Hi! On Wed, May 04, 2022 at 10:27:54PM +1000, Michael Ellerman wrote: > Segher Boessenkool <seg...@kernel.crashing.org> writes: > > Note that r1 is not the same as the CFA: r1 is the stack pointer, while > > the CFA is a DWARF concept. Often (but not always) they point to the > > same thing, for us. "When we change the stack pointer we should change > > the DWARF CFA as well"? > > Thanks, I reworded it a bit: > > DWARF has a concept called the CFA (Canonical Frame Address), which on > powerpc is calculated as an offset from the stack pointer (r1). That > means when the stack pointer is changed there must be a corresponding > CFI directive to update the calculation of the CFA.
The CFA only uses r1 if it does not have a separate frame pointer, in which case that is used. "... is usually calculated ..." maybe? A bit of handwaving is better than giving the impression you are stating something exact, if you don't. > >> 2) If a function changes LR or any non-volatile register, the save > >> location for those regs must be given. The cfi can be at any > >> instruction after the saves up to the point that the reg is > >> changed. (Exception: LR save should be described before a bl.) > > > > That isn't an exception? bl changes the current LR after all :-) > > As Alan mentioned the exception is the the CFI has to appear before the > bl not after, I noted that in the change log. You have a bit of freedom where you place your CFI statements, but you cannot place them too late (or too early). This is true for all CFI statements. I suppose the final consequence of that can be a bit surprising here :-) Segher
