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.
>
> 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
Don't show me Arm64 assembly when we're discussing Arm32.
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