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-Wconstant-conversion in lib/base64.c #2181

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@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/

Activity

  1. nathanchance commented on Sep 16, 2026

    @nathanchance
    MemberAuthor

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

  2. added
    [BUG] llvm (main)A bug in an unreleased version of LLVM (this label is appropriate for regressions)
    Reported upstreamThis bug was filed on LLVM’s issue tracker, Phabricator, or the kernel mailing list.
    and removed
    [BUG] linuxA bug that should be fixed in the mainline kernel.
    on Sep 16, 2026
  3. nathanchance commented on Oct 5, 2026

    @nathanchance
    MemberAuthor
  4. added
    [PATCH] SubmittedA patch has been submitted for review
    and removed
    Reported upstreamThis bug was filed on LLVM’s issue tracker, Phabricator, or the kernel mailing list.
    on Oct 5, 2026
  5. added and removed
    [PATCH] SubmittedA patch has been submitted for review
    on Oct 8, 2026
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