On 7/14/22 19:02, Linus Torvalds wrote:
And guess what? The code could just use roundup_pow_of_two(), which is
designed exactly like ilog2() to be used for compile-time constant
values.
So the code should just use
#define FASTOP_SIZE roundup_pow_of_two(FASTOP_LENGTH)
and be a lot more legible, wouldn't it?
Because I don't think there is anything magical about the length
"8/16/32". It's purely "aligned and big enough to contain
FASTOP_LENGTH".
roundup_pow_of_two unfortunately is not enough for stringizing
FASTOP_SIZE into an asm statement. :(
#define __FOP_FUNC(name) \
".align " __stringify(FASTOP_SIZE) " \n\t" \
".type " name ", @function \n\t" \
name ":\n\t" \
ASM_ENDBR
The shifts are what we came up with for the SETCC thunks when ENDBR and
SLS made them grew beyond 4 bytes; Peter's patch is reusing the trick
for the fastop thunks.
Because I don't think there is anything magical about the length
"8/16/32". It's purely "aligned and big enough to contain
FASTOP_LENGTH".
I agree with that, it's only limited to 8/16/32 to keep the macro to a
decent size.
And then the point of that
static_assert(FASTOP_LENGTH <= FASTOP_SIZE);
just goes away, because there are no subtle math issues there any more.
In fact, the remaining question is just "where did the 7 come from" in
#define FASTOP_LENGTH (7 + ENDBR_INSN_SIZE + RET_LENGTH)
The 7 is an upper limit to the length of the code between endbr and ret.
There's no particular reason to limit to 7, but it allows using an
alignment of 8 in the smallest case (no thunks, no SLS, no endbr) where
you just have ".align 8; ...; ret".
Paolo
