Try fixing c++ compiler warnings about unknown union size. Fix JuliaL…

…ang#13172

src/dump.c

@@ -527,7 +527,7 @@ static void jl_serialize_datatype(ios_t *s, jl_datatype_t *dt)
  write_int32(s, dt->alignment);
  write_int8(s, dt->haspadding);
  size_t fieldsize = jl_fielddesc_size(dt->fielddesc_type);
- ios_write(s, (char*)&dt->fields32[0], nf * fieldsize);
+ ios_write(s, (char*)dt->fields, nf * fieldsize);
  jl_serialize_value(s, dt->types);
  }
 
@@ -1129,7 +1129,7 @@ static jl_value_t *jl_deserialize_datatype(ios_t *s, int pos, jl_value_t **loc)
  dt->alignment = read_int32(s);
  dt->haspadding = read_int8(s);
  size_t fieldsize = jl_fielddesc_size(fielddesc_type);
- ios_read(s, (char*)&dt->fields32[0], nf * fieldsize);
+ ios_read(s, (char*)dt->fields, nf * fieldsize);
  dt->types = (jl_svec_t*)jl_deserialize_value(s, (jl_value_t**)&dt->types);
  jl_gc_wb(dt, dt->types);
  }

src/julia.h

@@ -290,11 +290,7 @@ typedef struct _jl_datatype_t {
  uint32_t uid;
  void *struct_decl; //llvm::Value*
  void *ditype; // llvm::MDNode* to be used as llvm::DIType(ditype)
- union {
- jl_fielddesc8_t fields8[0];
- jl_fielddesc16_t fields16[0];
- jl_fielddesc32_t fields32[0];
- };
+ size_t fields[];
 } jl_datatype_t;
 
 typedef struct {
@@ -718,26 +714,26 @@ STATIC_INLINE jl_value_t *jl_cellset(void *a, size_t i, void *x)
  static inline uint32_t jl_field_##f(jl_datatype_t *st, int i) \
  { \
  if (st->fielddesc_type == 0) { \
- return st->fields8[i].f;  \
+ return ((jl_fielddesc8_t*)st->fields)[i].f; \
  } \
  else if (st->fielddesc_type == 1) { \
- return st->fields16[i].f;  \
+ return ((jl_fielddesc16_t*)st->fields)[i].f; \
  } \
  else { \
- return st->fields32[i].f;  \
+ return ((jl_fielddesc32_t*)st->fields)[i].f; \
  } \
  } \
  static inline void jl_field_set##f(jl_datatype_t *st, int i, \
  uint32_t val) \
  { \
  if (st->fielddesc_type == 0) { \
- st->fields8[i].f = val;  \
+ ((jl_fielddesc8_t*)st->fields)[i].f = val; \
  } \
  else if (st->fielddesc_type == 1) { \
- st->fields16[i].f = val;  \
+ ((jl_fielddesc16_t*)st->fields)[i].f = val; \
  } \
  else { \
- st->fields32[i].f = val;  \
+ ((jl_fielddesc32_t*)st->fields)[i].f = val; \
  } \
  }