Re: [PATCH] Turn on LRA on all targets

["Maciej W. Rozycki" <macro@orcam.me.uk>]

On Fri, 16 Feb 2024, Jakub Jelinek wrote:

> >  There is no function prologue to optimise in the VAX case, because all 
> > the frame setup has already been made by the CALLS instruction itself in 
> > the caller.  The first machine instruction of the callee is technically 
> > already past the "prologue".  And then RET serves as the whole function 
> > "epilogue".
> 
> So, what is the problem with DWARF unwinding?  Just make sure to emit
> appropriate instructions describing the saving of the corresponding
> registers at specific points based on CFA at the start of the function
> (so that it appears in CIE instructions) and that should be all that is
> needed, no?

 I may not remember all the issues correctly offhand as it's been a while 
since I looked into it, but as I recall DWARF handling code has not been 
prepared for all the frame to have been already allocated and initialised 
at a function's entry point, and also at least DWARF-4 is IIRC required to 
have statics at offsets positive from FP (for a stack growing downwards).

 There was an issue about restoring the caller's value of SP too, which is 
callee's AP+4*(*AP)+4 (AP being the argument pointer, which is a hardreg, 
pointing at #arguments), or alternatively it can be calculated as the sum 
of FP, the fixed frame size (20), the variable frame size (4*#statics, 
recorded as a register mask in a 12-bit field at *(FP+6)), the #arguments 
slot (4), and the argument space size (4*#arguments).  Obviously the 
former calculation is easier to make (#arguments has to be zero-extended 
from bits 7:0).

 FAOD the #arguments slot is filled by the CALLS instruction at call time 
and can vary between callers obviously for varargs, but also cases such as 
open(2), so to reconstruct the caller's SP you always need to examine the 
actual stack frame, and it does not appear to be supported by libgcc DWARF 
EH code as it stands.

 I've left out stack realignment stuff or CALLG instruction support from 
the consideration above, as we don't handle these features in our psABI, 
all of which have provisions for from hardware in the stack frame.

 As I say I may have missed or confused something, and anyway I'll take 
another stab at it, hopefully soon.

  Maciej