Re: [PATCH] libgcc: Mostly vectorize CIE encoding extraction for FDEs

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Oct 17, 2022 at 3:01 PM Florian Weimer via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> "zR" and "zPLR" are the most common augmentations.  Use a simple
> SIMD-with-in-a-register technique to check for both augmentations,
> and that the following variable-length integers have length 1, to
> get more quickly at the encoding field.
>
> libgcc/
>
>         * unwind-dw2-fde.c (get_cie_encoding_slow): Rename from
>         get_cie_encoding.  Mark as noinline.
>         (get_cie_encoding): Add fast path for "zR" and "zPLR"
>         augmentations.  Call get_cie_encoding_slow as a fall-back.
>
> ---
>  libgcc/unwind-dw2-fde.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 59 insertions(+), 2 deletions(-)
>
> diff --git a/libgcc/unwind-dw2-fde.c b/libgcc/unwind-dw2-fde.c
> index 3c0cc654ec0..4e3a54c5a1a 100644
> --- a/libgcc/unwind-dw2-fde.c
> +++ b/libgcc/unwind-dw2-fde.c
> @@ -333,8 +333,10 @@ base_from_object (unsigned char encoding, const struct object *ob)
>  /* Return the FDE pointer encoding from the CIE.  */
>  /* ??? This is a subset of extract_cie_info from unwind-dw2.c.  */
>
> -static int
> -get_cie_encoding (const struct dwarf_cie *cie)
> +/* Disable inlining because the function is only used as a slow path in
> +   get_cie_encoding below.  */
> +static int __attribute__ ((noinline))
> +get_cie_encoding_slow (const struct dwarf_cie *cie)
>  {
>    const unsigned char *aug, *p;
>    _Unwind_Ptr dummy;
> @@ -389,6 +391,61 @@ get_cie_encoding (const struct dwarf_cie *cie)
>      }
>  }
>
> +static inline int
> +get_cie_encoding (const struct dwarf_cie *cie)
> +{
> +  /* Fast path for some augmentations and single-byte variable-length
> +     integers.  Do this only for targets that align struct dwarf_cie to 8
> +     bytes, which ensures that at least 8 bytes are available starting at
> +     cie->version.  */
> +#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__ \
> +  || __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> +  if (__alignof (*cie) == 8 && sizeof (unsigned long long) == 8)
> +    {
> +      unsigned long long value = *(const unsigned long long *) &cie->version;

TBAA?  Maybe use

           unsigned long long value;
           memcpy (&value, &cie->version, 8);

instead?

> +
> +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> +#define C(x) __builtin_bswap64 (x)
> +#else
> +#define C(x) x
> +#endif
> +
> +      /* Fast path for "zR".  Check for version 1, the "zR" string and that
> +        the sleb128/uleb128 values are single bytes.  In the comments
> +        below, '1', 'c', 'd', 'r', 'l' are version, code alignment, data
> +        alignment, return address column, augmentation length.  Note that
> +        with CIE version 1, the return address column is byte-encoded.  */
> +      unsigned long long expected =
> +       /*   1 z R 0 c d r l.  */
> +       C (0x017a520000000000ULL);
> +      unsigned long long mask =
> +       /*   1 z R 0 c d r l.  */
> +       C (0xffffffff80800080ULL);
> +
> +      if ((value & mask) == expected)
> +       return cie->augmentation[7];
> +
> +      /* Fast path for "zPLR".  */
> +      expected =
> +       /*   1 z P L R 0 c d.  */
> +       C (0x017a504c52000000ULL);
> +      mask =
> +       /*   1 z P L R 0 c d.  */
> +       C (0xffffffffffff8080ULL);
> +#undef C
> +
> +      /* Validate the augmentation length, and return the enconding after
> +        it.  No check for the return address column because it is
> +        byte-encoded with CIE version 1.  */
> +      if (__builtin_expect ((value & mask) == expected
> +                           && (cie->augmentation[8] & 0x80) == 0, 1))
> +         return cie->augmentation[9];
> +    }
> +#endif
> +
> +  return get_cie_encoding_slow (cie);
> +}
> +
>  static inline int
>  get_fde_encoding (const struct dwarf_fde *f)
>  {
>
> base-commit: de84a1e4b107b803ec3b064c3771a6ed8c0e201e
>