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From: "paisanafc at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/112666] New: Missed optimization: Value initialization
 zero-initializes members with user-defined constructor
Date: Wed, 22 Nov 2023 08:58:46 +0000
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X-Bugzilla-Component: c++
X-Bugzilla-Version: 11.4.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112666

            Bug ID: 112666
           Summary: Missed optimization: Value initialization
                    zero-initializes members with user-defined constructor
           Product: gcc
           Version: 11.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: paisanafc at gmail dot com
  Target Milestone: ---

Looking for the presence of "memset" instructions in the generated assembly=
, it
seems that gcc is zero-initializing class members with user-defined
constructors that shouldn't need to be zero-initialized.

I share below the example benchmark and a godbolt link for convenience
(https://godbolt.org/z/158q6sfen). I used the benchmark library as I didn't
know an easy way to reproduce the instruction `benchmark::DoNotOptimize`. I
hope that's ok.

---

#include <benchmark/benchmark.h>
#include <array>

struct A {
    A() =3D default;
    ~A() {
      benchmark::DoNotOptimize(c); // avoid inlining
    }
    std::array<char, 50000> member;
    char c;
};

struct B {
    B() {}  // user-defined ctor
    ~B() {
      benchmark::DoNotOptimize(c); // avoid inlining
    }
    std::array<char, 50000> member;
    char c;
};

struct C {
    // no user-defined ctor
    B b;
    int dummy;
};

// The benchmark code:

static void ACreation(benchmark::State& state) {
  for (auto _ : state) {
    A a{};
    benchmark::DoNotOptimize(a);
  }
}
BENCHMARK(ACreation);
static void BCreation(benchmark::State& state) {
  for (auto _ : state) {
    B b{};
    benchmark::DoNotOptimize(b);
  }
}
BENCHMARK(BCreation);
static void CCreation(benchmark::State& state) {
  for (auto _ : state) {
    C c{};
    benchmark::DoNotOptimize(c);
  }
}
BENCHMARK(CCreation);
BENCHMARK_MAIN();

---

When I run this with https://github.com/google/benchmark, I get the followi=
ng
results (with gcc++11.4 and above):

-----------------------------------------------------
Benchmark           Time             CPU   Iterations
-----------------------------------------------------
ACreation         736 ns          736 ns       933741
BCreation        3.62 ns         3.62 ns    191180154
CCreation         755 ns          754 ns       944906

The struct "C" which is just "B" and an int is much slower at being initial=
ized
than B when value initialization (via {}) is used. However, my understandin=
g of
the C++ standard is that members with a user-defined default constructor do=
 not
need to be zero-initialized in this situation. Looking at the godbolt assem=
bly
output, I see that both `A a{}` and `C c{}` generate a memset instruction,
while `B b{}` doesn't. Clang, on the other hand, seems to initialize C almo=
st
as fast as B.

This potentially missed optimization in gcc is particularly nasty for struc=
ts
with large embedded storage (e.g. structs that contain C-arrays, std::array=
s,
or static_vectors).=