Re: lib/base64.c:58:18: warning: implicit conversion from 'int' to 's8' (aka 'signed char') changes value from 131 to -125

[Kuan-Wei Chiu]

On Thu, Sep 10, 2026 at 03:35:20PM -0700, Nathan Chancellor wrote:
> On Fri, Sep 11, 2026 at 06:07:30AM +0800, kernel test robot wrote:
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > head:   50d05c7c76c96b90462f24debacca971d2e86713
> > commit: c4eb7ad32eab13ba64cc452c6f43d518b63f5e03 lib/base64: optimize base64_decode() with reverse lookup tables
> > date:   10 months ago
> > config: hexagon-allnoconfig (https://download.01.org/0day-ci/archive/20260911/202609110514.S0a0FD6a-lkp@xxxxxxxxx/config)
> > compiler: clang version 24.0.0git (https://github.com/llvm/llvm-project d3db069999553568848400c84cbf815c03f5470f)
> > reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260911/202609110514.S0a0FD6a-lkp@xxxxxxxxx/reproduce)
> > 
> > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > the same patch/commit), kindly add following tags
> > | Fixes: c4eb7ad32eab ("lib/base64: optimize base64_decode() with reverse lookup tables")
> > | Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > | Closes: https://lore.kernel.org/oe-kbuild-all/202609110514.S0a0FD6a-lkp@xxxxxxxxx/
> > 
> > All warnings (new ones prefixed by >>):
> > 
> > >> lib/base64.c:58:18: warning: implicit conversion from 'int' to 's8' (aka 'signed char') changes value from 131 to -125 [-Wconstant-conversion]
> >       58 |         [BASE64_IMAP] = BASE64_REV_INIT('+', ',')
> >          |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:52:2: note: expanded from macro 'BASE64_REV_INIT'
> >       48 | #define BASE64_REV_INIT(ch_62, ch_63) { \
> >          |                                       ~
> >       49 |         [0 ... 0x1f] = -1, \
> >       50 |         INIT_32(0x20, ch_62, ch_63), \
> >       51 |         INIT_32(0x40, ch_62, ch_63), \
> >       52 |         INIT_32(0x60, ch_62, ch_63), \
> >          |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:46:50: note: expanded from macro 'INIT_32'
> >       46 | #define INIT_32(v, ...) INIT_16(v, __VA_ARGS__), INIT_16((v) + 16, __VA_ARGS__)
> >          |                                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:45:49: note: expanded from macro 'INIT_16'
> >       45 | #define INIT_16(v, ...) INIT_8(v, __VA_ARGS__), INIT_8((v) + 8, __VA_ARGS__)
> >          |                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    note: (skipping 1 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
> >    lib/base64.c:43:48: note: expanded from macro 'INIT_4'
> >       43 | #define INIT_4(v, ...) INIT_2(v, __VA_ARGS__), INIT_2((v) + 2, __VA_ARGS__)
> >          |                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:42:48: note: expanded from macro 'INIT_2'
> >       42 | #define INIT_2(v, ...) INIT_1(v, __VA_ARGS__), INIT_1((v) + 1, __VA_ARGS__)
> >          |                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:34:42: note: expanded from macro 'INIT_1'
> >       34 |                 : (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
> >          |                                              ~~~~~~~~~~^~~~
> >    lib/base64.c:58:18: warning: implicit conversion from 'int' to 's8' (aka 'signed char') changes value from 130 to -126 [-Wconstant-conversion]
> >       58 |         [BASE64_IMAP] = BASE64_REV_INIT('+', ',')
> >          |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:52:2: note: expanded from macro 'BASE64_REV_INIT'
> >       48 | #define BASE64_REV_INIT(ch_62, ch_63) { \
> >          |                                       ~
> >       49 |         [0 ... 0x1f] = -1, \
> >       50 |         INIT_32(0x20, ch_62, ch_63), \
> >       51 |         INIT_32(0x40, ch_62, ch_63), \
> >       52 |         INIT_32(0x60, ch_62, ch_63), \
> >          |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:46:50: note: expanded from macro 'INIT_32'
> >       46 | #define INIT_32(v, ...) INIT_16(v, __VA_ARGS__), INIT_16((v) + 16, __VA_ARGS__)
> >          |                                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:45:49: note: expanded from macro 'INIT_16'
> >       45 | #define INIT_16(v, ...) INIT_8(v, __VA_ARGS__), INIT_8((v) + 8, __VA_ARGS__)
> >          |                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    note: (skipping 1 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
> >    lib/base64.c:43:48: note: expanded from macro 'INIT_4'
> >       43 | #define INIT_4(v, ...) INIT_2(v, __VA_ARGS__), INIT_2((v) + 2, __VA_ARGS__)
