On 20/08/14 16:22, Wilco Dijkstra wrote:
Hi,
Various targets implement -momit-leaf-frame-pointer to avoid using a frame
pointer in leaf
functions. Currently the GCC mid-end does not provide a way of doing this, so
targets have resorted
to hacks. Typically this involves forcing flag_omit_frame_pointer to be true in
the
<arch>_option_override callback. The issue is that this doesn't work as it
modifies the actual
option variable. As a result the callback is not idempotent, so option
save/restore when using
function attributes fail as the callback is called multiple times on the
modified options. Note this
bug exists on all targets which override options in <arch>_option_override (and
despite claims to
the contrary in BZ 60580 this bug exists on all targets that implement
-fomit-leaf-frame-pointer).
agree, current gcc don't support finer control of frame pointer.
currently all three targets i386/aarch64/bfin want finer control
of frame pointer for leaf function have bug.
for example, for a simple testcase:
__attribute__ ((optimize("no-omit-frame-pointer")))
int
cal (int a)
{
int b = a + 0x200;
foo(&b);
return a + b + 1;
}
__attribute__ ((optimize("omit-frame-pointer")))
int
cal1 (int a)
{
int b = a + 0x200;
foo(&b);
return a + b + 1;
}
./cc1-bfin -O0 hello.c -momit-leaf-frame-pointer
the attribute for "cal1" doesn't work.
2. Change the mid-end to call <arch>_frame_pointer_required even when
!flag_omit_frame_pointer. This
is a generic solution which allows targets to decide when exactly to optimize
frame pointers.
However it does mean all implementations of <arch>_frame_pointer_required must
be updated (the
trivial safe fix is to add "if (!flag_omit_frame_pointer) return true;" at the
start).
IMHO, this fix make sense, it will let the calculation for
frame_pointer_needed to be more
flexible.
remove the "! flag_omit_frame_pointer" when initialize frame_pointer_needed,
and let frame_pointer_required
hook to check flag_omit_frame_pointer and any other target frame pointer
control flags like omit-leaf-frame-pointer
etc to decide whether frame pointer needed.
frame_pointer_needed
= (! flag_omit_frame_pointer
|| (cfun->calls_alloca && EXIT_IGNORE_STACK)
/* We need the frame pointer to catch stack overflow exceptions
....
any comments?
thanks.
-- Jiong
