[PATCH 3/5] ihex: Simplify next record offset calculation

From: Andrey Smirnov
Date: Fri Dec 21 2018 - 02:29:04 EST


Next record calucaltion can be reduced to a much more tivial ALIGN
operation as follows:

1. Splitting 5 into 2 + 3 we get

next = ((be16_to_cpu(rec->len) + 2 + 3) & ~3) - 2 (1)

2. Using ALIGN macro we reduce (1) to:

ALIGN(be16_to_cpu(rec->len) + 2, 4) - 2 (2)

3. Subsituting 'next' in original next record calucation we get:

(void *)&rec->data[ALIGN(be16_to_cpu(rec->len) + 2, 4) - 2] (3)

4. Converting array index to pointer arithmetic we convert (3) into:

(void *)rec + sizeof(*rec) +
ALIGN(be16_to_cpu(rec->len) + 2, 4) - 2 (4)

5. Subsituting sizeof(*rec) with its value, 6, and substracting 2,
in (4) we get:

(void *)rec + ALIGN(be16_to_cpu(rec->len) + 2, 4) + 4 (5)

6. Since ALIGN(X, 4) + 4 == ALIGN(X + 4, 4), (5) can be converted to:

(void *)rec + ALIGN(be16_to_cpu(rec->len) + 6, 4) (6)

5. Subsituting 6 in (6) to sizeof(*rec) we get:

(void *)rec + ALIGN(be16_to_cpu(rec->len) + sizeof(*rec), 4) (7)

Using expression (7) should make it more clear that next record is
located by adding full size of the current record (payload + auxiliary
data) aligned to 4 bytes, to the location of the current one. No
functional change intended.

Cc: Chris Healy <cphealy@xxxxxxxxx>
Cc: Kyle McMartin <kyle@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: linux-kernel <linux-kernel@xxxxxxxxxxxxxxx>
Signed-off-by: Andrey Smirnov <andrew.smirnov@xxxxxxxxx>
---
include/linux/ihex.h | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/include/linux/ihex.h b/include/linux/ihex.h
index 9130f307a420..98cb5ce0b0a0 100644
--- a/include/linux/ihex.h
+++ b/include/linux/ihex.h
@@ -21,14 +21,18 @@ struct ihex_binrec {
uint8_t data[0];
} __attribute__((packed));

+static inline uint16_t ihex_binrec_size(const struct ihex_binrec *p)
+{
+ return be16_to_cpu(p->len) + sizeof(*p);
+}
+
/* Find the next record, taking into account the 4-byte alignment */
static inline const struct ihex_binrec *
__ihex_next_binrec(const struct ihex_binrec *rec)
{
- int next = ((be16_to_cpu(rec->len) + 5) & ~3) - 2;
- rec = (void *)&rec->data[next];
+ const void *p = rec;

- return rec;
+ return p + ALIGN(ihex_binrec_size(rec), 4);
}

static inline const struct ihex_binrec *
--
2.19.1