https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102355
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Severity|normal |enhancement
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I suspect is more related to how GCC handles frame pointers and stack pointers.
As seen by -fno-omit-frame-pointer with both GCC and LLVM.
The frame pointer needs to be 16 byte aligned and you can see LLVM generates
the same code as GCC does when this is true:
pushq %rbp
movq %rsp, %rbp
subq $16, %rsp
leaq -8(%rbp), %rdi
callq escape(unsigned long long&)
addq $16, %rsp
popq %rbp
retq
GCC omitting the frame pointer is not much difference than when it is not
omitting it except it just does not use the frame pointer at all.
subq $24, %rsp
leaq 8(%rsp), %rdi
call escape(unsigned long long&)
addq $24, %rsp
ret
So GCC stack frame is
GCC makes this stack layout:
8 bytes padding
8 bytes variable "local"
8 bytes padding (where the frame pointer would be stored)
8 bytes return address
It just happens LLVM decides to reuse that location for local variables.
