include/uapi/linux/swab.h: add __u16 cast to __swab16 conditional
PATCH 1/1 include/uapi/linux/swab.h: add __u16 cast to __swab16 conditional
if __HAVE_BUILTIN_BSWAP16__ is defined then __swab16 utilizes a __u16 cast. This same cast should be used if __HAVE_BUILTIN_BSWAP16__ is not defined as well. This should fix loads (at least a few) clang -Wformat warnings specifically with ntohs()
--- include/uapi/linux/swab.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
include/uapi/linux/swab.h +1 -1
@@ -102,7 +102,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)#define __swab16 (x) (__u16)__builtin_bswap16((__u16)(x))#else #define __swab16 (x) \ - (__builtin_constant_p((__u16)(x)) ? \ + (__u16)(__builtin_constant_p((__u16)(x)) ? \ ___constant_swab16(x) : \ __fswab16(x))#endif
Signature · 1 line
2.30.2
Quoted 10 lines · On Tue, 7 Jun 2022 15:20:06 -0700 Justin Stitt <[email protected]> wrote:
if
__HAVE_BUILTIN_BSWAP16__is defined then__swab16utilizes a__u16cast. This same cast should be used if__HAVE_BUILTIN_BSWAP16__is not defined as well. This should fix loads (at least a few) clang -Wformat warnings specifically withntohs()...
include/uapi/linux/swab.h static inline __attribute_const__ __u32 __fswahb32(__u32 val) · line 108
Quoted 8 lines
include/uapi/linux/swab.h +1
-1@@ -102,7 +102,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)#define __swab16 (x) (__u16)__builtin_bswap16((__u16)(x))#else #define __swab16 (x) \ - (__builtin_constant_p((__u16)(x)) ? \ + (__u16)(__builtin_constant_p((__u16)(x)) ? \ ___constant_swab16(x) : \ __fswab16(x))#endif
More explanation, please? Both ___constant_swab16() and __fswab16() return __u16, so why does this patch have any effect?
Quoted 10 lines · Andrew Morton
Quoted 7 lines · On Tue, 7 Jun 2022 15:20:06 -0700 Justin Stitt <[email protected]> wrote:
if
__HAVE_BUILTIN_BSWAP16__is defined then__swab16utilizes a__u16cast. This same cast should be used if__HAVE_BUILTIN_BSWAP16__is not defined as well. This should fix loads (at least a few) clang -Wformat warnings specifically withntohs()...
Quoted 15 lines
include/uapi/linux/swab.h static inline __attribute_const__ __u32 __fswahb32(__u32 val) · line 108
Quoted 11 lines
include/uapi/linux/swab.h +1
-1@@ -102,7 +102,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)#define __swab16 (x) (__u16)__builtin_bswap16((__u16)(x))#else #define __swab16 (x) \ - (__builtin_constant_p((__u16)(x)) ? \ + (__u16)(__builtin_constant_p((__u16)(x)) ? \ ___constant_swab16(x) : \ __fswab16(x))#endif More explanation, please? Both
___constant_swab16()and__fswab16()return__u16, so why does this patch have any effect?
See this example:
https://godbolt.org/z/fzE73jn13
And the ImplicitCastExpr nodes adding to the AST:
https://godbolt.org/z/oYeYxYdKW
Both the second and third operand are promoted to int.
C11: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf
6.5.15/5
Quoted 1 line
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If both the second and third operands have arithmetic type, the result type that would be determined by the usual arithmetic conversions, were they applied to those two operands, is the type of the result.
6.3.1.8/1
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Otherwise, the integer promotions are performed on both operands.
6.3.1.1/2
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If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions.
Signature · 2 lines
Thanks, ~Nick Desaulniers
Quoted 34 lines · On Tue, 7 Jun 2022 15:42:56 -0700 Nick Desaulniers <[email protected]>…
Quoted 10 lines · Andrew Morton
Quoted 7 lines · On Tue, 7 Jun 2022 15:20:06 -0700 Justin Stitt <[email protected]> wrote:
if
__HAVE_BUILTIN_BSWAP16__is defined then__swab16utilizes a__u16cast. This same cast should be used if__HAVE_BUILTIN_BSWAP16__is not defined as well. This should fix loads (at least a few) clang -Wformat warnings specifically withntohs()...
