On Fri, Oct 21, 2022 at 03:24:27AM -0400, Jason A. Donenfeld wrote: > Hi Greg, > > On Fri, Oct 21, 2022 at 2:48 AM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > > > On Thu, Oct 20, 2022 at 01:17:33PM -0700, Linus Torvalds wrote: > > > And in other cases, there's no actual difference at all, just > > > different register usage, so the diff looks fairly big, but doesn't > > > seem to be real. In one case I looked at, it started with a 'movzbl', > > > but it was that in both cases, because the type was actually 'unsigned > > > char' to begin with. But for some reason it just used different > > > registers. Example: > > > > > > - handle_control_request() in drivers/usb/gadget/udc/dummy_hcd.c > > > > > > The reason here *seems* to be that > > > > > > char *buf; > > > buf = (char *)urb->transfer_buffer; > > > > > > where it really probably should be 'u8 *buf', since it actually > > > does a cast to 'u8' in one place, but there isn't even any read of > > > that 'buf' pointer. So the difference seems to be entirely just some > > > "different type in assignment" cast internal to gcc that then > > > incidentally generated a random other choice in register allocation. > > > > I've send a patch for this now: > > https://lore.kernel.org/r/20221021064453.3341050-1-gregkh@xxxxxxxxxxxxxxxxxxx > > and will take it through the USB tree, unless Jason wants to grab it > > through his tree. > > This doesn't appear to have any actual effect, but just changes gcc's > register allocation unexpectedly. So feel free to take it, as it > doesn't seem like it's "one of those bad cases" that I'm keeping track > of. Great, will take it through my tree, thanks! greg k-h
