On Mon, Jul 7, 2025 at 11:08 PM H.J. Lu <hjl.to...@gmail.com> wrote: > > On Mon, Jul 7, 2025 at 11:08 PM Jason Merrill <ja...@redhat.com> wrote: > > > > On 7/1/25 5:36 PM, H.J. Lu wrote: > > > 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: > > > > Yes, I understand that function parameter alignment can be different > > from other objects of that type. > > > > But since we have a PARM_DECL to represent that particular function > > parameter, it seems natural to represent that difference in the > > DECL_ALIGN of the PARM_DECL. If you don't, its DECL_ALIGN is wrong. > >
The parameter visible to the program does not need to share storage with the parameter space used for parameter passing which can happen in registers, or even partial in memory/registers or - as in this case - in memory that is not aligned as the formal parameter is. In such a case GCC arranges for local storage (with proper alignment) that is populated from the argument storage (but it doesn't happen for the case in question due to a bug IMO). > __alignof returns TYPE_ALIGN, not DECL_ALIGN. For PARM_DECL, > TYPE_ALIGN may not be the same as DECL_ALIGN. I don't think it's wise to try to "fix" this in the frontend. A DECL_ALIGN that is smaller than TYPE_ALIGN sounds bogus to me - while DECL_ALIGN should always be the ultimate source of truth having a larger TYPE_ALIGN just means the TYPE_ALIGN is wrong. But the type is what the user controls here, so we can't support the case in question (lower DECL_ALIGN). Richard. > > -- > H.J.
