[cfe-commits] r117220 - in /cfe/trunk: lib/Analysis/CFG.cpp test/Sema/warn-unreachable.c

[Zhongxing Xu]

2010/10/27 Marcin Świderski <marcin.sfider at gmail.com>

> 2010/10/26 Frits van Bommel <fvbommel at gmail.com>
>
> On Tue, Oct 26, 2010 at 4:53 AM, Zhongxing Xu <xuzhongxing at gmail.com>
>> wrote:
>> > Only statements containing control flow could cause 'Block' to be NULL,
>> e.g.
>> > DoStmt, WhileStmt, CastStmt. They do occur in the RHS of assignments,
>> but
>> > they do not occur in the LHS of assignments. So I think it's safe here.
>> Or I
>> > could miss something?
>>
>> The following code is accepted by both gcc and clang in c++ mode, even
>> if -pedantic and/or -std=c++98 is passed:
>> =====
>> int* iftrue();
>> int* iffalse();
>>
>> void conditional_assign (int cond, int val) {
>>    (cond ? *iftrue() : *iffalse()) = val;
>> }
>> =====
>>
>> Neither will compile it in C mode as-is, but both are fine with it if
>> I put move the dereference to before the opening bracket. (Again, even
>> with -pedantic and/or (-std=c89 or -std=c99)
>> Apparently C doesn't preserve the lvalue-ness of the conditional
>> operator's operands, but C++ does. Not really surprising given that
>> C++ has an int& type but C doesn't; in C++ their types are int& while
>> in C they're probably plain old ints.
>>
>> Anyway, this is an example of code clang (like gcc) compiles without
>> complaining that has control flow in the LHS of an assignment
>> operator.
>>
>> I didn't check whether statement expressions are allowed in the LHS,
>> but that could be another case.
>>
>
> I've found code that did crash the CFGBuilder (with RHS then LHS visiting
> order for assignment operator) in test/Analysis/misc-ps-region-store.m:
>
> struct list_pr8141
> {
>   struct list_pr8141 *tail;
> };
>
> struct list_pr8141 *
> pr8141 (void) {
>   struct list_pr8141 *items;
>   for (;; items = ({ do { } while (0); items->tail; })) //
> expected-warning{{Dereference of undefined pointer value}}
>     {
>     }
> }
>
> After switching to LHS then RHS visiting order (as in repository) this will
> work fine, but it's just a one case. I did play with the code a little and
> came up with this piece:
>
> struct list_pr8141 *
> pr8141 (void) {
>   struct list_pr8141 *items;
>   while (items - ({ do { } while (0); items->tail; })) {}
> }
>
> This will crash the CFGBuilder as it is in repository. After fixing
> VisitBinaryOperator() to always return a block this will still crash, but in
> VisitWhileStmt on assert on line 1739. This have been run with command used
> for misc-ps-region-store.m test file.
>

Hi Marcin,

I fixed this case in r117436.
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