Quoted 15 lines
include/uapi/linux/swab.h static inline __attribute_const__ __u32 __fswahb32(__u32 val) · line 108
Quoted 11 lines
include/uapi/linux/swab.h +1
-1@@ -102,7 +102,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)#define __swab16 (x) (__u16)__builtin_bswap16((__u16)(x))#else #define __swab16 (x) \ - (__builtin_constant_p((__u16)(x)) ? \ + (__u16)(__builtin_constant_p((__u16)(x)) ? \ ___constant_swab16(x) : \ __fswab16(x))#endif More explanation, please? Both
___constant_swab16()and__fswab16()return__u16, so why does this patch have any effect?See this example:
https://godbolt.org/z/fzE73jn13
And the ImplicitCastExpr nodes adding to the AST:
https://godbolt.org/z/oYeYxYdKWBoth the second and third operand are promoted to int.
Quoted 8 lines
C11: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf
6.5.15/5
Quoted 1 line
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If both the second and third operands have arithmetic type, the result type that would be determined by the usual arithmetic conversions, were they applied to those two operands, is the type of the result.
6.3.1.8/1
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Otherwise, the integer promotions are performed on both operands.
6.3.1.1/2
Quoted 1 line
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If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions.
Geeze. Can we please turn this into English and add it to the changelog?
Is it saying that an expression
int ? u16 : u16
has type int? Or something else? What did we do wrong here and is it possible to correct our types rather than adding a cast?
Quoted 46 lines · Andrew Morton
Quoted 34 lines · On Tue, 7 Jun 2022 15:42:56 -0700 Nick Desaulniers <[email protected]>…
Quoted 10 lines · Andrew Morton
Quoted 7 lines · On Tue, 7 Jun 2022 15:20:06 -0700 Justin Stitt <[email protected]> wrote:
if
__HAVE_BUILTIN_BSWAP16__is defined then__swab16utilizes a__u16cast. This same cast should be used if__HAVE_BUILTIN_BSWAP16__is not defined as well. This should fix loads (at least a few) clang -Wformat warnings specifically withntohs()...
Quoted 15 lines
include/uapi/linux/swab.h static inline __attribute_const__ __u32 __fswahb32(__u32 val) · line 108
Quoted 11 lines
include/uapi/linux/swab.h +1
-1@@ -102,7 +102,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)#define __swab16 (x) (__u16)__builtin_bswap16((__u16)(x))#else #define __swab16 (x) \ - (__builtin_constant_p((__u16)(x)) ? \ + (__u16)(__builtin_constant_p((__u16)(x)) ? \ ___constant_swab16(x) : \ __fswab16(x))#endif More explanation, please? Both
___constant_swab16()and__fswab16()return__u16, so why does this patch have any effect?See this example:
https://godbolt.org/z/fzE73jn13
And the ImplicitCastExpr nodes adding to the AST:
https://godbolt.org/z/oYeYxYdKWBoth the second and third operand are promoted to int.
Quoted 8 lines
C11: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf
6.5.15/5
Quoted 1 line
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If both the second and third operands have arithmetic type, the result type that would be determined by the usual arithmetic conversions, were they applied to those two operands, is the type of the result.
6.3.1.8/1
Quoted 1 line
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Otherwise, the integer promotions are performed on both operands.
6.3.1.1/2
Quoted 1 line
Quoted 1 line
If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions.
Quoted 7 lines
Geeze. Can we please turn this into English and add it to the changelog?
Is it saying that an expression
int ? u16 : u16has type int?
Yep.
Quoted 1 line
Or something else?
Technically, the int in your example (the first operand) doesn't matter. Could be a long long or a char and it would not matter.
Quoted 1 line
What did we do wrong here and is it
Perhaps the simplest English explanation would be "ternary expressions with then/else clauses with types smaller than int undergo implicit promotion to int."
Quoted 1 line
possible to correct our types rather than adding a cast?
I think the cast is the explicit cast back to __u16 way to go here, IMO. I don't think anything within the ternary could be changed to avoid implicit promotions.
Justin, can you please send a v2 removing the casts withing __builtin_constant_p (as in the diff I posted previously in this thread) and with the below text added to the commit message:
Ternary expressions with then/else clauses with types smaller than int undergo implicit promotion to int. Cast the result of the ternary back to the expected __u16 to match the type when __HAVE_BUILTIN_BSWAP16__ is defined.
