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[PATCH] libdw: pre-compute leb128 loop limits
- From: Josh Stone <jistone at redhat dot com>
- To: elfutils-devel at lists dot fedorahosted dot org
- Date: Mon, 15 Dec 2014 12:18:25 -0800
- Subject: [PATCH] libdw: pre-compute leb128 loop limits
Signed-off-by: Josh Stone <jistone@redhat.com>
---
On Fedora 21, this appears to be slightly faster, although pretty close
to noise levels. Mark, can you see if this helps the performance slip
on your el7 system?
---
libdw/ChangeLog | 6 ++++++
libdw/memory-access.h | 17 +++++++++++++++--
2 files changed, 21 insertions(+), 2 deletions(-)
diff --git a/libdw/ChangeLog b/libdw/ChangeLog
index 2aa878bd8ad7..17b4db980278 100644
--- a/libdw/ChangeLog
+++ b/libdw/ChangeLog
@@ -1,3 +1,9 @@
+2014-12-15 Josh Stone <jistone@redhat.com>
+
+ * memory-access.h (__libdw_max_len_leb128): New.
+ (__libdw_get_uleb128): Use __libdw_max_len_leb128.
+ (__libdw_get_sleb128): Likewise.
+
2014-12-15 Mark Wielaard <mjw@redhat.com>
* dwarf_getpubnames.c (get_offsets): Make sure whole unit fall inside
diff --git a/libdw/memory-access.h b/libdw/memory-access.h
index 8226d00e9b33..99c827af22bd 100644
--- a/libdw/memory-access.h
+++ b/libdw/memory-access.h
@@ -39,6 +39,14 @@
#define len_leb128(var) ((8 * sizeof (var) + 6) / 7)
+static inline size_t
+__libdw_max_len_leb128 (const unsigned char *addr, const unsigned char *end)
+{
+ const size_t type_len = len_leb128 (uint64_t);
+ const size_t pointer_len = likely (addr < end) ? end - addr : 0;
+ return likely (type_len <= pointer_len) ? type_len : pointer_len;
+}
+
#define get_uleb128_step(var, addr, nth) \
do { \
unsigned char __b = *(addr)++; \
@@ -51,10 +59,13 @@ static inline uint64_t
__libdw_get_uleb128 (const unsigned char **addrp, const unsigned char *end)
{
uint64_t acc = 0;
+
/* Unroll the first step to help the compiler optimize
for the common single-byte case. */
get_uleb128_step (acc, *addrp, 0);
- for (unsigned int i = 1; i < len_leb128 (acc) && *addrp < end; ++i)
+
+ const size_t max = __libdw_max_len_leb128 (*addrp - 1, end);
+ for (size_t i = 1; i < max; ++i)
get_uleb128_step (acc, *addrp, i);
/* Other implementations set VALUE to UINT_MAX in this
case. So we better do this as well. */
@@ -82,8 +93,10 @@ static inline int64_t
__libdw_get_sleb128 (const unsigned char **addrp, const unsigned char *end)
{
int64_t acc = 0;
+
/* Unrolling 0 like uleb128 didn't prove to benefit optimization. */
- for (unsigned int i = 0; i < len_leb128 (acc) && *addrp < end; ++i)
+ const size_t max = __libdw_max_len_leb128 (*addrp, end);
+ for (size_t i = 0; i < max; ++i)
get_sleb128_step (acc, *addrp, i);
/* Other implementations set VALUE to INT_MAX in this
case. So we better do this as well. */
--
2.1.0