[PATCH v3 4/4] kallsyms: Unroll 24-bit sequence reconstruction in get_symbol_seq()

[Jim Cromie]

kallsyms_seqs_of_names[] stores 3-byte big-endian sequence indices that
map alphabetical symbol positions to address-ordered symbol records.
Currently, get_symbol_seq() reconstructs each 24-bit integer using a
3-iteration for-loop that shifts and bitwise-ORs each byte sequentially.
During binary search in kallsyms_lookup_names() and duplicate boundary
scans, this loop introduces branch and loop overhead on the hot lookup
path.

Mark get_symbol_seq() as static inline and unroll the 3-byte extraction
into direct byte shifts: (p[0] << 16) | (p[1] << 8) | p[2]. This
eliminates loop induction variable maintenance and allows the compiler
to generate direct loads and constant shifts.

Signed-off-by: Jim Cromie <jim.cromie@xxxxxxxxx>
---
Changes in v3:
- Added as a standalone micro-optimization patch (addresses David
  Laight review).
---
 kernel/kallsyms.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 06c79ad70cfd..8f76403e0e7b 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -281,14 +281,11 @@ unsigned long kallsyms_sym_address(int idx)
 	return (unsigned long)offset_to_ptr(kallsyms_offsets + idx);
 }
 
-static unsigned int get_symbol_seq(int index)
+static inline unsigned int get_symbol_seq(int index)
 {
-	unsigned int i, seq = 0;
+	const u8 *p = &kallsyms_seqs_of_names[3 * index];
 
-	for (i = 0; i < 3; i++)
-		seq = (seq << 8) | kallsyms_seqs_of_names[3 * index + i];
-
-	return seq;
+	return (p[0] << 16) | (p[1] << 8) | p[2];
 }
 
 static int kallsyms_lookup_names(const char *name,

-- 
2.55.0