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include/linux/kernel.h: change abs() macro so it uses consistent return type

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Rewrite abs() so that its return type does not depend on the
architecture and no unexpected type conversion happen inside of it.  The
only conversion is from unsigned to signed type.  char is left as a
return type but treated as a signed type regradless of it's actual
signedness.

With the old version, int arguments were promoted to long and depending
on architecture a long argument might result in s64 or long return type
(which may or may not be the same).

This came after some back and forth with Nicolas.  The current macro has
different return type (for the same input type) depending on
architecture which might be midly iritating.

An alternative version would promote to int like so:

	#define abs(x)	__abs_choose_expr(x, long long,			\
			__abs_choose_expr(x, long,			\
			__builtin_choose_expr(				\
				sizeof(x) <= sizeof(int),		\
				({ int __x = (x); __x<0?-__x:__x; }),	\
				((void)0))))

I have no preference but imagine Linus might.  :] Nicolas argument against
is that promoting to int causes iconsistent behaviour:

	int main(void) {
		unsigned short a = 0, b = 1, c = a - b;
		unsigned short d = abs(a - b);
		unsigned short e = abs(c);
		printf("%u %u\n", d, e);  // prints: 1 65535
	}

Then again, no sane person expects consistent behaviour from C integer
arithmetic.  ;)

Note:

  __builtin_types_compatible_p(unsigned char, char) is always false, and
  __builtin_types_compatible_p(signed char, char) is also always false.

Signed-off-by: Michal Nazarewicz <[email protected]>
Reviewed-by: Nicolas Pitre <[email protected]>
Cc: Srinivas Pandruvada <[email protected]>
Cc: Wey-Yi Guy <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Michal Nazarewicz 2016-01-15 16:57:58 -08:00 • committed by Linus Torvalds
commit 8f57e4d930
3 changed files with 23 additions and 24 deletions
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9
drivers/iio/industrialio-core.c
View file

@ -433,16 +433,15 @@ ssize_t iio_format_value(char *buf, unsigned int type, int size, int *vals)
scale_db = true;
case IIO_VAL_INT_PLUS_MICRO:
if (vals[1] < 0)
return sprintf(buf, "-%ld.%06u%s\n", abs(vals[0]),
-vals[1],
scale_db ? " dB" : "");
return sprintf(buf, "-%d.%06u%s\n", abs(vals[0]),
-vals[1], scale_db ? " dB" : "");
else
return sprintf(buf, "%d.%06u%s\n", vals[0], vals[1],
scale_db ? " dB" : "");
case IIO_VAL_INT_PLUS_NANO:
if (vals[1] < 0)
return sprintf(buf, "-%ld.%09u\n", abs(vals[0]),
-vals[1]);
return sprintf(buf, "-%d.%09u\n", abs(vals[0]),
-vals[1]);
else
return sprintf(buf, "%d.%09u\n", vals[0], vals[1]);
case IIO_VAL_FRACTIONAL: