Inhibit saving callee-saved registers for noreturn+nothrow functions

[Florian Weimer <fweimer@xxxxxxxxxx>]

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

Is there a way to avoid this?

This would allow us to reduce stack usage of every thread by a couple of
words.

Thanks,
Florian