https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92714
Bug ID: 92714
Summary: [missed-optimization] aggregate initialization of an
array fills the whole array with zeros first,
including non-zero elements
Product: gcc
Version: 8.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: lassie.darkorbit at gmail dot com
Target Milestone: ---
void *sink;
void bar() {
int a[100]{1,2,3,4};
sink = a; // a escapes the function
asm("":::"memory"); // and compiler memory barrier
// forces the compiler to materialize a[] in memory instead of optimizing
away
}
gcc 8.1 and gcc 9.2 both make asm like this (even with -O3):
bar():
push edi # save call-preserved EDI which rep stos
uses
xor eax, eax # eax=0
mov ecx, 100 # repeat-count = 100
sub esp, 400 # reserve 400 bytes on the stack
mov edi, esp # dst for rep stos
mov DWORD PTR sink, esp # sink = a
rep stosd # memset(a, 0, 400)
mov DWORD PTR [esp], 1 # then store the non-zero initializers
mov DWORD PTR [esp+4], 2 # over the zeroed part of the array
mov DWORD PTR [esp+8], 3
mov DWORD PTR [esp+12], 4
add esp, 400 # cleanup the stack
pop edi # and restore caller's EDI
ret
