https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72827
--- Comment #10 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> It does look possible that this is an LRA issue. Here's the code right
> before failure:
>
> 100dae08: f8 95 22 39 addi r9,r2,-27144
> 100dae0c: 01 00 40 39 li r10,1
> 100dae10: 00 00 49 99 stb r10,0(r9)
> 100dae14: b8 04 20 39 li r9,1208
> 100dae18: 14 4a 7f 7c add r3,r31,r9
> 100dae1c: 08 00 83 e8 ld r4,8(r3)
> 100dae20: 00 00 63 e8 ld r3,0(r3)
> 100dae24: 3d 86 14 48 bl 10223460
> <system__secondary_stack__ss_re
> lease>
> 100dae28: 00 00 00 60 nop
>
> At the call, r3 has a value of 0. It looks quite possible that the stack
> load at 0x100dae20 is from a spill slot. I don't see offset 1208 used for a
> corresponding store anywhere in the function. There is, however, some odd
> code that uses that constant to fiddle with the frame pointer and then set
> it back to where it was:
>
> 100da5d8: b8 04 20 39 li r9,1208
>
> 100da5e4: 14 4a ff 7f add r31,r31,r9
> 100da5e8: 50 f8 e9 7f subf r31,r9,r31
Here are the declarations of the relevant subroutines:
type Mark_Id is private;
-- Type used to mark the stack for mark/release processing
function SS_Mark return Mark_Id;
-- Return the Mark corresponding to the current state of the stack
procedure SS_Release (M : Mark_Id);
-- Restore the state of the stack corresponding to the mark M. If an
-- additional chunk have been allocated, it will never be freed during a
-- ??? missing comment here
type Mark_Id is record
Sstk : System.Address;
Sptr : SS_Ptr;
end record;
-- A mark value contains the address of the secondary stack structure,
-- as returned by System.Soft_Links.Get_Sec_Stack_Addr, and a stack
-- pointer value corresponding to the point of the mark call.
So the double-word load before the call to SS_Release should be from a Mark_Id
object obtained from a preceding call to SS_Mark. It indeed looks like the
double-word store to this Mark_Id object has been optimized away, possibly
because of the strange back-and-forth adjustment to the FP. Does passing
CFLAGS="-O2 -fno-dse" for the gnattools make a difference?
