Re: Cost of handling misaligned access

Anton Ertl <anton@mips.complang.tuwien.ac.at> schrieb:

BGB <cr88192@gmail.com> writes:

On 2/3/2025 12:55 AM, Anton Ertl wrote:
Rather, have something like an explicit "__unaligned" keyword or
similar, and then use the runtime call for these pointers.

>
There are people who think that it is ok to compile *p to anything if
p is not aligned, even on architectures that support unaligned
accesses.  At least one of those people recommended the use of
memcpy(..., ..., sizeof(...)).  Let's see what gcc produces on
rv64gc (where unaligned accesses are guaranteed to work):
>
[fedora-starfive:/tmp:111378] cat x.c
#include <string.h>
>
long uload(long *p)
{
  long x;
  memcpy(&x,p,sizeof(long));
  return x;
}
[fedora-starfive:/tmp:111379] gcc -O -S x.c
[fedora-starfive:/tmp:111380] cat x.s
        .file   "x.c"
        .option nopic
        .text
        .align  1
        .globl  uload
        .type   uload, @function
uload:
        addi    sp,sp,-16
        lbu     t1,0(a0)

[...]

With RISC-V, nobody ever knows what architecture he is compiling for...

Did you tell gcc specifically that unsigned access was supported in
the architecture you were using?