Also remove pointless casts within __builtin_constant_p argument lists. --
Thanks,
~Nick Desaulniers
Quoted 7 lines · Andrew Morton
Quoted 6 lines
6.5.15/5
Quoted 1 line
Quoted 1 line
If both the second and third operands have arithmetic type, the result type that would be determined by the usual arithmetic conversions, were they applied to those two operands, is the type of the result.
6.3.1.8/1
Quoted 1 line
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Otherwise, the integer promotions are performed on both operands.
6.3.1.1/2
Quoted 1 line
Quoted 1 line
If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions.
Quoted 8 lines
Geeze. Can we please turn this into English and add it to the changelog?
Is it saying that an expression
int ? u16 : u16has type int? Or something else? What did we do wrong here and is it possible to correct our types rather than adding a cast?
Not quite. Same rules as u16 + u16 - on architectures where int is wider than 16 bits it's (int)u16 + (int)u16 and yields int, on 16bit ones it's (unsigned int)u16 + (unsigned int)u16 and yields unsigned int.
You *can't* get smaller-than-int out of ? :, same as you can't get it out of addition, etc.
__builtin_choose_expr() would do it, but I would take a cast over that ugliness.
FWIW, it might make sense for clang to keep track of the following
property: expression has the same value as it would if integer promotions in it had been replaced with integer promotion of result.
Example: with
unsigned short x, y, mask;
expresion "x & y" is interpreted as and_int((int)x, (int)y), which is equal to (int)and_u16(x, y), so that expression has the property in question. "x != 12 ? x : y" has the same property. "x + y", OTOH, doesn't - if x and y are both 32768, x + y is add_int((int)x, (int)y), i.e. 65536, while (int)add_u16(x, y) would be 0.
For a somewhat more subtle example,
(x & ~mask) | (y & mask) is interpreted as or_int(and_int((int)x, not_int((int)mask)), and_int((int)y, (int)mask)) which is equal to (int)or_u16(and_u16(x,not_u16(mask)), and_u16(y, mask))
IOW, the property in question holds for that one, despite having a subexpression (~mask) that does *NOT* have that property. (int)not_u16(0) is 0xffff and not_int((int)0) is (assuming 32bit int) 0xffffffff. Upper 16 bits get fouled; applying & with known-16bit launders them off...
That predicate is behind the handling of small bitwise types in sparse; otherwise all operations on __be16 would trigger warnings due to promotions from __be16 to int. And aforementioned subtle example is common enough, so we had to deal with it. See commit d24967cb847b "[PATCH] handle fouled-bitwise" in sparse git...
Quoted 19 lines · Al Viro
Quoted 7 lines · Andrew Morton
Quoted 6 lines
6.5.15/5
Quoted 1 line
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If both the second and third operands have arithmetic type, the result type that would be determined by the usual arithmetic conversions, were they applied to those two operands, is the type of the result.
6.3.1.8/1
Quoted 1 line
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Otherwise, the integer promotions are performed on both operands.
6.3.1.1/2
Quoted 1 line
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If an int can represent all values of the original type (as restricted by the width, for a bit-field), the value is converted to an int; otherwise, it is converted to an unsigned int. These are called the integer promotions.
Quoted 8 lines
Geeze. Can we please turn this into English and add it to the changelog?
Is it saying that an expression
int ? u16 : u16has type int? Or something else? What did we do wrong here and is it possible to correct our types rather than adding a cast?
Quoted 6 lines
Not quite. Same rules as u16 + u16 - on architectures where int is wider than 16 bits it's (int)u16 + (int)u16 and yields int, on 16bit ones it's (unsigned int)u16 + (unsigned int)u16 and yields unsigned int.
You *can't* get smaller-than-int out of ? :, same as you can't get it out of addition, etc.
Exactly, and well put. More concise than I was able to express. I think that description will satisfy Andrew's request for additional context, so I'll recommend Justin add a blurb derived from what you said when sending a v3.
Quoted 7 lines
__builtin_choose_expr()would do it, but I would take a cast over that ugliness.FWIW, it might make sense for clang to keep track of the following
property: expression has the same value as it would if integer promotions in it had been replaced with integer promotion of result.
I'm not sure that's precisely the same issue here.
The issue we're facing is more so that ntohs is being used in printf-like expressions; clang's -Wformat warns about default argument promotion so we need to clean up cases where smaller-than-int format flags are being used for promoted-to-int params. While looking at that, Nathan noticed that __swab16 will return either a __u16 or an int based on whether __HAVE_BUILTIN_BSWAP16__ is defined, which depends on BOTH the compiler being used and target architecture. This patch from Justin just cleans that up.
