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add new macro BIT_S to include/linux/kernel.h

PATCH In review Michael Veeck 3 participants 5 messages Aug 13, 2005 – Aug 25, 2005 Kernel janitors

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Michael Veeck

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#0 lore raw

PATCH 1/1 add new macro BIT_S to include/linux/kernel.h

description:
adds a new macro for doing a bitshift. right now every subsystem defines its own macro (which all look the same) but this one also does overflow checking like it is already done in include/linux/input.h

patch is against latest git-tree from Linus

Signed-off-by Michael Veeck <[email protected]>
include/linux/kernel.h +5 -0
@@ -30,6 +30,11 @@ extern const char linux_banner[];
 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
 #define ALIGN(x,a) (((x)+(a)-1)&~((a)-1))
 
+/*
+ * BIT macro that also handles a possible overflow
+ */
+#define BITS(x) (1UL << ((x) % BITS_PER_LONG))
+
 #define	KERN_EMERG	"<0>"	/* system is unusable			*/
 #define	KERN_ALERT	"<1>"	/* action must be taken immediately	*/
 #define	KERN_CRIT	"<2>"	/* critical conditions			*/

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#1 lore raw
Quoted 3 lines · Michael Veeck

adds a new macro for doing a bitshift. right now every subsystem defines its own macro (which all look the same) but this one also does overflow checking

Checking?

$ cat -n test.c
     1	#define BIT(x) (1ULL << (x))
     2	#define BITS(x) (1ULL << ((x) % 64))
     3	
     4	int main(void)
     5	{
     6		BIT(65);
     7		BITS(65);
     8		return 0;
     9	}
$ gcc -o test test.c
test.c: In function `main':
test.c:6: warning: left shift count >= width of type
	<=== ???

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#2 lore raw
In reply to Michael Veeck
Quoted 9 lines · Alexey Dobriyan
Quoted 3 lines · Michael Veeck

adds a new macro for doing a bitshift. right now every subsystem defines its own macro (which all look the same) but this one also does overflow checking

Checking?

Quoted 15 lines
$ cat -n test.c
     1	#define BIT(x) (1ULL << (x))
     2	#define BITS(x) (1ULL << ((x) % 64))
     3	
     4	int main(void)
     5	{
     6		BIT(65);
     7		BITS(65);
     8		return 0;
     9	}
$ gcc -o test test.c
test.c: In function `main':
test.c:6: warning: left shift count >= width of type
	<== ???

okay, good point, but does that work even when the parameter of BIT isnt known at compile time?
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#3 lore raw
In reply to Michael Veeck
Quoted 26 lines · Michael Veeck
Quoted 9 lines · Alexey Dobriyan
Quoted 3 lines · Michael Veeck

adds a new macro for doing a bitshift. right now every subsystem defines its own macro (which all look the same) but this one also does overflow checking

Checking?

Quoted 15 lines
$ cat -n test.c
    1	#define BIT(x) (1ULL << (x))
    2	#define BITS(x) (1ULL << ((x) % 64))
    3	
    4	int main(void)
    5	{
    6		BIT(65);
    7		BITS(65);
    8		return 0;
    9	}
$ gcc -o test test.c
test.c: In function `main':
test.c:6: warning: left shift count >= width of type
	<=== ???
Quoted 2 lines

okay, good point, but does that work even when the parameter of BIT isnt known at compile time?

How about something like:
#define BITS(x) (1ULL << (__builtin_constant_p(x) ? (x) : (x) % 64)

Or maybe a WARN_ON would be nice, since (x >= 64) probably means there's a bug.

Domen

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#4 lore raw
In reply to Michael Veeck
Quoted 34 lines · Domen Puncer
Quoted 28 lines · Michael Veeck
Quoted 10 lines · Alexey Dobriyan
Quoted 3 lines · Michael Veeck

adds a new macro for doing a bitshift. right now every subsystem defines its own macro (which all look the same) but this one also does overflow checking

Checking?

Quoted 15 lines
$ cat -n test.c
   1	#define BIT(x) (1ULL << (x))
   2	#define BITS(x) (1ULL << ((x) % 64))
   3	
   4	int main(void)
   5	{
   6		BIT(65);
   7		BITS(65);
   8		return 0;
   9	}
$ gcc -o test test.c
test.c: In function `main':
test.c:6: warning: left shift count >= width of type
	<== ???
Quoted 2 lines

okay, good point, but does that work even when the parameter of BIT isnt known at compile time?

Quoted 8 lines

How about something like:
#define BITS(x) (1ULL << (__builtin_constant_p(x) ? (x) : (x) % 64)

Or maybe a WARN_ON would be nice, since (x >= 64) probably means there's a bug.

Domen

I would put your warning around BIT. When it is used wrongly (x to high) then it really seems to be a bug. Since most definitions use this, it maybe unravels one or two bugs.

BIT_S does not check, that might be misleading word, but rather do a wrap. Everybody who will use BIT_S should know that. Maybe BIT_WRAP would be a better word for the macro then.

Veeck

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