[missed optimization opportunity] clang still generates code for empty loop over an std::map object

[yurivict]

Bugzilla Link	51694
Version	11.0
OS	FreeBSD
Attachments	testcase.cpp
CC	@DougGregor,@zygoloid

Extended Description

The attached testcase iterates through the std::map object with empty for-body.

clang-14 still generates some code in this function when it should be empty.

Replacing the type of the argument with std::vector correctly leads to an empty function.

[wheatman]

This still reproduces on post 16 trunk(5cbcaf1678709e4da8b32f075a5c9a3cab028965) https://godbolt.org/z/vEcEfET1E

Slightly simpler code

#include <map>

void f(std::map<int, int> &v) {
	for (auto i : v)
		{ }
}

Produces the following assembly when compiled with -O3

f(std::map<int, int, std::less<int>, std::allocator<std::pair<int const, int> > >&): # @fff(std::map<int, int, std::less<int>, std::allocator<std::pair<int const, int> > >&)
        push    rbx
        mov     rbx, rdi
        mov     rax, qword ptr [rdi + 24]
        add     rbx, 8
        cmp     rax, rbx
        je      .LBB0_3
.LBB0_1:                                # =>This Inner Loop Header: Depth=1
        mov     rdi, rax
        call    std::_Rb_tree_increment(std::_Rb_tree_node_base*)@PLT
        cmp     rax, rbx
        jne     .LBB0_1
.LBB0_3:
        pop     rbx
        ret

If you replace the map with a unordered_map you instead get

        mov     rdi, qword ptr [rdi]
        test    rdi, rdi
        jne     .LBB0_1
        ret

And if you replace the map with a vector you get

f(std::vector<int, std::allocator<int> >&):                 # @f(std::vector<int, std::allocator<int> >&)
        ret

[antoniofrighetto]

Should be fixed by https://github.com/llvm/llvm-project/commit/970bf07d0b184c7ec356ae8f47b193a5e3ff0309. Please feel free to reopen this, should you experience an issue alike.