http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28896
--- Comment #8 from Larry Baker <baker at usgs dot gov> 2012-07-04 01:40:23 UTC
---
Andreas,
I posted my negative findings that -mcpu=68020 still causes the
-fstack-limit-symbol code path to fail. I believe the fundamental problem
there is the allocation of two different rtx registers that should actually
refer to the same rtx register. Consider the code in gcc/config/m68k/m68k.c:
> /* If the stack limit is a symbol, we can check it here,
> before actually allocating the space. */
> if (crtl->limit_stack
> && GET_CODE (stack_limit_rtx) == SYMBOL_REF)
> {
> limit = plus_constant (stack_limit_rtx, current_frame.size + 4);
> if (!m68k_legitimate_constant_p (Pmode, limit))
> {
> emit_move_insn (gen_rtx_REG (Pmode, D0_REG), limit);
> limit = gen_rtx_REG (Pmode, D0_REG);
> }
> emit_insn (gen_ctrapsi4 (gen_rtx_LTU (VOIDmode,
> stack_pointer_rtx, limit),
> stack_pointer_rtx, limit,
> const1_rtx));
> }
>
> fsize_with_regs = current_frame.size;
My analysis is (I am NOT a compiler developer, so I could be way off base here)
that gen_rtx_REG returns an rtx register reference to an unlimited pool of rtx
registers. Somehow, later, the binding of the physical register D0_REG takes
place -- I guess in the code generator. The emit_move_insn copies the limit
rtx expression into an unnamed rtx register. That rtx register is valid, i.e.,
has a value. The next line seems to be intended to modify limit to refer to
the rtx register containing the original limit value for use in emit_insn( trap
). However, the rtx expression assigned to limit is not the rtx register that
has been set up; it is a new rtx register that is not valid, i.e., has no
value.
This code gets skipped if the limit rtx expression is already a valid constant.
Which, I assume means it is a numeric constant, not a symbolic constant.
I think the proper code should name the rtx register allocated for the
emit_move_insn and then copy the value of that name into the limit rtx
expression:
> {
> rtx d0reg = gen_rtx_REG (Pmode, D0_REG);
> emit_move_insn (d0reg, limit);
> limit =d0reg;
> }
What do you think?
Larry Baker
US Geological Survey
650-329-5608
ba...@usgs.gov
On 3 Jul 2012, at 2:47 AM, sch...@linux-m68k.org wrote:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28896
>
> Andreas Schwab <sch...@linux-m68k.org> changed:
>
> What |Removed |Added
> ----------------------------------------------------------------------------
> Status|NEW |RESOLVED
> Resolution| |FIXED
> Target Milestone|--- |4.8.0
>
> --- Comment #6 from Andreas Schwab <sch...@linux-m68k.org> 2012-07-03
> 09:47:41 UTC ---
> Fixed in 4.8.
>
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