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default_init results in a memset with GCC16 #333
Description
It's perhaps a little early to open an issue for this but since I'm already toying with GCC16, I was surprised to see that it now generates a memset to zero init even when default_init is used.
boost::container::small_vector<float, 100> bar(100, boost::container::default_init);
for(auto b : bar)
std::cout << b;GCC16.0.1 (-std=c++26 -O3):
push r12
mov edx, 424
xor esi, esi
push rbp
push rbx
sub rsp, 432
mov rdi, rsp
lea r12, [rsp+24]
lea rbp, [rsp+424]
call "memset" ; zero init 400 bytes
mov eax, 100
mov QWORD PTR [rsp], r12
mov rbx, r12
movq xmm0, rax
punpcklqdq xmm0, xmm0
movups XMMWORD PTR [rsp+8], xmm032-bit (only the relevant bit):
lea eax, [ebp-440]
push 412 ; count
push 0 ; ch
push eax ; dest
call "memset"GCC15.2 (-std=c++26 -O3):
push r12
mov eax, 100
push rbp
movq xmm0, rax
push rbx
punpcklqdq xmm0, xmm0
sub rsp, 432
lea r12, [rsp+24]
movups XMMWORD PTR [rsp+8], xmm0
lea rbp, [rsp+424]
mov QWORD PTR [rsp], r12
mov rbx, r12Clang 22.1 (-std=c++26 -O3):
push r15
push r14
push rbx
sub rsp, 432
lea r14, [rsp + 32]
mov qword ptr [rsp + 24], 100
mov qword ptr [rsp + 8], r14
mov qword ptr [rsp + 16], 100
mov r15d, 24
mov rbx, qword ptr [rip + std::cout@GOTPCREL]I'm not yet sure if this is a compiler regression, an issue with the containers, a skill issue or something else. Again, happy to close it if it's too soon. :)
Activity
igaztanaga commentedon Jun 4, 2026
Could this be related to C++26's "erroneous-value initialization" implementation (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114457) ?
igaztanaga commentedon Jun 4, 2026
Maybe you can avoid the compiler-inserted memset with the "[[indeterminate]]" (https://en.cppreference.com/cpp/language/attributes/indeterminate) attribute:
boost::container::small_vector<float,100> bar [[indeterminate]] (100, boost::container::default_init);
or use the compiler flag to override the default behaviour for C++26
-ftrivial-auto-var-init=uninitialized
igaztanaga commentedon Oct 14, 2026
I've confirmed is related to C++26 erroneous behaviour, I'll updated the documentation with the explanation.