[Bug middle-end/104195] New: Fails to optimize nested array indexing p[i/N].data[i%N]

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104195

            Bug ID: 104195
           Summary: Fails to optimize nested array indexing
                    p[i/N].data[i%N]
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

GCC seems to be unable to optimize some nested array accesses of the form
p[i/N].data[i%N] into a simple ((T*)p)[i]. The C test case is

    struct CChunk { int data[4]; };
    int f(struct CChunk *p, unsigned long long i) { return p[i/4].data[i%4]; }

gcc -O2 currently produces:

    movq %rsi, %rax
    andl $3, %esi
    shrq $2, %rax
    salq $4, %rax
    addq %rax, %rdi
    movl (%rdi,%rsi,4), %eax
    ret

but I would prefer it to produce:

    movl (%rdi,%rsi,4), %eax
    retq

A more exhaustive C++ test follows — GCC can optimize a few of these, but not
all. (Clang can't optimize any of these; I've just filed
https://github.com/llvm/llvm-project/issues/53367 about that.)
https://godbolt.org/z/3E1e6c5e3

template<class T, int N>
struct Chunk {
    T data[N];
};

template<class T, int N, class IndexType>
int f(Chunk<T, N> *p, IndexType i) {
    return p[i/N].data[i%N];
}

template int f(Chunk<char,2>*, unsigned long long);  // GCC wins
template int f(Chunk<char,4>*, unsigned long long);  // GCC wins
template int f(Chunk<char,2>*, unsigned);
template int f(Chunk<char,4>*, unsigned);
template int f(Chunk<int,2>*, unsigned long long);  // GCC wins
template int f(Chunk<int,4>*, unsigned long long);
template int f(Chunk<int,2>*, unsigned);
template int f(Chunk<int,4>*, unsigned);