> On Jun 30, 2022, at 10:24 AM, Richard Biener <richard.guent...@gmail.com> > wrote: > > > >> Am 30.06.2022 um 16:08 schrieb Qing Zhao via Gcc-patches >> <gcc-patches@gcc.gnu.org>: >> >> >> >>> On Jun 29, 2022, at 5:14 PM, Martin Sebor <mse...@gmail.com> wrote: >>> >>> On 6/28/22 13:01, Qing Zhao wrote: >>>>> On Jun 28, 2022, at 2:49 PM, Jakub Jelinek <ja...@redhat.com> wrote: >>>>> >>>>> On Tue, Jun 28, 2022 at 06:29:01PM +0000, Qing Zhao wrote: >>>>>> >>>>>> >>>>>>> On Jun 28, 2022, at 2:22 PM, Jakub Jelinek <ja...@redhat.com> wrote: >>>>>>> >>>>>>>> On Tue, Jun 28, 2022 at 06:15:58PM +0000, Qing Zhao wrote: >>>>>>>>>> Because the flag just tells whether some array shouldn't be treated >>>>>>>>>> as (poor man's) >>>>>>>>>> flexible array member. We still need to find out if some FIELD_DECL >>>>>>>>>> is to >>>>>>>>>> be treated like a flexible array member, which is a minority of >>>>>>>>>> COMPONENT_REFs. >>>>>>>>>> struct S { int a; char b[0]; int c; } s; >>>>>>>>>> struct T { int d; char e[]; }; >>>>>>>>>> struct U { int f; struct T g; int h; } u; >>>>>>>>>> Neither s.b nor u.g.e is to be treated like flexible array member, >>>>>>>>>> no matter what -fstrict-flex-array= option is used. >>>>>>>>> >>>>>>>>> Then, to resolve this issue, we might need a opposite flag >>>>>>>>> DECL_IS_FLEXARRAY in FIELD_DECL? >>>>>>>>> >>>>>>>>> The default is FALSE for all FIELD_DECL. >>>>>>>> >>>>>>>> Doesn't matter whether it is positive or negative, you still need to >>>>>>>> analyze >>>>>>>> it. See the above example. If you have struct T t; and test t.e, >>>>>>>> then it >>>>>>>> is flexarray. But u.g.e is not, even when the COMPONENT_REF refers to >>>>>>>> the >>>>>>>> same FIELD_DECL. In the t.e case e is the very last field, in the >>>>>>>> latter >>>>>>>> case u.g.e is the last field in struct T, but struct U has the h field >>>>>>>> after >>>>>>> >>>>>>> So, do you mean that the current FE analysis will not be able to decide >>>>>>> whether a specific array field is at the end of the enclosing structure? >>>>>>> Only the middle end can decide this ? >>>>>> >>>>>> Well, anything that analyzes it, can be in the FE or middle-end, but >>>>>> there >>>>>> is no place to store it for later. >>>> Then I am a little confused: >>>> If the FE can decide wether an array field is at the end of the enclosing >>>> structure, then combined with whether it’s a [0], [1] or [], and which >>>> level of -fstrict-flex-array, >>>> The FE should be able to decide whether this array field is a flexible >>>> array member or not, then set the flag DECL_IS_FLEXARRAY (or >>>> DECL_NOT_FLEXARRAY). >>>> The new flag is the place to store such info, right? >>>> Do I miss anything here? >>> >>> I think the problem is that there is just one FIELD_DECL for member >>> M of a given type T but there can be more than one instance of that >>> member, one in each struct that has a subobject of T as its own >>> member. Whether M is or isn't a (valid) flexible array member >>> varies between the two instances. >> >> Okay, I see. >> A FIELD_DECL might be shared by multiple structure or unions, and whether >> it’s a flexible array member varies between different enclosing structures >> or unions. >> Therefore FIELD_DECL cannot carry the flexible array member information >> accurately. > > 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? 2. For the following example: struct AX { int n; short ax[];}; 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? Qing > > >> Then, how about encoding the flexible array member information into the >> enclosing structure or union? >> >> >> Another thing is: All this complexity is caused by GNU extension which >> permits the flexible array >> member not at the end of the struct. (As I mentioned in a previous email, I >> listed here again) >> >> For example the following two examples: >> >> 1. [opc@qinzhao-ol8u3-x86 trailing_array]$ cat t1.c >> struct AX >> { >> int n; >> short ax[]; >> int m; >> }; >> >> void warn_ax_local (struct AX *p) >> { >> p->ax[2] = 0; >> } >> >> 2. [opc@qinzhao-ol8u3-x86 trailing_array]$ cat t2.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]$ gcc -O2 -Wall t1.c -S >> t4.c:4:9: error: flexible array member not at end of struct >> 4 | short ax[]; >> >> [opc@qinzhao-ol8u3-x86 trailing_array]$ gcc -O2 -Wall t2.c -S >> >> It’s clear to see that in the above t1.c, GCC reports error when the >> flexible array member is Not at the end of the structure (AX) that >> immediately enclosing the field. >> However, for t2.c, when the flexible array member is Not at the end of the >> structure that does not immediately enclosing it (UX), then it’s accepted. >> >> I am very confused about t2.c, is the struct UX a correct declaration? >> >> Thanks. >> >> Qing >> >>> >>> Martin >>