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
>