Quoted 21 lines
Example: with
unsigned short x, y, mask;expresion "x & y" is interpreted as
and_int((int)x, (int)y), which is equal to (int)and_u16(x, y), so that expression has the property in question. "x != 12 ? x : y" has the same property. "x + y", OTOH, doesn't - if x and y are both 32768, x + y isadd_int((int)x, (int)y), i.e. 65536, while (int)add_u16(x, y) would be 0.For a somewhat more subtle example,
(x & ~mask) | (y & mask) is interpreted as or_int(and_int((int)x, not_int((int)mask)), and_int((int)y, (int)mask)) which is equal to (int)or_u16(and_u16(x,not_u16(mask)), and_u16(y, mask))IOW, the property in question holds for that one, despite having a subexpression (~mask) that does *NOT* have that property. (int)not_u16(0) is
0xffffand not_int((int)0) is (assuming 32bit int)0xffffffff. Upper 16 bits get fouled; applying & with known-16bit launders them off...
Quoted 5 lines
That predicate is behind the handling of small bitwise types in sparse; otherwise all operations on
__be16would trigger warnings due to promotions from__be16to int. And aforementioned subtle example is common enough, so we had to deal with it. See commitd24967cb847b"[PATCH] handle fouled-bitwise" in sparse git...
https://git.kernel.org/pub/scm/devel/sparse/sparse.git/commit/?id=d24967cb847b7a04920698a9053ea8195046a831
(For others' reference)
Signature · 2 lines
Thanks, ~Nick Desaulniers
From: Nick Desaulniers
Quoted 1 line
Sent: 08 June 2022 20:35
....
Quoted 8 lines
The issue we're facing is more so that
ntohsis being used in printf-like expressions; clang's -Wformat warns about default argument promotion so we need to clean up cases where smaller-than-int format flags are being used for promoted-to-int params. While looking at that, Nathan noticed that__swab16will return either a__u16or an int based on whether__HAVE_BUILTIN_BSWAP16__is defined, which depends on BOTH the compiler being used and target architecture. This patch from Justin just cleans that up.
The 'problem' is that the (__u16) cast is likely to add an extra '& 0xffff' instruction that is almost certainly not required.
OTOH the lack of this masking has always been a difference between htons() on BE and LE systems.
But clang is also being over-enthusiastic with its warnings. IIRC varargs parameters always get integer promotion. So if %hd ever makes sense for printf then it implies a mask inside printf.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
Quoted 8 lines · Justin Stitt
if
__HAVE_BUILTIN_BSWAP16__is defined then__swab16utilizes a__u16cast. This same cast should be used if__HAVE_BUILTIN_BSWAP16__is not defined as well. This should fix loads (at least a few) clang -Wformat warnings specifically withntohs()
I wonder why there's a cast inside the __builtin_constant_p parameter list? Can't imagine that changing anything. I wonder if this should be:
include/uapi/linux/swab.h +0 -0
@@ -102,7 +102,7 @@ static inline __attribute_const__ __u32
__fswahb32(__u32 val)
#define __swab16(x) (__u16)__builtin_bswap16((__u16)(x)) #else
#define __swab16(x) \
- (__builtin_constant_p((__u16)(x)) ? \
+ (__u16)(__builtin_constant_p(x) ? \
___constant_swab16(x) : \
__fswab16(x)) #endif
@@ -115,7 +115,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)
#define __swab32(x) (__u32)__builtin_bswap32((__u32)(x)) #else
#define __swab32(x) \
- (__builtin_constant_p((__u32)(x)) ? \
+ (__u32)(__builtin_constant_p(x) ? \
___constant_swab32(x) : \
__fswab32(x)) #endif
@@ -128,7 +128,7 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)
#define __swab64(x) (__u64)__builtin_bswap64((__u64)(x)) #else
#define __swab64(x) \
- (__builtin_constant_p((__u64)(x)) ? \
+ (__u64)(__builtin_constant_p(x) ? \
___constant_swab64(x) : \
__fswab64(x)) #endif
--
Thanks,
~Nick Desaulniers
PATCH v2 1/1 include/uapi/linux/swab.h: move explicit cast outside ternary
A cast inside __builtin_constant_p doesn't do anything since it should evaluate as constant at compile time irrespective of this cast. Instead, I moved this cast outside the ternary to ensure the return type is as expected.
