> 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?) 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? > >> 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? 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