https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102096
Bug ID: 102096
Summary: Gcc unnecessarily initializes indeterminate variables
passed across function boundaries
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: pskocik at gmail dot com
Target Milestone: ---
Compared to clang where:
long ret_unspec(void){ auto long rv; return rv; }
void take6(long,long,long,long,long,long);
void call_take6(void)
{
//6 unnecessary XORs on GCC
auto long id0; //indeterminate
auto long id1; //indeterminate
auto long id2; //indeterminate
auto long id3; //indeterminate
auto long id4; //indeterminate
auto long id5; //indeterminate
take6(id0,id1,id2,id3,id4,id5);
}
yields (x86_64):
ret_unspec: # @ret_unspec2
retq
call_take6: # @call_take6
jmp take6
(1+5 bytes), GCC compiles the above to
ret_unspec2:
xorl %eax, %eax
ret
call_take6:
xorl %r9d, %r9d
xorl %r8d, %r8d
xorl %ecx, %ecx
xorl %edx, %edx
xorl %esi, %esi
xorl %edi, %edi
jmp take6
(3+19 bytes), unnecessarily 0-initializing the indeterminate
return-value/arguments.
Type casting the called function can often be hackishly used to get the same
assembly but doing so is technically UB and not as generic as supporting the
passing of unspecified arguments/return values, which can be used to omit
argument register initializations not just for arguments at the end of an
argument pack but also in the middle.
TL;DR: Allowing to passing/return indeterminate variables without generating
initializing code for them would be nice. Clang already does it.
