Hi Jeff,
I may be missing something, but isn't it the transition to an FP
relative address rather than a SP relative address that's the problem
here?
Yes, I believe so.
Where does that happen?
I think that it happens in dwarf2out.c:based_loc_descr() which
detects the use of the frame pointer and works out that it is going
to be eliminated to the stack pointer:
/* We only use "frame base" when we're sure we're talking about the
post-prologue local stack frame. We do this by *not* running
register elimination until this point, and recognizing the special
argument pointer and soft frame pointer rtx's. */
if (reg == arg_pointer_rtx || reg == frame_pointer_rtx)
{
rtx elim = (ira_use_lra_p
? lra_eliminate_regs (reg, VOIDmode, NULL_RTX)
: eliminate_regs (reg, VOIDmode, NULL_RTX));
if (elim != reg)
.....
The problem, I believe, is that based_loc_descr() is only called
from mem_loc_descriptor when the mode of the rtl concerned is an
MODE_INT. For example:
case REG:
if (GET_MODE_CLASS (mode) != MODE_INT
[...]
else
if (REGNO (rtl) < FIRST_PSEUDO_REGISTER)
mem_loc_result = based_loc_descr (rtl, 0, VAR_INIT_STATUS_INITIALIZED);
or, (this is another one that I found whilst investigating this
problem further):
case PLUS:
plus:
if (is_based_loc (rtl)
&& (GET_MODE_SIZE (mode) <= DWARF2_ADDR_SIZE
|| XEXP (rtl, 0) == arg_pointer_rtx
|| XEXP (rtl, 0) == frame_pointer_rtx)
&& GET_MODE_CLASS (mode) == MODE_INT)
mem_loc_result = based_loc_descr (XEXP (rtl, 0),
INTVAL (XEXP (rtl, 1)),
VAR_INIT_STATUS_INITIALIZED);
else
There are quite a few places in mem_loc_descriptor where the code checks
for the mode being in the MODE_INT class. I am not exactly sure why. I
think that it might be that the programmer thought that any expression that
does not involve integer based arithmetic cannot be expressed in DWARF CFA
notation and so would have to be handled specially. If I am correct,
then it seems to me that the proper fix would be to use SCALAR_INT_MODE_P
instead.
I tried out the extended patch (attached) and it gave even better GDB
results for the MSP430 and still no regressions (GCC or GDB) for MSP430 or
x86_64.
Is this enough justification ?