RISC-V double float alignment

Hesham Almatary hesham.almatary at cl.cam.ac.uk
Fri Feb 1 16:37:31 UTC 2019

On Fri, 1 Feb 2019 at 15:08, Jiri Gaisler <jiri at gaisler.se> wrote:
> Hello,
> As far as I understand, RISC-V does not require any particular alignment
> of data structures in memory. Nevertheless, the compiler automatically
> aligns data structures on their natural sizes, i.e. ints are aligned on
> 4-byte and doubles on 8-bytes boundaries. sis-riscv with support for
> doubles (extension D) checks that load and store double are properly
> aligned on 8-byte boundaries. This works fine for all compiler-generated
> code, (e.g. paranoia) but fails for spcontext01, where the context save
> function in riscv-exception-handler.S tries to save 64-bit floating
> point registers on a non-aligned stack address.
> Is this an oversight in the code, or does the hardware (and simulators)
> have to support unaligned accesses?
Spike can be built to support either aligned or unaligned accesses. This
includes floating points and integers. It's not in the spec that a RISC-V
implementation has to support it or not (it's up to the platform),
but if it doesn't, it should trigger an unaligned access trap and
should be handled.

That said, I am not sure whether the intention of the RISC-V FPU
implementation in RTEMS is assuming everything will be aligned
(in which case some alignment checks should be added) or not.

I had another a look at the RISC-V Context_Control struct. I assume this only
happens on rv32 with D extension?

> Jiri.
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