ACLE and UBSan disagree about clz with input zero #63113
Description
D152023 made calls to __builtin_clz and __builtin_ctz with input 0 to be undefined behaviour (RFC here). However we use them in the arm-acle.h to implement __clz:
llvm-project/clang/lib/Headers/arm_acle.h
Line 143 in c91246b
The ACLE documentation for __clz states:
When x is zero it returns the argument width, i.e. 32 or 64.
The definitions in the header will need to be updated to handle the zero case.