For instance, if __HAVE_BUILTIN_BSWAP16__ was not defined then __swab16 is actually returning an int not a u16 due to integer promotion as described by Nick in this thread. This has repercussions when building with clang -Wformat. This fix should solve many of these warnings.
--- include/uapi/linux/swab.h | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-)
include/uapi/linux/swab.h +6 -6
@@ -99,10 +99,10 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)* @x: value to byteswap */ #ifdef __HAVE_BUILTIN_BSWAP16__ -#define __swab16 (x) (__u16)__builtin_bswap16((__u16)(x)) +#define __swab16 (x) (__u16)__builtin_bswap16(x)#else #define __swab16 (x) \ - (__u16)(__builtin_constant_p((__u16)(x)) ? \ + (__u16)(__builtin_constant_p(x) ? \ ___constant_swab16(x) : \ __fswab16(x))#endif
@@ -112,10 +112,10 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)* @x: value to byteswap */ #ifdef __HAVE_BUILTIN_BSWAP32__ -#define __swab32 (x) (__u32)__builtin_bswap32((__u32)(x)) +#define __swab32 (x) (__u32)__builtin_bswap32(x)#else #define __swab32 (x) \ - (__builtin_constant_p((__u32)(x)) ? \ + (__u32)(__builtin_constant_p(x) ? \ ___constant_swab32(x) : \ __fswab32(x))#endif
@@ -125,10 +125,10 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)* @x: value to byteswap */ #ifdef __HAVE_BUILTIN_BSWAP64__ -#define __swab64 (x) (__u64)__builtin_bswap64((__u64)(x)) +#define __swab64 (x) (__u64)__builtin_bswap64(x)#else #define __swab64 (x) \ - (__builtin_constant_p((__u64)(x)) ? \ + (__u64)(__builtin_constant_p(x) ? \ ___constant_swab64(x) : \ __fswab64(x))#endif
Signature · 1 line
2.30.2
Re: [PATCH v2] include/uapi/linux/swab.h: move explicit cast outside ternary
Quoted 9 lines · On Tue, 7 Jun 2022 17:14:22 -0700 Justin Stitt <[email protected]> wrote:
A cast inside
__builtin_constant_pdoesn't do anything since it should evaluate as constant at compile time irrespective of this cast. Instead, I moved this cast outside the ternary to ensure the return type is as expected.For instance, if
__HAVE_BUILTIN_BSWAP16__was not defined then__swab16is actually returning anintnot au16due to integer promotion as described by Nick in this thread. This has repercussions when building with clang -Wformat. This fix should solve many of these warnings.
ARM allmodconfig:
In file included from ./include/linux/swab.h:5,
from ./arch/arm/include/asm/opcodes.h:86,
from ./arch/arm/include/asm/bug.h:7,
from ./include/linux/bug.h:5,
from ./include/linux/mmdebug.h:5,
from ./include/linux/gfp.h:5,
from ./include/linux/slab.h:15,
from ./fs/xfs/kmem.h:9,
from ./fs/xfs/xfs_linux.h:24,
from ./fs/xfs/xfs.h:22,
from fs/xfs/scrub/agheader.c:6:fs/xfs/scrub/agheader.c: In function 'xchk_superblock':./include/uapi/linux/byteorder/little_endian.h:42:52: error: unsigned conversion from 'int' to 'short unsigned int' changes value from '-49265' to '16271' [-Werror=overflow]
42 | #define __cpu_to_be16(x) ((__force __be16)__swab16((x)))
| ^~~./include/uapi/linux/swab.h:102:46: note: in definition of macro '__swab16'
102 | #define __swab16(x) (__u16)__builtin_bswap16(x)
| ^./include/linux/byteorder/generic.h:96:21: note: in expansion of macro '__cpu_to_be16'
96 | #define cpu_to_be16 __cpu_to_be16
| ^~~~~~~~~~~~~
fs/xfs/scrub/agheader.c:158:23: note: in expansion of macro 'cpu_to_be16'
158 | vernum_mask = cpu_to_be16(~XFS_SB_VERSION_OKBITS |
| ^~~~~~~~~~~cc1: all warnings being treated as errors
make[2]: *** [scripts/Makefile.build:249: fs/xfs/scrub/agheader.o] Error 1 make[1]: *** [scripts/Makefile.build:466: fs/xfs] Error 2
make: *** [Makefile:1839: fs] Error 2
Re: [PATCH v2] include/uapi/linux/swab.h: move explicit cast outside ternary
Quoted 35 lines · Andrew Morton
Quoted 9 lines · On Tue, 7 Jun 2022 17:14:22 -0700 Justin Stitt <[email protected]> wrote:
A cast inside
