[Bug libstdc++/107921] New: Overflow warnings in libsupc++/hash_bytes.cc for msp430-elf -mlarge

[Bug libstdc++/107921] New: Overflow warnings in libsupc++/hash_bytes.cc for msp430-elf -mlarge

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107921

            Bug ID: 107921
           Summary: Overflow warnings in libsupc++/hash_bytes.cc for
                    msp430-elf -mlarge
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: build
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---
            Target: msp430-elf

When building libstdc++ for msp430-elf the "large" multilib shows:

/home/jwakely/src/gcc/gcc/libstdc++-v3/libsupc++/hash_bytes.cc: In function
'std::size_t std::_Hash_bytes(const void*, size_t, size_t)':
/home/jwakely/src/gcc/gcc/libstdc++-v3/libsupc++/hash_bytes.cc:76:22: warning:
unsigned conversion from 'long int' to 'std::size_t' {aka '__int20 unsigned'}
changes value from '1540483477' to '125333' [-Woverflow]
   76 |     const size_t m = 0x5bd1e995;
      |                      ^~~~~~~~~~
/home/jwakely/src/gcc/gcc/libstdc++-v3/libsupc++/hash_bytes.cc:85:16: warning:
right shift count >= width of type [-Wshift-count-overflow]
   85 |         k ^= k >> 24;
      |              ~~^~~~~


I think this is because size_t is an alias for __int20 which means we use this
preprocessor branch:

#if __SIZEOF_SIZE_T__ == 4

  // Implementation of Murmur hash for 32-bit size_t.
  size_t
  _Hash_bytes(const void* ptr, size_t len, size_t seed)
  {
    const size_t m = 0x5bd1e995;

sizeof(__int20) is 4, but it is not a 32-bit type.

We should probably be using __SIZE_WIDTH__ == 32 instead.