[cfe-dev] [analyzer] Temporaries.

Artem Dergachev via cfe-dev cfe-dev at lists.llvm.org
Thu Feb 15 19:01:07 PST 2018


While all three more or less work, a combination of the three - 
temporary destructors, broken lifetime extension, and copy elision - 
causes massive false positives at the moment. Consider:

      1    #include <memory>
      2
      3    void use(const char *);
      4
      5    void foo() {
      6      char *p = new char[10];
      7      std::unique_ptr<char []> x = std::unique_ptr<char[]>(p);
      8      use(p);
      9    }

This causes a use-after free warning, even though everything we ever 
wanted was inlined. Here's the AST for line 7:

  DeclStmt 0x7fddb999d8a8 <line:7:3, col:58>
  `-VarDecl 0x7fddba8ad738 <col:3, col:57> col:28 x 
'std::unique_ptr<char []>':'std::__1::unique_ptr<char [], 
std::__1::default_delete<char []> >' cinit
    `-ExprWithCleanups 0x7fddb999d890 <col:32, col:57> 
'std::unique_ptr<char []>':'std::__1::unique_ptr<char [], 
std::__1::default_delete<char []> >'
      `-CXXConstructExpr 0x7fddb999d858 <col:32, col:57> 
'std::unique_ptr<char []>':'std::__1::unique_ptr<char [], 
std::__1::default_delete<char []> >' 'void (std::__1::unique_ptr<char 
[], std::__1::default_delete<char []> > &&) noexcept' elidable
        `-MaterializeTemporaryExpr 0x7fddb999d840 <col:32, col:57> 
'std::unique_ptr<char []>':'std::__1::unique_ptr<char [], 
std::__1::default_delete<char []> >' xvalue
          `-CXXFunctionalCastExpr 0x7fddb999b7f0 <col:32, col:57> 
'std::unique_ptr<char []>':'std::__1::unique_ptr<char [], 
std::__1::default_delete<char []> >' functional cast to 
std::unique_ptr<char []> <ConstructorConversion>
            `-CXXBindTemporaryExpr 0x7fddb999b7d0 <col:32, col:57> 
'std::unique_ptr<char []>':'std::__1::unique_ptr<char [], 
std::__1::default_delete<char []> >' (CXXTemporary 0x7fddb999b7c8)
              `-CXXConstructExpr 0x7fddb999b6a0 <col:32, col:57> 
'std::unique_ptr<char []>':'std::__1::unique_ptr<char [], 
std::__1::default_delete<char []> >' 'void (char *, typename 
enable_if<__same_or_less_cv_qualified<char *, pointer>::value, 
__nat>::type) noexcept'
                |-ImplicitCastExpr 0x7fddb999ad88 <col:56> 'char *' 
<LValueToRValue>
                | `-DeclRefExpr 0x7fddba8ad958 <col:56> 'char *' lvalue 
Var 0x7fddba8ad1d0 'p' 'char *'
                `-CXXDefaultArgExpr 0x7fddb999b680 <<invalid sloc>> 
'typename enable_if<__same_or_less_cv_qualified<char *, pointer>::value, 
__nat>::type':'std::__1::unique_ptr<char [], 
std::__1::default_delete<char []> >::__nat'

So we're constructing a temporary unique_ptr, binding the temporary for 
subsequent destruction, functional-casting it (no-op), materializing it 
for elidable move-construction, doing elidable move-construction into 
variable 'x', then destroying the original temporary.

During move-construction, we're erasing our pointer from the temporary 
and transfer it into 'x'.

However, due to the previous MaterializeTemporaryExpr, which "is" "the" 
"lifetime extension" ("through elidable move"), we have incorrectly 
created a copy of the temporary. So during move-construction we've 
erased our pointer in the copy of the temporary, but not in the original 
temporary.

The destructor, however, is deleting the original correct temporary, not 
the erroneous copy! And the original temporary still owns the pointer.

It's not possible for MaterializeTemporaryExpr to inform the destructor 
that the address has changed because our MaterializeTemporaryExpr 
doesn't know the old address that needs to be replaced.

Sounds like it's time to do something about lifetime extension.

