[Bug c++/96197] Excess memory consumption, positive correlation with the size of a constexpr array

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96197

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2020-07-15
      Known to fail|                            |10.1.0, 11.0
           Keywords|                            |memory-hog

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Does it help if you replace your simplistic O(n) algorithms with O(log n) ones?
That is, 

constexpr const struct word_type &WORD_LOOKUP(WORD index, const struct
word_type *table_row =word_table) {
    for (;;) {
        if (table_row->index == index) return *table_row;

        if (table_row->index == WORD::NONE) return word_lookup(WORD::NONE);

        ++table_row;
    }

//    return table_row->index == index ? *table_row : (table_row->index ==
WORD::NONE ? word_lookup(WORD::NONE) : WORD_LOOKUP(index, ++table_row));
}

replace the for (;;) loop with table_row[index] (OK, that's maybe too much
guessing into your data structure) or with a binary search over the table
which you keep sorted?

It's probably simply garbage that accumulates during constexpr evaluation.
-ftime-report shows

 constant expression evaluation     :  28.45 ( 92%)   1.57 ( 96%)  32.97 ( 92%)
5127882 kB (100%)

that is 5GB of GC memory from constexpr evaluation.