https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108268
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Looking at GCC 12, I think no target actually stored anything into that last
element
unless it was a return or alternate return column.
It is true that expand_builtin_init_dwarf_reg_sizes had:
if (!init_state.wrote_return_column)
init_return_column_size (mode, mem, DWARF_FRAME_RETURN_COLUMN);
#ifdef DWARF_ALT_FRAME_RETURN_COLUMN
init_return_column_size (mode, mem, DWARF_ALT_FRAME_RETURN_COLUMN);
#endif
which is what the new gcc tries to preserve.
init_one_dwarf_reg_size had:
const unsigned int dnum = DWARF_FRAME_REGNUM (regno);
const unsigned int rnum = DWARF2_FRAME_REG_OUT (dnum, 1);
const unsigned int dcol = DWARF_REG_TO_UNWIND_COLUMN (rnum);
...
if (rnum >= DWARF_FRAME_REGISTERS)
return;
though, and seems DWARF_REG_TO_UNWIND_COLUMN always uses the default definition
of
just returning its argument, so I think init_one_dwarf_reg_size would never
store to
the last element. init_return_column_size can though. And
TARGET_INIT_DWARF_REG_SIZES_EXTRA is only defined on msp430 and rs6000 and
neither writes at offsets >= DWARF_FRAME_REGISTERS.
So, I think the DWARF_FRAME_REGISTERS element should be considered into whether
everything is constant/same size only if it is DWARF_FRAME_RETURN_COLUMN or
DWARF_ATL_FRAME_RETURN_COLUMN.
