pjrc.com/threads/54711-Teensy-4-0-First-Beta-Test?p=198210&viewfull=1#post198210

Handler survives LTO pruning Remove Printing when PRINT_DEBUG_STUFF #ifdef off

[Defragster]

Paul, LTO may not be working - at least executing - but without this added change it won't compile:

Add line 266:: void HardFault_HandlerC(unsigned int *hardfault_args) attribute((used));

[Defragster]

Cross ref - LTO compile/upload on T4 required 15s reset as noted here: https://forum.pjrc.com/threads/54711-Teensy-4-0-First-Beta-Test?p=202037&viewfull=1#post202037

[Defragster]

Paul with the ASM code embedding the call to the Fault_Handler it is missed when 'weak' by LTO parsing. This (used) decoration with an added 'prototype' suggested by FrankB prevents that loss.