Re: [PATCH] asm-generic: fix -Wtype-limits compiler warnings

Andrew Morton
2019-07-23T21:27:51Z

On Tue, 23 Jul 2019 16:49:46 -0400 Qian Cai <cai@lca.pw> wrote:

> The commit d66acc39c7ce ("bitops: Optimise get_order()") introduced a
> compilation warning because "rx_frag_size" is an "ushort" while
> PAGE_SHIFT here is 16. The commit changed the get_order() to be a
> multi-line macro where compilers insist to check all statements in the
> macro even when __builtin_constant_p(rx_frag_size) will return false as
> "rx_frag_size" is a module parameter.
>
> In file included from ./arch/powerpc/include/asm/page_64.h:107,
> from ./arch/powerpc/include/asm/page.h:242,
> from ./arch/powerpc/include/asm/mmu.h:132,
> from ./arch/powerpc/include/asm/lppaca.h:47,
> from ./arch/powerpc/include/asm/paca.h:17,
> from ./arch/powerpc/include/asm/current.h:13,
> from ./include/linux/thread_info.h:21,
> from ./arch/powerpc/include/asm/processor.h:39,
> from ./include/linux/prefetch.h:15,
> from drivers/net/ethernet/emulex/benet/be_main.c:14:
> drivers/net/ethernet/emulex/benet/be_main.c: In function
> 'be_rx_cqs_create':
> ./include/asm-generic/getorder.h:54:9: warning: comparison is always
> true due to limited range of data type [-Wtype-limits]
> (((n) < (1UL << PAGE_SHIFT)) ? 0 : \
> ^
> drivers/net/ethernet/emulex/benet/be_main.c:3138:33: note: in expansion
> of macro 'get_order'
> adapter->big_page_size = (1 << get_order(rx_frag_size)) * PAGE_SIZE;
> ^~~~~~~~~
>
> Fix it by moving almost all of this multi-line macro into a proper
> function __get_order(), and leave get_order() as a single-line macro in
> order to avoid compilation errors.
>
> ...
>
> --- a/include/asm-generic/getorder.h
> +++ b/include/asm-generic/getorder.h
> @@ -15,6 +15,16 @@ int __get_order(unsigned long size)
> {
> int order;
>
> + if (__builtin_constant_p(size)) {
> + if (!size)
> + return BITS_PER_LONG - PAGE_SHIFT;
> +
> + if (size < (1UL << PAGE_SHIFT))
> + return 0;
> +
> + return ilog2((size) - 1) - PAGE_SHIFT + 1;
> + }
> +
> size--;
> size >>= PAGE_SHIFT;
> #if BITS_PER_LONG == 32
> @@ -49,11 +59,6 @@ int __get_order(unsigned long size)
> */
> #define get_order(n) \
> ( \
> - __builtin_constant_p(n) ? ( \
> - ((n) == 0UL) ? BITS_PER_LONG - PAGE_SHIFT : \
> - (((n) < (1UL << PAGE_SHIFT)) ? 0 : \
> - ilog2((n) - 1) - PAGE_SHIFT + 1) \
> - ) : \
> __get_order(n) \
> )

So we can remove __get_order() altogether now?

--- a/include/asm-generic/getorder.h~asm-generic-fix-wtype-limits-compiler-warnings-fix
+++ a/include/asm-generic/getorder.h
@@ -7,11 +7,29 @@
#include <linux/compiler.h>
#include <linux/log2.h>

-/*
- * Runtime evaluation of get_order()
+/**
+ * get_order - Determine the allocation order of a memory size
+ * @size: The size for which to get the order
+ *
+ * Determine the allocation order of a particular sized block of memory. This
+ * is on a logarithmic scale, where:
+ *
+ * 0 -> 2^0 * PAGE_SIZE and below
+ * 1 -> 2^1 * PAGE_SIZE to 2^0 * PAGE_SIZE + 1
+ * 2 -> 2^2 * PAGE_SIZE to 2^1 * PAGE_SIZE + 1
+ * 3 -> 2^3 * PAGE_SIZE to 2^2 * PAGE_SIZE + 1
+ * 4 -> 2^4 * PAGE_SIZE to 2^3 * PAGE_SIZE + 1
+ * ...
+ *
+ * The order returned is used to find the smallest allocation granule required
+ * to hold an object of the specified size.
+ *
+ * The result is undefined if the size is 0.
+ *
+ * This function may be used to initialise variables with compile time
+ * evaluations of constants.
*/
-static inline __attribute_const__
-int __get_order(unsigned long size)
+static inline __attribute_const__ int get_order(unsigned long size)
{
int order;

@@ -35,33 +53,6 @@ int __get_order(unsigned long size)
return order;
}

-/**
- * get_order - Determine the allocation order of a memory size
- * @size: The size for which to get the order
- *
- * Determine the allocation order of a particular sized block of memory. This
- * is on a logarithmic scale, where:
- *
- * 0 -> 2^0 * PAGE_SIZE and below
- * 1 -> 2^1 * PAGE_SIZE to 2^0 * PAGE_SIZE + 1
- * 2 -> 2^2 * PAGE_SIZE to 2^1 * PAGE_SIZE + 1
- * 3 -> 2^3 * PAGE_SIZE to 2^2 * PAGE_SIZE + 1
- * 4 -> 2^4 * PAGE_SIZE to 2^3 * PAGE_SIZE + 1
- * ...
- *
- * The order returned is used to find the smallest allocation granule required
- * to hold an object of the specified size.
- *
- * The result is undefined if the size is 0.
- *
- * This function may be used to initialise variables with compile time
- * evaluations of constants.
- */
-#define get_order(n) \
-( \
- __get_order(n) \
-)
-
#endif /* __ASSEMBLY__ */

#endif /* __ASM_GENERIC_GETORDER_H */
_