On Fri, Jul 1, 2022 at 2:55 PM Qing Zhao <qing.z...@oracle.com> wrote:
>
>
>
> > On Jul 1, 2022, at 2:49 AM, Richard Biener <richard.guent...@gmail.com>
> > wrote:
> >
> > On Thu, Jun 30, 2022 at 9:30 PM Qing Zhao <qing.z...@oracle.com> wrote:
> >>
> >>
> >>
> >>> On Jun 30, 2022, at 1:03 PM, Jakub Jelinek <ja...@redhat.com> wrote:
> >>>
> >>> On Thu, Jun 30, 2022 at 03:31:00PM +0000, Qing Zhao wrote:
> >>>>> No, that’s not true. A FIELD_DELC is only shared for cv variants of a
> >>>>> structure.
> >>>>
> >>>> Sorry for my dump questions:
> >>>>
> >>>> 1. What do you mean by “cv variants” of a structure?
> >>>
> >>> const/volatile qualified variants. So
> >> Okay. I see. thanks.
> >>>
> >>>> 2. For the following example:
> >>>>
> >>>> struct AX { int n; short ax[];};
> >>>
> >>> struct AX, const struct AX, volatile const struct AX etc. types will share
> >>> the FIELD_DECLs.
> >>
> >> Okay.
> >>>
> >>>> struct UX {struct AX b; int m;};
> >>>>
> >>>> Are there two different FIELD_DECLs in the IR, one for AX.ax, the other
> >>>> one is for UX.b.ax?
> >>>
> >>> No, there are just n and ax FIELD_DECLs with DECL_CONTEXT of struct AX and
> >>> b and m FIELD_DECLs with DECL_CONTEXT of struct UX.
> >>
> >> Ah, right.
> >>
> >>
> >>>
> >>> But, what is important is that when some FIELD_DECL is last in some
> >>> structure and has array type, it doesn't mean it should have an
> >>> unconstrained length.
> >>> In the above case, when struct AX is is followed by some other member, it
> >>> acts as a strict short ax[0]; field (even when that is an exception), one
> >>> can tak address of &UX.b.ax[0], but can't dereference that, or
> >>> &UX.b.ax[1].
> >>
> >> So, is this a GNU extension. I see that CLANG gives a warning by default
> >> and GCC gives a warning when specify -pedantic:
> >> [opc@qinzhao-ol8u3-x86 trailing_array]$ cat t3.c
> >> struct AX
> >> {
> >> int n;
> >> short ax[];
> >> };
> >>
> >> struct UX
> >> {
> >> struct AX b;
> >> int m;
> >> };
> >>
> >> void warn_ax_local (struct AX *p, struct UX *q)
> >> {
> >> p->ax[2] = 0;
> >> q->b.ax[2] = 0;
> >> }
> >> [opc@qinzhao-ol8u3-x86 trailing_array]$ clang -O2 -Wall t3.c -S
> >> t3.c:9:13: warning: field 'b' with variable sized type 'struct AX' not at
> >> the end of a struct or class is a GNU extension
> >> [-Wgnu-variable-sized-type-not-at-end]
> >> struct AX b;
> >> ^
> >> [opc@qinzhao-ol8u3-x86 trailing_array]$ gcc -O2 -Wall t3.c -pedantic -S
> >> t3.c:9:13: warning: invalid use of structure with flexible array member
> >> [-Wpedantic]
> >> 9 | struct AX b;
> >> | ^
> >>
> >> But, Yes, I agree, even though this is only a GNU extension, We still need
> >> to handle it and accept it as legal code.
> >>
> >> Then, yes, I also agree that encoding the info of is_flexible_array into
> >> FIELD_DECL is not good.
> >
> > Which is why I suggested to encode 'not_flexible_array'. This way the
> > FE can mark all a[1] this way in some mode
> > but leave a[] as possibly flexarray (depending on context).
>
> Then, FE marking (not_flexible_array) can not do the complete job to mark
> whether a field array is flexible array member or not, Middle end still need
> to
> check the “context” (i.e, whether the array ref is at the end of a structure?)
Yes, but at the very "root" the frontend get's to say whether char[1]
is possibly
flexarray or if only char[] is.
> So, only FE marking + Middle-end “context checking” together will decide a
> REAL flex array?
>
> If so, comparing to the current implemenation to have all the checking in
> middle-end, what’s the
> major benefit of moving part of the checking into FE, and leaving the other
> part in middle-end?
Because a frontend might decide based on language rules that char[1]
is never a flexarray and
in particular it can decide to do that only for user declared
structures. In particular the latter is
difficult for the middle-end where some aggregates are built by the
middle-end (gcov) or the
targets.
> >
> >> How about encoding the info of “has_flexible_array” into the enclosing
> >> RECORD_TYPE or UNION_TYPE node?
> >
> > But that has the same issue. Consider
> >
> > struct A { int n; int a[1]; };
> >
> > where a is considered possibly a flexarray vs.
> >
> > struct B { struct A a; int b; };
> >
> > where B.a would be not considered to have a flexarray (again note
> > 'possibly' vs. 'actually does').
> >
> > Also
> >
> > struct A a;
> >
> > has 'a' as _not_ having a flexarray (because it's size is statically
> > allocated) but
> >
> > struct A *a;
> > struct B *b;
> >
> > a->a[n];
> >
> > as possibly accessing the flexarray portion of *a while
> >
> > b->a.a[n]
> >
> > is not accessing a flexarray because there's a member after a in b.
> >
> > For your original proposal it's really the field declaration itself
> > which changes so annotating the FIELD_DECL
> > seems correct to me.
>
> Then middle-end still need to check the context, and combined
> with the “not_flexible_array” flag that is encoded in FIELD_DECL
> to make the final decision?
Yes.
> Thanks.
>
> Qing
> >
> >> For example, in the above example, the RECORD_TYPE for “struct AX” will
> >> be marked as “has_flexible_array”, but that for “struct UX” will not.
> >>
> >>>
> >>> I believe pedantically flexible array members in such cases don't
> >>> necessarily mean zero length array, could be longer, e.g. for the usual
> >>> x86_64 alignments
> >>> struct BX { long long n; short o; short ax[]; };
> >>> struct VX { struct BX b; int m; };
> >>> I think it acts as short ax[3]; because the padding at the end of struct
> >>> BX
> >>> is so long that 3 short elements fit in there.
> >>> While if one uses
> >>> struct BX bx = { 1LL, 2, { 3, 4, 5, 6, 7, 8, 9, 10 } };
> >>> (a GNU extension), then it acts as short ax[11]; - the initializer is 8
> >>> elements and after short ax[8]; is padding for another 3 full elemenets.
> >>> And of course:
> >>> struct BX *p = malloc (offsetof (struct BX, ax) + n * sizeof (short));
> >>> means short ax[n].
> >>> Whether struct WX { struct BX b; };
> >>> struct WX *p = malloc (offsetof (struct WX, b.ax) + n * sizeof (short));
> >>> is pedantically acting as short ax[n]; is unclear to me, but we are
> >>> generally allowing that and people expect it.
> >>
> >> Okay, I see now.
> >>>
> >>> Though, on the GCC side, I think we are only treating like flexible arrays
> >>> what is really at the end of structs, not followed by other members.
> >>
> >> My understanding is, Permitting flexible array to be followed by other
> >> members is a GNU extension. (Actually, it’s not allowed by standard?).
> >>
> >> Thanks a lot for your patience and help.
> >>
> >> Qing
> >>>
> >>> Jakub
>
