libgcc: Mostly vectorize CIE encoding extraction for FDEs
Checks
Commit Message
"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(-)
base-commit: de84a1e4b107b803ec3b064c3771a6ed8c0e201e
Comments
* Florian Weimer via Libc-alpha:
> "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.
Sorry, this should have to gcc-patches. Will resend there.
Thanks,
Florian
On Okt 17 2022, Florian Weimer via Libc-alpha wrote:
> + unsigned long long value = *(const unsigned long long *) &cie->version;
Is that strict-aliasing safe?
I guess you wanted to send that to gcc-patches@.
* Andreas Schwab:
> On Okt 17 2022, Florian Weimer via Libc-alpha wrote:
>
>> + unsigned long long value = *(const unsigned long long *) &cie->version;
>
> Is that strict-aliasing safe?
Should be, the data is never written to during the program's life-time.
libgcc doesn't use memcpy consistently for reading this constant data.
Instead it uses non-char types to read things.
> I guess you wanted to send that to gcc-patches@.
Right.
Thanks,
Florian
@@ -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;
+
+#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)
{