Re: libcpp: Improve search_line_sve2

> On 19 Aug 2026, at 23:09, Wilco Dijkstra <[email protected]> wrote:
> 
> 
> Improve search_line_sve2() - like the AdvSIMD version, loop until a match is
> found even if the input pointer is unaligned or close to the end.  This relies
> on CPP_BUFFER_PADDING >= 256 (maximum SVE vector length) and *end containing a
> match.  The resulting code is both simpler and faster.
> 
> Performance is ~1-2% better on a huge trace that replays the calls from libcpp.
> 
> Passes bootstrap, OK for commit?

Ok.
Thanks for improving it.
Kyrill

> 
> libcpp/ChangeLog:
> 
>        * lex.cc (search_line_sve): Improve.
> 
> ---
> 
> diff --git a/libcpp/lex.cc b/libcpp/lex.cc
> index bd35e28ce8f40a33e94f560b510107d49477d44d..047765fe733f028a663fe95c6bc206c9c4ee228f 100644
> --- a/libcpp/lex.cc
> +++ b/libcpp/lex.cc
> @@ -758,18 +758,19 @@ done:
>    into a byte index, in place of the bitmask the Neon version has to
>    build and move to a general register.
> 
> -   Unlike the Neon version this one needs neither alignment nor a
> -   page-crossing test.  Full-vector loads run while S is at or below
> -   END rounded down to a vector boundary, so they may extend a little
> -   past *END into the tail padding; a single predicated vector then
> -   covers any remainder.  The newline that _cpp_convert_input forces
> -   at *END still terminates the scan.
> +   Loop until a match is found even if the input pointer is unaligned
> +   or close to the end.  Since this may overread, it relies on *end
> +   containing a match and CPP_BUFFER_PADDING >= 256 (maximum SVE vector
> +   length).
> 
>    The loop consumes svcntb () bytes per iteration, so it scales with the
>    implemented vector length.  */
> 
> +/* Unaligned loads, potentially using padding after the final newline.  */
> +static_assert (CPP_BUFFER_PADDING >= 256, "");
> +
> static const uchar * __attribute__ ((target ("+sve2")))
> -search_line_sve2 (const uchar *s, const uchar *end)
> +search_line_sve2 (const uchar *s, const uchar *end ATTRIBUTE_UNUSED)
> {
>   /* Order within a segment is irrelevant to MATCH, which tests set
>      membership, so this needs no adjustment for big-endian.  */
> @@ -777,34 +778,20 @@ search_line_sve2 (const uchar *s, const uchar *end)
>  | ((uint32_t) '\\' << 16) | ((uint32_t) '?' << 24));
>   const svuint8_t needles = svreinterpret_u8_u32 (svdup_n_u32 (chars));
>   const svbool_t all = svptrue_b8 ();
> -  const uint64_t vl = svcntb ();
> -  svuint8_t data;
> -  svbool_t match;
> -  uintptr_t limit;
> -
> -  /* Unaligned loads, potentially using padding after the final newline.  */
> -  static_assert (CPP_BUFFER_PADDING >= 256, "");
> 
> -  /* Align END down to a vector boundary so the loop can consume on
> -     average half a vector more near the end.  */
> -  limit = (uintptr_t) end & -vl;
> +  svuint8_t data = svld1_u8 (all, s);
> +  svbool_t match = svmatch_u8 (all, data, needles);
> +  if (__builtin_expect (svptest_any (all, match), 1))
> +    return s + svcntp_b8 (all, svbrkb_b_z (all, match));
> 
> -  while ((uintptr_t) s <= limit)
> +  while (1)
>     {
> +      s += svcntb ();
>       data = svld1_u8 (all, s);
>       match = svmatch_u8 (all, data, needles);
> -      if (svptest_any (all, match))
> +      if (__builtin_expect (svptest_any (all, match), 1))
> return s + svcntp_b8 (all, svbrkb_b_z (all, match));
> -      s += vl;
>     }
> -
> -  /* What is left, up to and including *END, which _cpp_convert_input
> -     forces to a newline.  That guarantees a match, so no further test is
> -     needed.  */
> -  svbool_t pg = svwhilele_b8_u64 ((uintptr_t) s, (uintptr_t) end);
> -  data = svld1_u8 (pg, s);
> -  match = svmatch_u8 (pg, data, needles);
> -  return s + svcntp_b8 (pg, svbrkb_b_z (pg, match));
> }
> 
> static bool lexer_has_sve2;
>