> >          |                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:42:24: note: expanded from macro 'INIT_2'
> >       42 | #define INIT_2(v, ...) INIT_1(v, __VA_ARGS__), INIT_1((v) + 1, __VA_ARGS__)
> >          |                        ^~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:34:42: note: expanded from macro 'INIT_1'
> >       34 |                 : (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
> >          |                                              ~~~~~~~~~~^~~~
> >    lib/base64.c:58:18: warning: implicit conversion from 'int' to 's8' (aka 'signed char') changes value from 129 to -127 [-Wconstant-conversion]
> >       58 |         [BASE64_IMAP] = BASE64_REV_INIT('+', ',')
> >          |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:52:2: note: expanded from macro 'BASE64_REV_INIT'
> >       48 | #define BASE64_REV_INIT(ch_62, ch_63) { \
> >          |                                       ~
> >       49 |         [0 ... 0x1f] = -1, \
> >       50 |         INIT_32(0x20, ch_62, ch_63), \
> >       51 |         INIT_32(0x40, ch_62, ch_63), \
> >       52 |         INIT_32(0x60, ch_62, ch_63), \
> >          |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:46:50: note: expanded from macro 'INIT_32'
> >       46 | #define INIT_32(v, ...) INIT_16(v, __VA_ARGS__), INIT_16((v) + 16, __VA_ARGS__)
> >          |                                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:45:49: note: expanded from macro 'INIT_16'
> >       45 | #define INIT_16(v, ...) INIT_8(v, __VA_ARGS__), INIT_8((v) + 8, __VA_ARGS__)
> >          |                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    note: (skipping 1 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
> >    lib/base64.c:43:24: note: expanded from macro 'INIT_4'
> >       43 | #define INIT_4(v, ...) INIT_2(v, __VA_ARGS__), INIT_2((v) + 2, __VA_ARGS__)
> >          |                        ^~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:42:48: note: expanded from macro 'INIT_2'
> >       42 | #define INIT_2(v, ...) INIT_1(v, __VA_ARGS__), INIT_1((v) + 1, __VA_ARGS__)
> >          |                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:34:42: note: expanded from macro 'INIT_1'
> >       34 |                 : (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
> >          |                                              ~~~~~~~~~~^~~~
> >    lib/base64.c:58:18: warning: implicit conversion from 'int' to 's8' (aka 'signed char') changes value from 128 to -128 [-Wconstant-conversion]
> >       58 |         [BASE64_IMAP] = BASE64_REV_INIT('+', ',')
> >          |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:52:2: note: expanded from macro 'BASE64_REV_INIT'
> >       48 | #define BASE64_REV_INIT(ch_62, ch_63) { \
> >          |                                       ~
> >       49 |         [0 ... 0x1f] = -1, \
> >       50 |         INIT_32(0x20, ch_62, ch_63), \
> >       51 |         INIT_32(0x40, ch_62, ch_63), \
> >       52 |         INIT_32(0x60, ch_62, ch_63), \
> >          |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:46:50: note: expanded from macro 'INIT_32'
> >       46 | #define INIT_32(v, ...) INIT_16(v, __VA_ARGS__), INIT_16((v) + 16, __VA_ARGS__)
> >          |                                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    lib/base64.c:45:49: note: expanded from macro 'INIT_16'
> >       45 | #define INIT_16(v, ...) INIT_8(v, __VA_ARGS__), INIT_8((v) + 8, __VA_ARGS__)
> >          |                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    note: (skipping 1 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
> >    lib/base64.c:43:24: note: expanded from macro 'INIT_4'
> >       43 | #define INIT_4(v, ...) INIT_2(v, __VA_ARGS__), INIT_2((v) + 2, __VA_ARGS__)
> 
> I have not looked into the legitimacy of this warning yet but it comes
> about from a recent change to fix -Wconstant-conversion in clang:
> 
>   https://github.com/llvm/llvm-project/commit/a5ef934a8d295dc03be3960f2b3744ec2e53238e
> 
This looks like a false positive rather than a real bug in the C code.
The warning triggers because (v) - '0' + 52 evaluates to >= 128 during
macro expansion which overflows s8.

However, that branch is guarded by (v) >= '0' && (v) <= '9', which is a
compile time constant false for those values, so the branch is never
taken. It seems clang is diagnosing constant conversions in
unreachable branches of compile time conditional expressions.

Regards,
Kuan-Wei