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.
With 9+ arguments they spill to see https://godbolt.org/z/Yj3ovd8bY
Where the compiler generates:
f9:
cmp w0, 0
ble .L2
sub sp, sp, #32
mov w7, w0
mov w6, w0
mov w5, w0
mov w4, w0
mov w3, w0
stp x29, x30, [sp, 16]
add x29, sp, 16
mov w2, w0
mov w1, w0
str w0, [sp]
bl f
.L2:
ret
A traceback from inside f() definitely needs to use LR-4
for the stack offset.
(arm64 doesn't seem to support -mno-sched-prolog).
I've failed to get different sized stack frames for the true/false
sides of the branch.
The compiler seems to pre-allocate the space for extra args rather
than using 'push' type instructions.
This was certainly better for some x86 cpu (p-pro?) but has now
gone out of fashion.
David
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