https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97151
Bug ID: 97151
Summary: GCC fails to optimize away uselessly allocated arrays
(C++)
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: marat at slonopotamus dot org
Target Milestone: ---
A test program:
int main()
{
int *a = new int[10]();
delete[] a;
return 0;
}
Tested against: gcc 10.2.0 and gcc trunk as of 20200920 on codebolt.org with
-O2, -O3 and -Ofast
Expected: new/delete are optimized away under -O2 and higher because "a" is
never used for anything useful (like, side effects or whatever).
Actual: new/delete are present in assembly:
main:
sub rsp, 8
mov edi, 40
call operator new[](unsigned long)
pxor xmm0, xmm0
mov QWORD PTR [rax+32], 0
mov rdi, rax
movups XMMWORD PTR [rax], xmm0
movups XMMWORD PTR [rax+16], xmm0
call operator delete[](void*)
xor eax, eax
add rsp, 8
ret
Interesting observations:
1. if "delete[]" is commented out, whole program is optimized away
2. if "new int[10]()" is replaced with "new int[10]", whole program is
optimized away
3. if new/delete is replaced with matching malloc/free, whole program is
optimized away
If someone wants to play with this example online, here it is:
https://godbolt.org/z/YoGzxo
