https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71165
Bug ID: 71165 Summary: std::array with aggregate initialization generates huge code Product: gcc Version: 5.3.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: personalmountains at gmail dot com Target Milestone: --- [mostly copied from a thread on stackoverflow at http://stackoverflow.com/questions/37260097] This code takes several seconds to compile and produces a 52,776 byte executable: #include <array> #include <iostream> int main() { constexpr std::size_t size = 4096; struct S { float f; S() : f(0.0f) {} }; std::array<S, size> a = {}; // <-- note aggregate initialization for (auto& e : a) std::cerr << e.f; return 0; } Increasing 'size' seems to increase compilation time and executable size linearly. I cannot reproduce this behaviour with either clang 3.5 or Visual C++ 2015. Using -Os makes no difference. I've reproduced this on g++ 4.9.2 and 5.3.1, but 6.1 also seems to do the same thing. $ time g++ -O2 -std=c++11 test.cpp real 0m4.178s user 0m4.060s sys 0m0.068s Inspecting the assembly code reveals that the initialization of 'a' is unrolled, generating 4096 movl instructions: main: .LFB1313: .cfi_startproc pushq %rbx .cfi_def_cfa_offset 16 .cfi_offset 3, -16 subq $16384, %rsp .cfi_def_cfa_offset 16400 movl $0x00000000, (%rsp) movl $0x00000000, 4(%rsp) movq %rsp, %rbx movl $0x00000000, 8(%rsp) movl $0x00000000, 12(%rsp) movl $0x00000000, 16(%rsp) [...skipping 4000 lines...] movl $0x00000000, 16376(%rsp) movl $0x00000000, 16380(%rsp) This only happens when T has a non-trivial constructor and the array is initialized using {}. If I do any of the following, g++ generates a simple loop: 1) Remove S::S(); 2) Remove S::S() and initialize S::f in-class; 3) Remove the aggregate initialization (= {}); 4) Compile without -O2. Bugs 59659, 56671 may be relevant. In particular, some of the test cases in bug 59659 seem to still be failing.