On 14/02/2018 1:20 PM, Artem Dergachev wrote:
> More explanations on how the analyzer keeps making its way around the 
> C++ AST.
>
> == Lifetime extension ==
>
> This is a brain dump of how (and how much) lifetime extension of 
> temporary objects is currently broken in the static analyzert. 
> Spoilers: not too much, it seems, but annoying nevertheless.
>
> Consider an example:
>
>      1    class C {
>      2    public:
>      3      C() {}
>      4      ~C() {}
>      5    };
>      6
>      7    void foo() {
>      8      const C &c = C();
>      9    }
>
> With the AST for the variable declaration:
>
>       DeclStmt 0x7fa5ac85cba0 <line:8:3, col:19>
>       `-VarDecl 0x7fa5ac85c878 <col:3, col:18> col:12 c 'const C &' cinit
>         `-ExprWithCleanups 0x7fa5ac85cb30 <col:16, col:18> 'const C' 
> lvalue
>           `-MaterializeTemporaryExpr 0x7fa5ac85cb00 <col:16, col:18> 
> 'const C' lvalue extended by Var 0x7fa5ac85c878 'c' 'const C &'
>             `-ImplicitCastExpr 0x7fa5ac85cae8 <col:16, col:18> 'const 
> C' <NoOp>
>               `-CXXBindTemporaryExpr 0x7fa5ac85cac8 <col:16, col:18> 
> 'C' (CXXTemporary 0x7fa5ac85cac0)
>                 `-CXXTemporaryObjectExpr 0x7fa5ac85ca88 <col:16, 
> col:18> 'C' 'void ()'
>
> *here goes a periodic reminder that CXXTemporaryObjectExpr is a 
> sub-class of CXXConstructExpr*
>
> Notice how MaterializeTemporaryExpr is the innermost expression (the 
> first one in the order of execution) that is an lvalue. Essentially, 
> you can think of it as the mythical "rvalue-to-lvalue" cast that takes 
> in a C++ object rvalue and returns the this-value for that object. 
> Because all C++ objects have a this-value that never changes 
> throughout their lifetime, it is essentially their identity. Otherwise 
> you can't call methods on them.
>
> MaterializeTemporaryExpr also contains information about the lifetime 
> extension process: we needed the this-value in order to bind it to 
> variable 'c'. You see that in the AST.
>
> In the analyzer, however, MaterializeTemporaryExpr does a different 
> thing, as a temporary solution (no pun intended). It constructs a new 
> temporary region out of thin air and binds the rvalue object to that 
> temporary in the Store. The respective function in our code is called 
> "createTemporaryRegionIfNeeded". It also has a separate purpose of 
> converting rvalue sub-object adjustments into lvalue sub-object 
> adjustments, which we wouldn't discuss this time.
>
> Now that we learned how to inline temporary constructors and 
> destructors, it essentially means that the this-value in the 
> constructor and in the destructor would be different. Because first we 
> construct the object into temporary region R1, then we take 
> lazyCompoundVal{R1} to represent the value of CXXTemporaryObjectExpr, 
> then we materialize lazyCompoundVal{R1} to R2, then we bind R2 to 
> variable 'c', then we call the automatic(!) destructor for 'c' which 
> contains R2. To be clear, the region store at the time of destruction 
> would be:
>
>   c: R2,
>   R2: lazyCompoundVal{R1}.
>
> It means that fields of the object would contain the correct values, 
> there would be the correct number of destructors called (no temporary 
> destructors, just one automatic destructor), but the identity of the 
> object (this-value) would change in the process. Unless the object 
> actually makes any decisions or does any manipulations that involve 
> its this-value, the modeling should be correct. When the object starts 
> to actively use its this-value in its inlined methods, the analysis 
> would do wrong stuff. Additionally, it causes a mess in the checkers 
> when they try to track objects by their this-values - i.e. 
> IteratorChecker has a lot of additional code to work around the fact 
> that the region for the object constantly changes.
>
> From the above it is clear that MaterializeTemporaryExpr should not 
> construct any new regions, at least not in this case. We already have 
> the correct region, namely R1, which should be re-used.
>
