[PATCH v2] ARM: unwind: improve unwinders for noreturn case

[David Laight]

From: Russell King
> Sent: 21 March 2024 14:56
> 
> On Thu, Mar 21, 2024 at 02:37:28PM +0000, David Laight wrote:
> > From: Russell King
> > > Sent: 21 March 2024 13:08
> > >
> > > On Thu, Mar 21, 2024 at 12:57:07PM +0000, David Laight wrote:
> > > > From: Russell King
> > > > > Sent: 21 March 2024 12:23
> > > > ...
> > > > > > That might mean you can get the BL in the middle of a function
> > > > > > but where the following instruction is for the 'no stack frame'
> > > > > > side of the branch.
> > > > > > That is very likely to break any stack offset calculations.
> > > > >
> > > > > No it can't. At any one point in the function, the stack has to be in
> > > > > a well defined state, so that access to local variables can work, and
> > > > > also the stack can be correctly unwound. If there exists a point in
> > > > > the function body which can be reached where the stack could be in two
> > > > > different states, then the stack can't be restored to the parent
> > > > > context.
> > > >
> > > > Actually you can get there with a function that has a lot of args.
> > > > So you can have:
> > > > 	if (...) {
> > > > 		push x
> > > > 		bl func
> > > > 		add %sp, #8
> > > > 	}
> > > > 	code;
> > > > which is fine.
> > >
> > > No you can't.... and that isn't even Arm code. Arm doesn't use %sp.
> > > Moreover, that "bl" will stomp over the link register, meaning this
> > > function can not return.
> >
...
> 
> Don't show me Arm64 assembly when we're discussing Arm32.

Oops - I'd assumed no one did 32bit :-)
In any case it is much the same, see https://godbolt.org/z/7dcbKrs76

f4:
        push    {r3, lr}
        subs    r3, r0, #0
        ble     .L2
        mov     r2, r3
        mov     r1, r3
        bl      f
.L2:
        pop     {r3, pc}

f5:
        subs    r3, r0, #0
        ble     .L6
        push    {lr}
        sub     sp, sp, #12
        mov     r2, r3
        mov     r1, r3
        str     r3, [sp]
        bl      f
.L6:
        bx      lr

That is with -mno-sched-prolog but with 5+ args they spill to stack
and the %sp change is pulled into the conditional.

It does look like %lr is being saved (and for arm64 I think).

	David

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