[PATCH 1/2] riscv: Fix memmove and optimise memcpy when misalign

[Leo Liang]

On Thu, May 13, 2021 at 04:46:17PM +0800, Bin Meng wrote:
> At present U-Boot SPL fails to boot on SiFive Unleashed board, due
> to a load address misaligned exception happens when loading the FIT
> image in spl_load_simple_fit(). The exception happens in memmove()
> which is called by fdt_splice_().
> 
> Commit 8f0dc4cfd106 introduces an assembly version of memmove but
> it does take misalignment into account (it checks if length is a
> multiple of machine word size but pointers need also be aligned).
> As a result it will generate misaligned load/store for the majority
> of cases and causes significant performance regression on hardware
> that traps misaligned load/store and emulate them using firmware.
> 
> The current behaviour of memcpy is that it checks if both src and
> dest pointers are co-aligned (aka congruent modular SZ_REG). If
> aligned, it will copy data word-by-word after first aligning
> pointers to word boundary. If src and dst are not co-aligned,
> however, byte-wise copy will be performed.
> 
> This patch was taken from the Linux kernel patch [1], which has not
> been applied at the time being. It fixes the memmove and optimises
> memcpy for misaligned cases. It will first align destination pointer
> to word-boundary regardless whether src and dest are co-aligned or
> not. If they indeed are, then wordwise copy is performed. If they
> are not co-aligned, then it will load two adjacent words from src
> and use shifts to assemble a full machine word. Some additional
> assembly level micro-optimisation is also performed to ensure more
> instructions can be compressed (e.g. prefer a0 to t6).
> 
> With this patch, U-Boot boots again on SiFive Unleashed board.
> 
> [1] https://patchwork.kernel.org/project/linux-riscv/patch/20210216225555.4976-1-gary@garyguo.net/
> 
> Fixes: 8f0dc4cfd106 ("riscv: assembler versions of memcpy, memmove, memset")
> Signed-off-by: Bin Meng <bmeng.cn at gmail.com>
> ---

Reviewed-by: Leo Yu-Chi Liang <ycliang at cs.nctu.edu.tw>