https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106977
--- Comment #23 from Iain Sandoe <iains at gcc dot gnu.org> --- (In reply to ibuclaw from comment #21) > There is something about the Darwin ABI I'm just not getting from looking at > the front-end alone though: > > C++ Darwin 32-bit calling a method that returns an 8 byte struct: > ``` > __Z4testP3Bar: > subl $24, %esp > movl 28(%esp), %eax > movl (%eax), %edx > pushl %eax // <-- > call *(%edx) > ``` Darwin does not need an "sret" parameter since it will return the struct in registers. So it only passes "this". > C++ Linux 32-bit calling a method that returns an 8 byte struct: > ``` > _Z4testP3Bar: > subl $44, %esp > movl %gs:20, %edx > movl %edx, 28(%esp) > xorl %edx, %edx > movl 48(%esp), %eax > leal 8(%esp), %ecx > subl $8, %esp > movl (%eax), %edx > pushl %eax // <-- > pushl %ecx // <-- > call *(%edx) > ``` The Linux (SysV ABI arch-specific addition) says that structures are returned via an sret param - for all size of struct. So the ABIs differ in this (as noted on IRC, the Darwin 32b ABIs are not the same as Linux).
