https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92086
Bug ID: 92086
Summary: Provide way to avoid saving callee-saved registers in
functions without callers
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: enhancement
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: fw at gcc dot gnu.org
Target Milestone: ---
In some cases, it is desirable as an optimization not to save any callee-saved
registers in the function prologue. This is common for functions which are at
the lowest frame, where there is nothing to return to, and unwinding cannot
proceed, either. However, GCC seems to generate code for saving registers even
for such functions, for example:
int f1 (int);
__attribute__ ((noreturn, nothrow))
void
f2 (void)
{
int x1 = f1 (1);
int x2 = f1 (2);
int x3 = f1 (3);
int x4 = f1 (4);
f1 (x1);
f1 (x2);
f1 (x3);
f1 (x4);
__builtin_unreachable ();
}
yields this on x86-64 (with GCC 9):
f2:
pushq %r14
movl $1, %edi
pushq %r13
pushq %r12
pushq %rbp
subq $8, %rsp
call f1@PLT
movl $2, %edi
movl %eax, %r14d
call f1@PLT
movl $3, %edi
movl %eax, %r13d
call f1@PLT
movl $4, %edi
movl %eax, %r12d
call f1@PLT
movl %r14d, %edi
movl %eax, %ebp
call f1@PLT
movl %r13d, %edi
call f1@PLT
movl %r12d, %edi
call f1@PLT
movl %ebp, %edi
call f1@PLT
If it is not possible to unwind into the caller of f2 (say because it does not
exist), there is no impact on debugging experience because the saved values are
useless even for debugging.
I've reported this bug against the C front end because we may need a new
attribute for this. (If noreturn+nothrown cannot be repurposed.)
Mailing list discussion: https://gcc.gnu.org/ml/gcc-help/2019-10/msg00052.html
