The flag is for generating correct arm-thumb interwork veneers in the
assembly code __aeabi_{memcpy,memset,etc} functions.
Armv6m only does thumb code generation regardless of whether arm or
thumb is selected and armv6t2 uses the newer thumb 2 set. All other
versions that zig supports pre-armv7 need the veneers and hence the
flag. Armv5 is actually armv5t.
Relevant code from clang/lib/Basic/Targets/Arm.cpp
```c
bool ARMTargetInfo::isThumb() const {
return ArchISA == llvm::ARM::ISAKind::THUMB;
}
bool ARMTargetInfo::supportsThumb() const {
return CPUAttr.count('T') || ArchVersion >= 6;
}
bool ARMTargetInfo::supportsThumb2() const {
return CPUAttr.equals("6T2") ||
(ArchVersion >= 7 && !CPUAttr.equals("8M_BASE"));
}
```
Also see
http://www.llvm.org/svn/llvm-project/llvm/trunk/lib/Target/ARM/ARM.td
aff2e47821