On 09/01/2023 14:58, Andrea Corallo via Gcc-patches wrote:
Andrea Corallo via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
Richard Earnshaw <richard.earns...@foss.arm.com> writes:
On 27/09/2022 16:24, Kyrylo Tkachov via Gcc-patches wrote:
-----Original Message-----
From: Andrea Corallo <andrea.cora...@arm.com>
Sent: Tuesday, September 27, 2022 11:06 AM
To: Kyrylo Tkachov <kyrylo.tkac...@arm.com>
Cc: Andrea Corallo via Gcc-patches <gcc-patches@gcc.gnu.org>; Richard
Earnshaw <richard.earns...@arm.com>; nd <n...@arm.com>
Subject: Re: [PATCH 9/15] arm: Set again stack pointer as CFA reg when
popping if necessary
Kyrylo Tkachov <kyrylo.tkac...@arm.com> writes:
Hi Andrea,
-----Original Message-----
From: Gcc-patches <gcc-patches-
bounces+kyrylo.tkachov=arm....@gcc.gnu.org> On Behalf Of Andrea
Corallo via Gcc-patches
Sent: Friday, August 12, 2022 4:34 PM
To: Andrea Corallo via Gcc-patches <gcc-patches@gcc.gnu.org>
Cc: Richard Earnshaw <richard.earns...@arm.com>; nd <n...@arm.com>
Subject: [PATCH 9/15] arm: Set again stack pointer as CFA reg when
popping
if necessary
Hi all,
this patch enables 'arm_emit_multi_reg_pop' to set again the stack
pointer as CFA reg when popping if this is necessary.
From what I can tell from similar functions this is correct, but could you
elaborate on why this change is needed for my understanding please?
Thanks,
Kyrill
Hi Kyrill,
sure, if the frame pointer was set, than it is the current CFA register.
If we request to adjust the current CFA register offset indicating it
being SP (while it's actually FP) that is indeed not correct and the
incoherence we will be detected by an assertion in the dwarf emission
machinery.
Thanks, the patch is ok
Kyrill
Best Regards
Andrea
Hmm, wait. Why would a multi-reg pop be updating the stack pointer?
Hi Richard,
not sure I understand, isn't any pop updating SP by definition?
Back on this,
compiling:
=======
int i;
void foo (int);
int bar()
{
foo (i);
return 0;
}
=======
With -march=armv8.1-m.main+fp -mbranch-protection=pac-ret+leaf -mthumb -O0 -g
Produces the following asm for bar.
bar:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 1, uses_anonymous_args = 0
pac ip, lr, sp
push {r3, r7, ip, lr}
add r7, sp, #0
ldr r3, .L3
ldr r3, [r3]
mov r0, r3
bl foo
movs r3, #0
mov r0, r3
pop {r3, r7, ip, lr}
aut ip, lr, sp
bx lr
The offending instruction causing the ICE (without this patch) when
emitting dwarf is "pop {r3, r7, ip, lr}".
The current CFA reg when emitting the multipop is R7 (the frame
pointer). If is not the multipop that has the duty to restore SP as
current CFA here which other instruction should do it?
Digging a bit deeper, I'm now even more confused. arm_expand_epilogue
contains (parphrasing the code):
if frame_pointer_needed
{
if arm
{}
else
{
if adjust
r7 += adjust
mov sp, r7 // Reset CFA to SP
}
}
so there should always be a move of r7 into SP, even if this is strictly
redundant. I don't understand why this doesn't happen for your
testcase. Can you dig a bit deeper? I wonder if we've (probably
incorrectly) assumed that this function doesn't need an epilogue but can
use a simple return? I don't think we should do that when
authentication is needed: a simple return should really be one instruction.
Best Regards
Andrea
R.