> It is tempting to take R1 directly from lazyCompoundVal{R1} - it 
> already has memories about once being a value of R1. I'm not sure it's 
> not going to work - it may work, at least i'm not ready to come up 
> with a counterexample. But the meaning of LazyCompoundVal's parent 
> region is different and coincides with what we want just accidentally. 
> Namely, lazyCompoundVal{R1} is a value of an object that was bound to 
> R1 in some particular moment of time in the past, without any 
> explanation of when this moment of time was - but there's no 
> indication if R1 is the region of the temporary we've just 
> constructed, or a region of an unrelated object that used to have the 
> same value back then. As we'd see later, MaterializeTemporaryExpr 
> doesn't always contain a constructor within it - there are a lot of 
> cases to cover, and if the trick doesn't work even once, it's going to 
> be hard, so i'd probably not going to commit into maintaining this 
> invariant. Though it might be plausible to modify add an SVal kind 
> that does exactly what we mean here - i.e. a value that does indeed 
> correspond to a specific C++ object identified by region. It might be 
> a beautiful solution, but it may also fail miserably if tricky 
> cornercases show up - so i'm not ready to commit to that. Also the 
> correct way of dealing with such values (i.e. for how long are they 
> relevant?) would be extremely difficult to explain to checker developers.
>
> The more straightforward approach here is to include 
> MaterializeTemporaryExpr (hereinafter MTE) into the construction 
> context. It means, if a temporary that we're about to construct would 
> be lifetime-extended later, we'd rather know about that during 
> construction, and maintain a map in the program state from MTE to 
> their respective temporary regions that were used for representing the 
> respective construction targets. Upon encountering the respective MTE 
> we'd simply retrieve the implicit temporary storage for the value from 
> the program state and declare that temporary region to be the value of 
> the MTE. This would mimic the approach we have with 
> CXXBindTemporaryExprs (hereinafter BTE) and their respective regions 
> that allows temporary destructors to work - but this time it's going 
> to be about MaterializeTemporaryExprs and automatic destructors. I 
> imagine that on the checker side this can potentially be exposed via 
> some sort of State->getTemporaryStorage(Expr) call, but i believe that 
> generally this process should be as transparent to the checkers as 
> possible.
>
> It sounds as if both of these maps could be eliminated by always 
> making sure that the target temporary is constructed "with" the MTE 
> (for lifetime-extended temproraries) or BTE (for temporaries that 
> require destruction at the end of full-expression). In this case, with 
> the help of construction context-assisted lookahead, we declare that 
> the target of the construction is CXXTempObjectRegion(MTE, LC) or 
> CXXTempObjectRegion(BTE, LC) respectively, rather than 
> CXXTempObjectRegion(CXXConstructExpr). Then during evaluation of MTE 
> or BTE we'd simply construct the same region with the expression we're 
> currently evaluating, and it's automagically going to be the correct 
> region. This approach, however, becomes confusing when we start 
> dealing with elidable constructors (explained below). So for now i 
> believe that it is quite irrelevant which expression is identifying 
> the temporary region.
>
> == Elidable constructors ==
>
> While it doesn't sound like an immediately important task to implement 
> copy elision in the analyzer, it may help with making some things 
> easier. And it'd also make some reports fancier, as mentioned in 
> https://reviews.llvm.org/D43144.
>
> Elidable copy-constructors can be explained as a form of lifetime 
> extension. Instead of copying the temporary, they keep using the 
> original value of the temporary, which in some pretty twisted sense 
> means that they are lifetime-extending it to be able to use it. For 
