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). > 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. > 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 > > >