Re: [PATCH] c-family: Check backend for argument alignment on stack

[H.J. Lu]

On Tue, Jul 1, 2025 at 9:37 PM Jason Merrill <ja...@redhat.com> wrote:
>
> On 6/30/25 7:03 PM, H.J. Lu wrote:
> > On Mon, Jun 30, 2025 at 10:36 PM Jason Merrill <ja...@redhat.com> wrote:
> >>
> >> On 6/28/25 7:00 AM, H.J. Lu wrote:
> >>> Since a backend may ignore user type alignment for arguments passed on
> >>> stack, check backend for argument alignment on stack when evaluating
> >>> __alignof.
> >>
> >> I assume that's reflected in DECL_ALIGN, so could we just add PARM_DECL to
> >
> > No.  targetm.calls.function_arg_boundary may have special handling for it.
>
> Why wouldn't we adjust DECL_ALIGN of the PARM_DECL to reflect the actual
> alignment of the argument?  Are you saying it could be different from
> one call to another?

Function argument alignment is different from other places in memory if
the main variant type alignment is different:

tatic unsigned int
ix86_function_arg_boundary (machine_mode mode, const_tree type)
{
  unsigned int align;
  if (type)
    {
      /* Since the main variant type is used for call, we convert it to
         the main variant type.  */
      type = TYPE_MAIN_VARIANT (type);
      align = TYPE_ALIGN (type);
      if (TYPE_EMPTY_P (type))
        return PARM_BOUNDARY;
    }

> >>>    else if (VAR_OR_FUNCTION_DECL_P (expr))
> >>>      t = size_int (DECL_ALIGN_UNIT (expr));
> >>
> >> ?
> >>
> >> Jason
> >>
> >
> >
>


-- 
H.J.