Re: [PATCH] libcpp: add an AArch64 SVE2 search_line_fast

Hi Wilco,

> On 7 Aug 2026, at 14:10, Wilco Dijkstra <[email protected]> wrote:
> 
> Hi Kyrill,
> 
>> The AArch64 helper that finds the next '\n', '\r', '\\' or '?' scans a
>> fixed 16 bytes per iteration and spends more than half of its per-call
>> cost outside the loop, turning a vector comparison into a byte index.
>> Each iteration needs four CMEQs, three ORRs and an ADDP/FMOV pair to get
>> the comparison into a general register for the loop branch, and the exit
>> path builds a 16-bit mask with AND/UADDLP/USHL/ADDV/FMOV before it can
>> count trailing zeros.  Both sequences also transfer from SIMD to GP
>> regs, which can be expensive.
> 
> This sounds like an ancient sequence - we know how to do this fast by
> using SHRN or ADDHN to narrow the mask so that it can both be tested
> and immediately used in CTZ to get the bit position. We should fix this in
> another patch.
> 
> From a high level perspective, I'm wondering why SVE2 would use a pagecross
> check when you have a start and end pointer? The aligning and the pagecross
> check add extra overhead, so looping without aligning would be faster if we
> check that there is at least 1 whole vector left to do (then only the last partial
> vector needs special treatment).
> 
> Alternatively, if a pagecross check is useful (eg. if input buffer is frequently small)
> then it's better to process at least 2 and possibly 4 vectors before either aligning or
> falling back to the above loop.
> 
> It would be good to list the statistics that resulted in this design. Lines are short on
> average, but there are also lots of lines that need 4 or more vectors.
> 
> As for dispatch, I'm assuming HAVE_SVE2 would be true in almost all cases, so
> we get overhead from the indirect call (and no more inlining) in all cases. I think it
> would be better if we wrote:
> 
> static bool has_sve2;
> 
> static const uchar *
> search_line_fast (const uchar *s, const uchar *end ATTRIBUTE_UNUSED)
> {
>  if (has_sve2)
>    return search_line_sve2 (s, end);
>  // Neon version
> }
> 
> Although this likely can't inline the SVE2 version, the dispatch is very cheap and the
> Neon version can still be inlined as before - so you either get existing performance or
> a speedup from SVE2.

Thanks for having a look!
Indeed the page cross checks were not necessary, I overengineered it by looking at the old Neon version.
The new version as I think you described it is simpler and a bit faster (90% gain vs 80% gain). I’ve sent a v2 with your suggestions incorporated.
Kyrill

> 
> Cheers,
> Wilco