[Clang] [CodeGen] Wrong code generation for `extern reference`

[16bit-ykiko]

Minimal Reproducible Example:

/// src.cpp
#include <vector>

struct S {
    std::vector<int> c;
};

static S ls = {{1, 2, 3}};

S& s = ls;

/// main.cpp
#include <cstdio>
#include <vector>

struct S {
    std::vector<int> c;
};

extern S& s;

int main() {
    for (size_t i = 0; i < s.c.size(); i++) {
        printf("aaa\n");
    }

    for (auto& i : s.c) {
        printf("bbb\n");
    }
    return 0;
}

https://godbolt.org/z/W1YoYsMYh

[dtcxzyw]

Reduced: https://godbolt.org/z/TMMhcEo7s

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: ykiko (16bit-ykiko)

Minimal Reproducible Example:

/// src.cpp
#include <vector>

struct S {
    std::vector<int> c;
};

static S ls = {{1, 2, 3}};

S& s = ls;

/// main.cpp
#include <cstdio>
#include <vector>

struct S {
    std::vector<int> c;
};

extern S& s;

int main() {
    for (size_t i = 0; i < s.c.size(); i++) {
        printf("aaa\n");
    }

    for (auto& i : s.c) {
        printf("bbb\n");
    }
    return 0;
}

https://godbolt.org/z/W1YoYsMYh

[sakria9]

I use git bisect and find commit 0a9c08c , which emits

  store ptr getelementptr (i8, ptr @s, i64 1), ptr %__range1, align 8
  %3 = load ptr, ptr %__range1, align 8
  %call1 = call ptr @_ZNSt6vectorIiSaIiEE5beginEv(ptr noundef nonnull align 8 dereferenceable(24) %3) #1

then commit is c9637af (which try to fix 0a9c08c #95474 ) , which emits:

  store ptr @s, ptr %__range1, align 8
  %3 = load ptr, ptr %__range1, align 8
  %call1 = call ptr @_ZNSt6vectorIiSaIiEE5beginEv(ptr noundef nonnull align 8 dereferenceable(24) %3) #1

[dtcxzyw]

Related code:

llvm-project/clang/lib/CodeGen/CGDecl.cpp

Lines 1988 to 1992 in fb29f19

	llvm::Constant *constant = nullptr;
	if (emission.IsConstantAggregate ||
	D.mightBeUsableInConstantExpressions(getContext())) {
	assert(!capturedByInit && "constant init contains a capturing block?");
	constant = ConstantEmitter(*this).tryEmitAbstractForInitializer(D);