https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85697
Bug ID: 85697
Summary: At -Os nontrivial ctor does not use SSE to zero
Product: gcc
Version: 8.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: msharov at users dot sourceforge.net
Target Milestone: ---
struct alignas(16) A {
A (void) :a(0),b(0),c(0),d(0) {}
int a,b,c,d;
};
__attribute__((noinline)) void UseA (A& a) { a.a=1; }
int main (void)
{
A a {};
UseA (a);
return a.a;
}
-Os -march=native on Haswell, generates:
main:
subq $16, %rsp
movq %rsp, %rdi
movq $0, (%rsp)
movq $0, 8(%rsp)
call _Z4UseAR1A
movl (%rsp), %eax
addq $16, %rsp
ret
Using 16 bytes to zero A with 2 movq. With -O3:
main:
subq $24, %rsp
vpxor %xmm0, %xmm0, %xmm0
movq %rsp, %rdi
vmovaps %xmm0, (%rsp)
call _Z4UseAR1A
movl (%rsp), %eax
addq $24, %rsp
ret
using only 9 bytes for pxor/movaps. With -mno-avx it is 7 bytes for
xorps/movaps. With multiple objects of type A, the savings would be even
greater, since only one pxor would be needed for all and only 4 bytes per
object for zeroing.
Removing A constructor also results in SSE instruction use.
