It hass been a longstanding principle in LLVM that the presence of
debug info shall not affect code generation. This patch brings the
Swift frontend closer to this ideal:
- unconditionally emit shadow copies
- unconditionally bind type metadata
The extra allocas, bitcasts, geps, and stores being emitted get
optimized away when compiling at anything but -Onone. There are few
use-cases for compiling at -Onone without -g, so this shouldn't affect
performance for any real-world use-cases.
This is in some code to initialize variables for the debugger.
From the original commit
"Zero-initialize uninitialized Dictionary variables for the debugger.
To avoid the debugger displaying garbage or having expressions crash
when inspecting an uninitialized dictionary variable, zero-initialize
the first word of the alloca at -Onone.
"
Unfortunately, we are comparing different quantities in that code.
rdar://40043512
