Justin Lebar <justin.lebar@xxxxxxxxx> writes: > I'm confused about line 10, |movzx ecl, cl|. As I understand, this is > truncating ecx so it's 0 everywhere except the least-significant byte. > But we can only get to line 10 if the last write to ecx was in line > 23, which already truncated the top bits of the register. > > If I change |str| in the C code to a signed char, then line 10 becomes > movsx, which is sensible. That the instruction has a parallel in the > signed char case suggests to me that it's probably not an intentional > nop. > > I'd appreciate any help understanding what's going on here. It's a missed optimization bug. There are known problems in this area, although I couldn't turn up a bug report for this specific case. Ian
