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See
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96963

In file included from src/bpol/bitpol-normal.cc:7:
In function ‘void bitmat_companion(ulong, ulong, ulong*)’,
    inlined from ‘bool bitpol_normal_q(ulong, ulong, ulong, ulong*)’ at src/bpol/bitpol-normal.cc:70:25:
src/bmat/bitmat-inline.h:137:10: error: writing 8 bytes into a region of size 0 [-Werror=stringop-overflow=]
  137 |     M[0] = r;
      |     ~~~~~^~~
In file included from src/bpol/bitpol-normal.cc:13:
src/bpol/bitpol-normal.cc: In function ‘bool bitpol_normal_q(ulong, ulong, ulong, ulong*)’:
src/bpol/bitpol-normal.cc:69:23: note: destination object ‘C.35’ of size 0
   69 |         ALLOCA(ulong, C, n);
      |                       ^
src/fxtalloca.h:23:34: note: in definition of macro ‘ALLOCA’
   23 | #define ALLOCA(Type, v, n)  Type v[n]
      |                                  ^

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For the next one, see
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100417
and
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96295

src/fht/recfht2.cc: In function ?void recursive_fht_dit2_core(const double*, ulong, double*)?:
src/fht/recfht2.cc:31:28: error: ?<unknown>? may be used uninitialized [-Werror=maybe-uninitialized]
   31 |     recursive_fht_dit2_core(ev, nh, b);
      |     ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
src/fht/recfht2.cc:15:1: note: by argument 1 of type ?const double*? to ?void recursive_fht_dit2_core(const double*, ulong, double*)? declared here
   15 | recursive_fht_dit2_core(const double *a, ulong n, double *x)
      | ^~~~~~~~~~~~~~~~~~~~~~~
src/fht/recfht2.cc:32:28: error: ?<unknown>? may be used uninitialized [-Werror=maybe-uninitialized]
   32 |     recursive_fht_dit2_core(od, nh, c);
      |     ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
src/fht/recfht2.cc:15:1: note: by argument 1 of type ?const double*? to ?void recursive_fht_dit2_core(const double*, ulong, double*)? declared here
   15 | recursive_fht_dit2_core(const double *a, ulong n, double *x)
      | ^~~~~~~~~~~~~~~~~~~~~~~
src/fht/recfht2.cc: In function ?void recursive_fht_dif2_core(const double*, ulong, double*)?:
src/fht/recfht2.cc:82:28: error: ?<unknown>? may be used uninitialized [-Werror=maybe-uninitialized]
   82 |     recursive_fht_dif2_core(le, nh, b);
      |     ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
src/fht/recfht2.cc:57:1: note: by argument 1 of type ?const double*? to ?void recursive_fht_dif2_core(const double*, ulong, double*)? declared here
   57 | recursive_fht_dif2_core(const double *a, ulong n, double *x)
      | ^~~~~~~~~~~~~~~~~~~~~~~

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