| Message ID | CA+55aFwfN1L6U=6+_a-Ue_Z6tPNfphKkpidtdD06zs+tHLPyJQ@mail.gmail.com (mailing list archive) |
|---|---|
| State | Not Applicable, archived |
| Headers | show |
On Fri, Aug 1, 2014 at 6:16 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote: > > Actually, sadly, while that worked for my test-cases (and for Josh's > case), it turns out to really *not* work in general: the dummy > EXPR_IDENTIFIER that we create may end up referring to a unnamed > union, which doesn't *have* an ident associated with it. > > This only really triggers when you mix named initializers with then > relying on the whole "initialize the next one without a name", but > that does happen. And it fundamentally breaks that whole "use > find_identifier()" approach. > > So convert_ident() really cannot use find_indent(), and really does > have to use "sym->offset". But that doesn't work for anonymous union > members, because that offset was the offset within the anonymous > union, not the parent structure. > > However, the fix is "simple". We can just say that anonymous > union/structure member offsets have to be the offsets within the > parent union instead, and fix things up. So how about a patch like > this *instead* of the previous patch.. Sorry I am late for the party. This approach assume the anonymous structure is only referenced inside the structure. There is not other external reference. However, that assumption is not true if "-fplan9-extensions" or "-fms-extensions" was enabled. Sparse current does not support these two extensions. But it will be a problem if we support them down the road. According to this gcc document: https://gcc.gnu.org/onlinedocs/gcc/Unnamed-Fields.html One of the example is this: struct s1 { int a; }; struct s2 { struct s1; }; You see, "struct s1" does not have a name, however "struct s1" can be referenced from other symbol. So update the symbol offset in "struct s1" will be wrong for the other reference of "struct s1". Actually, the "find_identifier" call inside function "check_designators" already calculate the correct offset. Only if we can pass that offset to "convert_ident()". Another idea is that, instead of using ident in EXPR_IDENTIFIER, we can do one time find_identifier(). Then convert the EXPR_IDENTIFIER to an index to the struct. It might take more than one index level for the anonymous struct or union. BTW, that is very similar to the LLVM performing the "Get Element Pointer" instruction. Using the index, we can still reference the anonymous struct. This is not a trivial change. However it might be the right thing to do if we want to support "-fplan9-extensions" Chris -- To unsubscribe from this list: send the line "unsubscribe linux-sparse" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Fri, Aug 1, 2014 at 10:16 PM, Christopher Li <sparse@chrisli.org> wrote: > > Sorry I am late for the party. This approach assume the anonymous > structure is only referenced inside the structure. It does indeed. I thought that was ok. > There is not other > external reference. However, that assumption is not true if "-fplan9-extensions" > or "-fms-extensions" was enabled. Sparse current does not support these > two extensions. But it will be a problem if we support them down the road. .. and in fact I think even without those things, you can just make the unnamed union have a type name, ie struct S { union T { int a; } }; the union T has a typename, but is a unnamed member of struct S. We could use "union T" later. > Actually, the "find_identifier" call inside function "check_designators" already > calculate the correct offset. Only if we can pass that offset to > "convert_ident()". I actually tried that initially, and wanted to get rid of the whole "check_designators()" vs "convert_designators()", but couldn't really find a sane way to make that work. I'll look at it again, maybe I missed something. LInus -- To unsubscribe from this list: send the line "unsubscribe linux-sparse" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Linus, > .. and in fact I think even without those things, you can just make > the unnamed union have a type name, ie > > struct S { > union T { > int a; > } > }; > > the union T has a typename, but is a unnamed member of struct S. We > could use "union T" later. No such luck, not allowed by the C Standard. 6.7.2.1p13 "An unnamed member whose type specifier is a structure specifier with no tag is called an anonymous structure; an unnamed member whose type specifier is a union specifier with no tag is called an anonymous union" But you can do this sort of thing when the type specifier is not anonymous.
On Sat, Aug 2, 2014 at 11:31 AM, Derek M Jones <derek@knosof.co.uk> wrote: >> .. and in fact I think even without those things, you can just make >> the unnamed union have a type name, ie >> >> struct S { >> union T { >> int a; >> } >> }; >> >> the union T has a typename, but is a unnamed member of struct S. We >> could use "union T" later. > > > No such luck, not allowed by the C Standard. > > 6.7.2.1p13 > "An unnamed member whose type specifier is a structure specifier with > no tag is called an anonymous structure; an unnamed member whose type > specifier is a union specifier with no tag is called an anonymous union" > > But you can do this sort of thing when the type specifier is not anonymous. That is what I recall as well, not part of stander C. If you enable the "-fplan9-extensions", Linus' example can pass. Chris -- To unsubscribe from this list: send the line "unsubscribe linux-sparse" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/evaluate.c b/evaluate.c index 03992d03ae1d..78150f83a19f 100644 --- a/evaluate.c +++ b/evaluate.c @@ -1898,7 +1898,7 @@ static struct symbol *find_identifier(struct ident *ident, struct symbol_list *_ sub = find_identifier(ident, ctype->symbol_list, offset); if (!sub) continue; - *offset += sym->offset; + *offset = sym->offset; return sub; } } diff --git a/symbol.c b/symbol.c index a2731348a011..32af64050a64 100644 --- a/symbol.c +++ b/symbol.c @@ -116,6 +116,18 @@ static int bitfield_base_size(struct symbol *sym) return sym->bit_size; } +static void update_symbol_offsets(struct symbol *sym, unsigned int offset) +{ + struct symbol *field; + + if (sym->type != SYM_STRUCT && sym->type != SYM_UNION) + return; + FOR_EACH_PTR(sym->symbol_list, field) { + field->offset += offset; + update_symbol_offsets(field, offset); + } END_FOR_EACH_PTR(field); +} + /* * Structures are a bit more interesting to lay out */ @@ -174,6 +186,10 @@ static void lay_out_struct(struct symbol *sym, struct struct_union_info *info) bit_size = (bit_size + align_bit_mask) & ~align_bit_mask; sym->offset = bits_to_bytes(bit_size); + /* If it's an unnamed struct or union, update the offsets of the sub-members */ + if (sym->offset && !sym->ident) + update_symbol_offsets(sym, sym->offset); + info->bit_size = bit_size + base_size; // warning (sym->pos, "regular: offset=%d", sym->offset); }