[U-Boot] [PATCH] arm: arm64: only use general regs

Alexander Graf
2017-12-01T21:46:29Z

On 28.11.17 03:09, Peng Fan wrote:
> When compiling with android toolchain, there is an instruction
> "str q0, [x8],#16", but x8 is not 16bytes aligned,
> this instruction will trigger sync abort.
>
> So, following Linux kernel, only use general regs for arm64.
> If not, compiler may use simd registers Q[x]. We need to avoid
> using simd registers in U-Boot, because load/store Q[x] has
> restriction that 128bits aligned when str/ldr.
>
> Signed-off-by: Peng Fan <peng.fan@nxp.com>

The compiler should only output 16-byte-alignemnt-requiring instructions
when it can safely assume that the variable in question is 16 byte aligned.

Where did x8 come from? That was probably just an unsafe cast?

For reference, I ran into something similar recently on 32bit ARM:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82445

And this indeed turned out to be a compiler bug.

FWIW the main reason Linux doesn't want to use FPU registers in kernel
space is simply that it doesn't want to bother saving/restoring them on
syscalls or interrupts. But I don't quite see why we would care in U-Boot.

Alex