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

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

[Qian Cai]

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.

Fixes: d66acc39c7ce ("bitops: Optimise get_order()")
Signed-off-by: Qian Cai <cai@lca.pw>
---
 include/asm-generic/getorder.h | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/include/asm-generic/getorder.h b/include/asm-generic/getorder.h
index c64bea7a52be..c6a6d3cd7007 100644
--- 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)						\
 )
 
-- 
1.8.3.1

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

[Andrew Morton]

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 */
_

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

[Nathan Chancellor]

On Tue, Jul 23, 2019 at 04:49:46PM -0400, Qian Cai 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.

Wouldn't it just be better to rename __get_order to get_order?

> Fixes: d66acc39c7ce ("bitops: Optimise get_order()")
> Signed-off-by: Qian Cai <cai@lca.pw>

Other than that, LGTM.

Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>

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

[David Howells]

Qian Cai <cai@lca.pw> wrote:

> 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.

The idea was that you could compile-time initialise a global variable with
get_order():

	int a = get_order(SOME_MACRO);

This is the same reason that ilog2() is a macro:

	int a = ilog2(SOME_MACRO);

See the banner comment on get_order():

 * This function may be used to initialise variables with compile time
 * evaluations of constants.

If you're moving the constant branch into __get_order(), an inline function,
then we'll no longer be able to do this and you need to modify the comment
too.  In fact, would there still be a point in having the get_order() macro?

Also, IIRC, older versions of gcc see __builtin_constant_p(n) == 0 inside an
function, inline or otherwise, even if the passed-in argument *is* constant.

David

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

[Qian Cai]

On Wed, 2019-07-24 at 08:49 +0100, David Howells wrote:
> Qian Cai <cai@lca.pw> wrote:
> 
> > 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.
> 
> The idea was that you could compile-time initialise a global variable with
> get_order():
> 
> 	int a = get_order(SOME_MACRO);
> 
> This is the same reason that ilog2() is a macro:
> 
> 	int a = ilog2(SOME_MACRO);
> 
> See the banner comment on get_order():
> 
>  * This function may be used to initialise variables with compile time
>  * evaluations of constants.
> 
> If you're moving the constant branch into __get_order(), an inline function,
> then we'll no longer be able to do this and you need to modify the comment
> too.  In fact, would there still be a point in having the get_order() macro?
> 
> Also, IIRC, older versions of gcc see __builtin_constant_p(n) == 0 inside an
> function, inline or otherwise, even if the passed-in argument *is* constant.

I have GCC 8.2.1 which works fine.

# cat const.c 
#include <stdio.h>

static int i = 0;

static inline void check()
{
	if (__builtin_constant_p(i))
		printf("i is a const.\n");
}

void main()
{
	check();
}

# gcc -O2 const.c -o const

# ./const 
i is a const.

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

[David Howells]

Qian Cai <cai@lca.pw> wrote:

> I have GCC 8.2.1 which works fine.

But you need to check the minimum version, i.e. 4.6:

	#if GCC_VERSION < 40600

David

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

[Qian Cai]

> On Jul 24, 2019, at 5:49 PM, David Howells <dhowells@redhat.com> wrote:
> 
> Qian Cai <cai@lca.pw> wrote:
> 
>> I have GCC 8.2.1 which works fine.
> 
> But you need to check the minimum version, i.e. 4.6:
> 
> 	#if GCC_VERSION < 40600
> 

I did check gcc version 4.1.2 20080704 and it works fine.