Re: [PATCH 4/8] add functions for memory-efficient bitmaps
From: Jeff King <hidden>
Date: 2016-06-15 23:01:45
Subsystem:
the rest · Maintainer:
Linus Torvalds
On Thu, Jun 26, 2014 at 05:15:05AM +0200, Torsten Bögershausen wrote:
quoted
+ */ +static inline int bitset_sizeof(int num_bits) +{ + return (num_bits + CHAR_BIT - 1) / CHAR_BIT; +}Just a general question about the usage of "int" here (and at other places): Is there a special reason for new code to allow num_bits to be negative ?
No. I usually choose "int" when the word size is not likely to matter
(i.e., we do not expect it to overflow a 32-bit integer, nor to have so
many that I need to be careful not to waste space).
Probably "unsigned int" would be a more descriptive choice.
It may also help the compiler optimize better. Assuming CHAR_BIT is 8
(i.e., most everywhere), we get:
(num_bits + 7) / 8
Presumably the compiler implements the division with a right-shift.
Marking num_bits as unsigned should let us do just a logical shift,
without worrying about the sign. And indeed, here are the signed and
unsigned versions produced by "gcc -S -O2" (for an equivalent
non-inlined function):
[signed]
leal 14(%rdi), %edx
movl %edi, %eax
addl $7, %eax
cmovs %edx, %eax
sarl $3, %eax
ret
[unsigned]
leal 7(%rdi), %eax
shrl $3, %eax
ret
Much simpler, though see below for practical considerations.
To my knowledge all the size_t definitions these days are positive, because a size can not be negative.
size_t is perhaps a reasonable choice for the return value, given the name "sizeof". But if you really care about using the whole range of bits there, you need a data type for num_bits that is CHAR_BIT times larger.
Should we use "unsigned" here ? or "unsigned int" ?
Yes, I think so. Both are the same to the compiler. I have a vague recollection that we prefer one over the other, but grepping seems to find many examples of each in our code. I'm squashing in the patch below. I couldn't measure any speed improvement. I'm guessing because the functions are all inlined, which means we likely get away with calculating bitset_sizeof once outside of our loop. I think the result is still more obvious to read, though. -Peff ---
diff --git a/bitset.h b/bitset.h
index 5fbc956..268545b 100644
--- a/bitset.h
+++ b/bitset.h@@ -45,7 +45,7 @@ * * bits = xcalloc(1, bitset_sizeof(nr)); */ -static inline int bitset_sizeof(int num_bits) +static inline unsigned bitset_sizeof(unsigned num_bits) { return (num_bits + CHAR_BIT - 1) / CHAR_BIT; }
@@ -54,7 +54,7 @@ static inline int bitset_sizeof(int num_bits) * Set the bit at position "n". "n" is counted from zero, and must be * smaller than the num_bits given to bitset_sizeof when allocating the bitset. */ -static inline void bitset_set(unsigned char *bits, int n) +static inline void bitset_set(unsigned char *bits, unsigned n) { bits[n / CHAR_BIT] |= 1 << (n % CHAR_BIT); }
@@ -62,7 +62,7 @@ static inline void bitset_set(unsigned char *bits, int n) /* * Return the bit at position "n" (see bitset_set for a description of "n"). */ -static inline int bitset_get(unsigned char *bits, int n) +static inline unsigned bitset_get(unsigned char *bits, unsigned n) { return !!(bits[n / CHAR_BIT] & (1 << (n % CHAR_BIT))); }
@@ -75,9 +75,10 @@ static inline int bitset_get(unsigned char *bits, int n) * "max" (we assume any bits beyond "max" up to the next CHAR_BIT boundary are * zeroed padding). */ -static inline int bitset_equal(unsigned char *a, unsigned char *b, int max) +static inline int bitset_equal(unsigned char *a, unsigned char *b, + unsigned max) { - int i; + unsigned i; for (i = bitset_sizeof(max); i > 0; i--) if (*a++ != *b++) return 0;
@@ -89,9 +90,10 @@ static inline int bitset_equal(unsigned char *a, unsigned char *b, int max) * * See bitset_equal for the definition of "max". */ -static inline void bitset_or(unsigned char *dst, const unsigned char *src, int max) +static inline void bitset_or(unsigned char *dst, const unsigned char *src, + unsigned max) { - int i; + unsigned i; for (i = bitset_sizeof(max); i > 0; i--) *dst++ |= *src++; }
@@ -101,9 +103,9 @@ static inline void bitset_or(unsigned char *dst, const unsigned char *src, int m * * See bitset_equal for the definition of "max". */ -static inline int bitset_empty(const unsigned char *bits, int max) +static inline int bitset_empty(const unsigned char *bits, unsigned max) { - int i; + unsigned i; for (i = bitset_sizeof(max); i > 0; i--, bits++) if (*bits) return 0;