Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG

Sami Tolvanen
2022-05-17T20:36:51Z

On Mon, May 16, 2022 at 11:44 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, May 16, 2022 at 03:03:02PM -0700, Sami Tolvanen wrote:
> > On Mon, May 16, 2022 at 2:44 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Mon, May 16, 2022 at 09:32:55PM +0000, David Laight wrote:
> > >
> > > > > The compiler always generates this specific instruction sequence.
> > > >
> > > > Yes, but there are several ways to encode 'cmpl imm,-6(reg)'.
> > >
> > > Yes, but we don't care. This *always* uses the 32bit immediate form.
> > > Even if the immediate is small.
> >
> > Yes, that part is not a problem, but it's a valid point that LLVM
> > might not always use r8-r15 here, so I will have to check for the REX
> > prefix before blindly attempting to decode the instruction.
>
> LLVM has always used r11 for indirect calls, will that change?

No, this won't change register allocation, but I will have to ensure
that the compiler won't do any unnecessary register shuffling for the
check itself.

Sami