-Wconstant-conversion in lib/base64.c

[nathanchance]

After llvm/llvm-project@a5ef934:

lib/base64.c:58:18: error: implicit conversion from 'int' to 's8' (aka 'signed char') changes value from 131 to -125 [-Werror,-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: error: implicit conversion from 'int' to 's8' (aka 'signed char') changes value from 130 to -126 [-Werror,-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 \
      |                                              ~~~~~~~~~~^~~~

This comes from a relatively recent Linux commit:

c4eb7ad32eab ("lib/base64: optimize base64_decode() with reverse lookup tables")

https://lore.kernel.org/202609110514.S0a0FD6a-lkp@intel.com/

[nathanchance]

I've reported this upstream, as this should be provably unreachable and ignored: llvm/llvm-project#223923

[nathanchance]

llvm/llvm-project#226775