Introduce new macros min_lt and max_lt for comparing with larger type

A useful use case for min_t and max_t is comparing two values with larger type. For example comparing an u64 and an u32, usually we do not want to truncate the u64, so we need use min_t or max_t with u64.

To simplify the usage introducing two more macros min_lt and max_lt, 'lt' means larger type.

Signed-off-by Dave Young <[email protected]>

---
 include/linux/kernel.h |   13 +++++++++++++
 1 file changed, 13 insertions(+)

include/linux/kernel.h +6 -0

@@ -798,6 +798,19 @@ static inline void ftrace_dump(enum ftra
        type __max2 = (y);                      \
        __max1 > __max2 ? __max1: __max2; })
 
+/*
+ * use type of larger size in min_lt and max_lt
+ */
+#define min_lt(x, y) ({                        \
+       int sx = sizeof(typeof(x));             \
+       int sy = sizeof(typeof(y));             \

Quoted 26 lines · On Fri, Mar 11, 2016 at 2:21 PM, <[email protected]> wrote:

A useful use case for min_t and max_t is comparing two values with larger type. For example comparing an u64 and an u32, usually we do not want to truncate the u64, so we need use min_t or max_t with u64.

To simplify the usage introducing two more macros min_lt and max_lt, 'lt' means larger type.

Signed-off-by Dave Young <[email protected]>

---
 include/linux/kernel.h |   13 +++++++++++++
 1 file changed, 13 insertions(+)

include/linux/kernel.h +6 -0

@@ -798,6 +798,19 @@ static inline void ftrace_dump(enum ftra
        type __max2 = (y);                      \
        __max1 > __max2 ? __max1: __max2; })
 
+/*
+ * use type of larger size in min_lt and max_lt
+ */
+#define min_lt(x, y) ({                        \
+       int sx = sizeof(typeof(x));             \
+       int sy = sizeof(typeof(y));             \

include/linux/kernel.h static inline void ftrace_dump(enum ftra · line 816

Quoted 10 lines

include/linux/kernel.h +7 -0

+       sx > sy ? min_t(typeof(x), x, y) : min_t(typeof(y), x, y); })
+
+#define max_lt(x, y) ({                        \
+       int sx = sizeof(typeof(x));             \
+       int sy = sizeof(typeof(y));             \
+       sx > sy ? max_t(typeof(x), x, y) : max_t(typeof(y), x, y); })
+
 /**
  * clamp_t - return a value clamped to a given range using a given type
  * @type: the type of variable to use

No no!

C standard has defined "usual arithmetic conversions" rules[1], which decides the type promotion rules in binary operators.

The interfaces in this patch just bluntly overrides this rule to choose the bigger type size
for operation. Most of time it might work well, because most time the operands used in min_t()/max_t() in Linux kernel
have same sign'ness and this rule works.

But if two operands have same size type but have different different sign'ness, this interfaces will exhibit Undefind Behavior, i.e. you choose the typeof(y) as the final type to use in operation when they have the same type size, so it might be signed or unsigned, depending on the type of y.

So, in this /proc/fs/vmcore case you should rather just explicit cast the operand to avoid truncation.