https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89820
Bug ID: 89820
Summary: Returning empty type produces unnecessary instructions
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
Consider the following code
struct my_type_impl {};
my_type_impl foo0() { return {}; }
my_type_impl foo1() { my_type_impl tmp; return tmp; }
For `foo0` and `foo1` GCC generates the following assembly:
xor eax, eax
ret
However xoring the `eax` seems unnecessary and some of the other compilers just
generate the `ret` instruction.
The additional `xor` instruction could significantly increase the code size for
generic C++ programs. For example in Bug 89819 and Bug 89816 each of he 36 jump
table entries has that additional instruction.
