It seems __dtoi64 function is just not implemented, but I classify it as a bug report since it causes application crash.
Steps to reproduce:
- You will need Windows on ARM, see https://reactos.org/wiki/ARM_Port#Debugging_steps
- If you intend to work on this, but don't have required environment, you can ask for help in the Mattermost chat
- Build calc application for ARM architecture and start it with WinDbg in WoA environment
- Switch calculator to Scientific mode (View => Scientific)
- Switch from Decimal to Hexadecimal numbers (Hex) - assertion and crash will be triggered
Full debug log from WinDbg: calc_arm_dbg.txt