On Fri, May 03, 2019 at 09:29:11AM +0000, Matthew Malcomson wrote:
> On 02/05/19 16:33, Segher Boessenkool wrote:
> > Because other things want to use it as the place to put stack checking,
> > for example. And that cannot be after this note, but it can also not
> > be before it.
> >
>
> I figured the stack checking could be after one note (end of argument
> setup), and before the other (start of user code)?
I don't think that can work. It needs to do its thing before anything
else (that can overflow, at least) touches the stack.
> > Is there any reason the stack checking code is inserted way before the
> > prologue/epilogue are, btw?
>
> I don't know, I'm especially curious why the prologue & epilogue are
> emitted in assembly with the TARGET_ASM_FUNCTION_{EPILOGUE,PROLOGUE}
> macros instead of emitted as rtl somewhere earlier in the pipeline.
They are emitted as RTL on most targets. Some targets *also* use some
asm text output for special things, but the usual prologue code is done
as RTL.
> I'd guess the stack checking code is earlier since it would like to be
> optimised by the RTL passes.
It cannot, or *should* not, be optimised much. Hrm.
> Are you thinking there's call for a more thorough handling of the
> introduction and placement of these almost-prologue parts like
> stack-checking, non-local goto save areas and the like?
I think we should try to no longer rely on these magic notes, which only
still work by luck, or even do nbot really work anymore.
Segher
