On 8/20/19 1:26 AM, Richard Biener wrote:
> On Tue, Aug 20, 2019 at 4:32 AM Martin Sebor <mse...@gmail.com> wrote:
>>
>> On 8/19/19 8:10 AM, Richard Biener wrote:
>>> On Sat, Aug 17, 2019 at 12:43 AM Martin Sebor <mse...@gmail.com> wrote:
>>>>
>>>> With the recent enhancement to the strlen handling of multibyte
>>>> stores the g++.dg/warn/Warray-bounds-4.C for zero-length arrays
>>>> started failing on hppa (and probably elsewhere as well). This
>>>> is partly the result of the added detection of past-the-end
>>>> writes into the strlen pass which detects more instances of
>>>> the problem than -Warray-bounds. Since the IL each warning
>>>> works with varies between targets, the same invalid code can
>>>> be diagnosed by one warning one target and different warning
>>>> on another.
>>>>
>>>> The attached patch does three things:
>>>>
>>>> 1) It enhances compute_objsize to also determine the size of
>>>> a flexible array member (and its various variants), including
>>>> from its initializer if necessary. (This resolves 91457 but
>>>> introduces another warning where was previously just one.)
>>>> 2) It guards the new instance of -Wstringop-overflow with
>>>> the no-warning bit on the assignment to avoid warning on code
>>>> that's already been diagnosed.
>>>> 3) It arranges for -Warray-bounds to set the no-warning bit on
>>>> the enclosing expression to keep -Wstringop-overflow from issuing
>>>> another warning for the same problem.
>>>>
>>>> Testing the compute_objsize enhancement to bring it up to par
>>>> with -Warray-bounds in turn exposed a weakness in the latter
>>>> warning for flexible array members. Rather than snowballing
>>>> additional improvements into this one I decided to put that
>>>> off until later, so the new -Warray-bounds test has a bunch
>>>> of XFAILs. I'll see if I can find the time to deal with those
>>>> either still in stage 1 or in stage 3 (one of them is actually
>>>> an ancient regression).
>>>
>>> +static tree
>>> +get_initializer_for (tree init, tree decl)
>>> +{
>>>
>>> can't you use fold_ctor_reference here?
>>
>> Yes, but only with an additional enhancement. Char initializers
>> for flexible array members aren't transformed to STRING_CSTs yet,
>> so without the size of the initializer specified, the function
>> returns the initializer for the smallest subobject, or char in
>> this case. I've enhanced the function to handle them.
>
> So at the moment it returns an empty array constructor, correct?
> Isn't that the more canonical representation? The STRING_CST
> index type doesn't really handle "empty" strings and I expect code
> more confused about that than about an empty CTOR?
>
>>>
>>> +/* Determine the size of the flexible array FLD from the initializer
>>> + expression for the struct object DECL in which the meber is declared
>>> + (possibly recursively). Return the size or zero constant if it isn't
>>> + initialized. */
>>> +
>>> +static tree
>>> +get_flexarray_size (tree decl, tree fld)
>>> +{
>>> + if (tree init = DECL_INITIAL (decl))
>>> + {
>>> + init = get_initializer_for (init, fld);
>>> + if (init)
>>> + return TYPE_SIZE_UNIT (TREE_TYPE (init));
>>> + }
>>> +
>>> + return integer_zero_node;
>>>
>>> so you're hoping that the (sub-)CONSTRUCTOR get_initializer_for
>>> returns has a complete type but the initialized object didn't get it
>>> completed. Isnt that wishful thinking?
>>
>> I don't know what you mean. When might a CONSTRUCTOR not have
>> a complete type, and if/when it doesn't, why would that be
>> a problem here? TYPE_SIZE_UNIT will evaluate to null meaning
>> "don't know" and that's fine. Could you try to be more specific
>> about the problem you're pointing out?
>>
>>> And why return integer_zero_node
>>> rather than NULL_TREE here?
>>
>> Because the size of a flexible array member with no initializer
>> is zero.
>>
>>>
>>> + if (TREE_CODE (dest) == COMPONENT_REF)
>>> + {
>>> + *pdecl = TREE_OPERAND (dest, 1);
>>> +
>>> + /* If the member has a size return it. Otherwise it's a flexible
>>> + array member. */
>>> + if (tree size = DECL_SIZE_UNIT (*pdecl))
>>> + return size;
>>>
>>> because here you do.
>>
>> Not sure what you mean here either. (This code was also a bit
>
> You return NULL_TREE.
>
>> out of date WRT to the patch I had tested. Not sure how that
>> happened. The attached patch is up to date.)
>>
>>>
>>> Also once you have an underlying VAR_DECL you can compute
>>> the flexarray size by DECL_SIZE (var) - offset-of flexarray member.
>>> Isn't that way cheaper than walking the initializer (possibly many
>>> times?)
>>
>> It would be nice if it were this easy. Is the value of DECL_SIZE
>> (var) supposed to include the size of the flexible array member?
>
> Yes, DECL_SIZE of the VAR_DECL is the size we use for assembling.
> It is usually only the types that remain incomplete (or too small) since
> the FE doesn't create many variants of a struct S { int n; char x[]; }
> when "instantiating" it via struct S s = { 5, "abcde" }; that then holds
> true for the DECL_SIZE of the FIELD_DECL of x as well.
>
>> I don't see it mentioned in the comments in tree.h and in my tests
>> it only does in C but not in C++. Is that a bug that in C++ it
>> doesn't?
>
> tree.h dcoumentation isn't complete. And for me the C++ FE for
> the above simple example has
>
> <var_decl 0x7ffff7fedb40 s
> type <record_type 0x7ffff6d94f18 S cxx-odr-p type_5 type_6 BLK
> size <integer_cst 0x7ffff6c5e0a8 constant 32>
> unit-size <integer_cst 0x7ffff6c5e0c0 constant 4>
> align:32 warn_if_not_align:0 symtab:0 alias-set -1
> canonical-type 0x7ffff6d94f18
> fields <function_decl 0x7ffff6d98b00 __dt type <method_type
> 0x7ffff6da8150>
> public abstract external autoinline decl_3 QI t.ii:1:8
> align:16 warn_if_not_align:0 context <record_type 0x7ffff6d94f18 S>
> full-name "S::~S() noexcept (<uninstantiated>)"
> not-really-extern chain <function_decl 0x7ffff6d98d00
> __dt_base >> context <translation_unit_decl 0x7ffff6c4a168 t.ii>
> full-name "struct S"
> X() X(constX&) this=(X&) n_parents=0 use_template=0 interface-unknown
> pointer_to_this <pointer_type 0x7ffff6da85e8>
> reference_to_this <reference_type 0x7ffff6da8a80> chain <type_decl
> 0x7ffff6c6b850 S>>
> public static tree_1 tree_2 tree_3 BLK t.ii:3:3 size <integer_cst
> 0x7ffff6c5e0a8 32> unit-size <integer_cst 0x7ffff6c5e0c0 4>
> align:32 warn_if_not_align:0 context <translation_unit_decl
> 0x7ffff6c4a168 t.ii> initial <constructor 0x7ffff6da52a0> chain
> <type_decl 0x7ffff6c6b850 S>>
>
> and we assemble it like
>
> .globl s
> .data
> .align 4
> .type s, @object
> .size s, 4
> s:
> .long 5
> .string "abcde"
>
> note how we have .size s, 4 here...
>
> of course in the end nobody cares about a symbols size ...(?)
The linker may care. I've certainly run across those which would
warn/error when the size of an object with global scope differed across
TUs. I wouldn't expect to be running into that kind of issue in the
modern world though.
Jeff
