[Bug tree-optimization/105883] New: Memcmp folded only when size is a power of two

[denis.campredon at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105883

            Bug ID: 105883
           Summary: Memcmp folded only when size is a power of two
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: denis.campredon at gmail dot com
  Target Milestone: ---

Compiled with trunk and -O2, on x86-64, the testcase bellow shows, the calls to
memcmp are folded just when the size is a power of two. Other archs seems to
produce unoptimal code too.

If the arrays are declared const, everything is optimized as expected.


-------
template <int I, typename T>
bool equals() {
    T a[16] = {1,2,3,4,5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16};
    T b[16] = {1,2,3,4,5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16};
    static_assert(I <= sizeof a);
    return !__builtin_memcmp(a, b, I);
}

template <int I, typename T>
bool not_equals() {
    T a[16] = {1,2,3,4,5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16};
    T b[16] = {0,2,3,4,5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16};
    static_assert(I <= sizeof a);
    return !__builtin_memcmp(a, b, I);
}

#define TEST \
template bool equals<1, char>(); \
template bool equals<2, char>(); \
template bool equals<3, char>(); \
template bool equals<4, char>(); \
template bool equals<5, char>(); \
template bool equals<7, char>(); \
template bool equals<8, char>(); \
template bool equals<15, char>(); \
template bool equals<16, char>(); \
template bool equals<1, long long>(); \
template bool equals<2, long long>(); \
template bool equals<3, long long>(); \
template bool equals<4, long long>(); \
template bool equals<5, long long>(); \
template bool equals<7, long long>(); \
template bool equals<8, long long>(); \
template bool equals<16, long long>(); \
template bool equals<17, long long>(); \
template bool equals<31, long long>(); \
template bool equals<32, long long>(); \
template bool equals<63, long long>(); \
template bool equals<64, long long>(); \
template bool equals<127, long long>(); \
template bool equals<128, long long>();

TEST

#define equals not_equals
TEST