__builtin_constant_pdoesn't do anything since it should evaluate as constant at compile time irrespective of this cast. Instead, I moved this cast outside the ternary to ensure the return type is as expected.For instance, if
__HAVE_BUILTIN_BSWAP16__was not defined then__swab16is actually returning anintnot au16due to integer promotion as described by Nick in this thread. This has repercussions when building with clang -Wformat. This fix should solve many of these warnings.ARM allmodconfig:
In file included from
./include/linux/swab.h:5,from ./arch/arm/include/asm/opcodes.h:86, from ./arch/arm/include/asm/bug.h:7, from ./include/linux/bug.h:5, from ./include/linux/mmdebug.h:5, from ./include/linux/gfp.h:5, from ./include/linux/slab.h:15, from ./fs/xfs/kmem.h:9, from ./fs/xfs/xfs_linux.h:24, from ./fs/xfs/xfs.h:22, from fs/xfs/scrub/agheader.c:6:
fs/xfs/scrub/agheader.c: In function 'xchk_superblock':./include/uapi/linux/byteorder/little_endian.h:42:52: error: unsigned conversion from 'int' to 'short unsigned int' changes value from '-49265' to '16271' [-Werror=overflow]
42 | #define__cpu_to_be16(x)((__force__be16)__swab16((x)))
| ^~~./include/uapi/linux/swab.h:102:46: note: in definition of macro '__swab16'
102 | #define__swab16(x)(__u16)__builtin_bswap16(x)
| ^./include/linux/byteorder/generic.h:96:21: note: in expansion of macro '__cpu_to_be16'
96 | #definecpu_to_be16__cpu_to_be16
Quoted 9 lines
| ^~~~~~~~~~~~~ fs/xfs/scrub/agheader.c:158:23: note: in expansion of macro 'cpu_to_be16' 158 | vernum_mask = cpu_to_be16(~XFS_SB_VERSION_OKBITS | | ^~~~~~~~~~~cc1: all warnings being treated as errors
make[2]: *** [scripts/Makefile.build:249:fs/xfs/scrub/agheader.o] Error 1 make[1]: *** [scripts/Makefile.build:466: fs/xfs] Error 2
make: *** [Makefile:1839: fs] Error 2
Ah, I see what went wrong.
Justin added more than I requested.
Compare the diffstat between:
https://lore.kernel.org/llvm/CAKwvOd=-NXR5HoHwwEUZMsCt90oaADH6XHifje9n-8S8rj9SFw@mail.gmail.com/
with the v2:
https://lore.kernel.org/llvm/[email protected]/
Justin, please send a v3 dropping the changes made to the "__HAVE_BUILTIN_BSWAP*__ is defined" case; just make the changes I suggested.
In addition, please add to the commit message this snippet from Al:
``` As Al Viro notes: You *can't* get smaller-than-int out of ? :, same as you can't get it out of addition, etc. ```
Finally, Justin, you can retest this by running:
$ ARCH=arm make LLVM=1 -j72 allmodconfig fs/xfs/scrub/agheader.o with your patch applied to linux-next. (It wouldn't hurt to build the whole thing...but it's next, which is red today for ARCH=arm so nvm: https://github.com/ClangBuiltLinux/continuous-integration2/runs/6796317443?check_suite_focus=true). https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
Signature · 2 lines
Thanks, ~Nick Desaulniers
Re: [PATCH v2] include/uapi/linux/swab.h: move explicit cast outside ternary
In reply to Justin StittQuoted 8 lines · Justin Stitt
A cast inside
__builtin_constant_pdoesn't do anything since it should evaluate as constant at compile time irrespective of this cast. Instead, I moved this cast outside the ternary to ensure the return type is as expected.For instance, if
__HAVE_BUILTIN_BSWAP16__was not defined then__swab16is actually returning anintnot au16due to integer promotion as described by Nick in this thread. This has repercussions when building with clang
Also, "this thread" won't make much sense when applied if someone is looking at git log. Consider phrasing this instead as "in the lore link below" then include another link tag to
Though, I think it's simply more concise to just include what Al said, and drop this sentence altogether. You can send me v3 privately as an RFC and I'll greenlight it before you resend to the list.
Signature · 2 lines
Thanks, ~Nick Desaulniers