On Thu, Aug 1, 2024 at 12:28 PM Peter Zijlstra [off-list ref] wrote:

On Wed, Jul 31, 2024 at 11:34:13PM +0200, Alice Ryhl wrote:

quoted

quoted

Please work harder to not have to duplicate stuff like this.

I really didn't want to duplicate it, but it's very hard to find a
performant alternative. Is there any way we could accept duplication
only in the cases where an 'i' parameter is used? I don't have the
choice of using a Rust helper for 'i' parameters.

Perhaps one option could be to put the Rust code inside jump_label.h
and have the header file evaluate to either C or Rust depending on the
value of some #ifdefs?

#ifndef RUST_ASM
/* existing C code goes here */
#endif
#ifdef RUST_ASM
// rust code goes here
#endif

That way the duplication is all in a single file. It would also avoid
the need for duplicating the nop5 string, as the Rust case is still
going through the C preprocessor and can use the existing #define.

I suppose that is slightly better, but ideally you generate the whole of
the Rust thing from the C version. After all, Clang can already parse
this.

That said, with the below patch, I think you should be able to reuse the
JUMP_TABLE_ENTRY macro like:

        JUMP_TABLE_ENTRY({0}, {1}, {2} + {3})

Yeah, I think this can work. I will submit a follow-up patch that
removes the duplication soon.

Alice