> example, if we modify our example by replacing the lifetime-extending 
> reference variable with a value-type variable:
>
>      1    class C {
>      2    public:
>      3      C() {}
>      4      ~C() {}
>      5    };
>      6
>      7    void foo() {
>      8      C c = C();
>      9    }
>
> ...then we'd still have an MTE, even though lifetime extension would 
> seem to be gone:
>
>       DeclStmt 0x7fb8f005afb8 <line:8:3, col:12>
>       `-VarDecl 0x7fb8f005ac50 <col:3, col:11> col:5 c 'C' cinit
>         `-ExprWithCleanups 0x7fb8f005afa0 <col:9, col:11> 'C'
>           `-CXXConstructExpr 0x7fb8f005af68 <col:9, col:11> 'C' 'void 
> (const C &) noexcept' elidable
>             `-MaterializeTemporaryExpr 0x7fb8f005af00 <col:9, col:11> 
> 'const C' lvalue
>               `-ImplicitCastExpr 0x7fb8f005aee8 <col:9, col:11> 'const 
> C' <NoOp>
>                 `-CXXBindTemporaryExpr 0x7fb8f005aec8 <col:9, col:11> 
> 'C' (CXXTemporary 0x7fb8f005aec0)
>                   `-CXXTemporaryObjectExpr 0x7fb8f005ae88 <col:9, 
> col:11> 'C' 'void ()'
>
> In this case the MTE is expressing the fact that the temporary 
> constructed via CXXTemporaryObjectExpr can be "lifetime-extended" (by 
> actually merging it with the stack variable) rather than copied, if 
> the CXXConstructExpr surrounding it would be chosen to be elided. The 
> AST doesn't make the elision choice for us - but is compatible with 
> both choices. The MTE essentially overrides the necessity of immediate 
> destruction provided by the BTE, and lets the surrounding AST decide 
> upon the lifetime of the object.
>
> Because the analyzer currently does not do copy elision, it will use 
> the MTE only to compute the argument for the elidable 
> copy-constructor, and then perform the copy-construction, and then 
> destroy the original temporary at the end of the full-expression. 
> Note, however, that in this case we need to properly support both the 
> BTE (for the temporary destructor to work) and the MTE (for computing 
> its value). We need to implement the MTE's ability to perform 
> "rvalue-to-lvalue-cast" even if the temporary destruction is still 
> necessary. For this reason, if we rely on constructing temporary 
> regions with the correct BTEs or MTEs, at least one of these tasks 
> becomes impossible to perform.
>
> If we were to support copy elision, then the CXXTemporaryObjectExpr 
> constructor would go directly into the variable region. For the 
> purposes of modeling, it'd mean that only CXXTemporaryObjectExpr would 
> actually need to be modeled. But this would require additional coding 
> in the construction context to be able to realize that the target is 
> the variable while modeling the CXXTemporaryObjectExpr.
>
> For the sake of completeness, let's consider the ternary operator 
> example:
>
>      1    class C {
>      2    public:
>      3      C(int) {}
>      4      ~C() {}
>      5    };
>      6
>      7    void foo(int coin) {
>      8      const C &c = coin ? C(1) : C(2);
>      9    }
>
> The respective AST would be:
>
>       DeclStmt 0x7fc1e20023e0 <line:8:3, col:34>
>       `-VarDecl 0x7fc1e2001dc8 <col:3, col:33> col:12 c 'const C &' cinit
>         `-ExprWithCleanups 0x7fc1e2002370 <col:16, col:33> 'const C' 
> lvalue
>           `-MaterializeTemporaryExpr 0x7fc1e2002340 <col:16, col:33> 
> 'const C' lvalue extended by Var 0x7fc1e2001dc8 'c' 'const C &'
>             `-ImplicitCastExpr 0x7fc1e2002328 <col:16, col:33> 'const 
> C' <NoOp>
>               `-ConditionalOperator 0x7fc1e20022f8 <col:16, col:33> 'C'
>                 |-ImplicitCastExpr 0x7fc1e2002170 <col:16> 'bool' 
> <IntegralToBoolean>
>                 | `-ImplicitCastExpr 0x7fc1e2002158 <col:16> 'int' 
> <LValueToRValue>
>                 |   `-DeclRefExpr 0x7fc1e2001e28 <col:16> 'int' lvalue 
> ParmVar 0x7fc1e2001c18 'coin' 'int'
>                 |-CXXBindTemporaryExpr 0x7fc1e2002248 <col:23, col:26> 
> 'C' (CXXTemporary 0x7fc1e2002240)
>                 | `-CXXConstructExpr 0x7fc1e2002208 <col:23, col:26> 
> 'C' 'void (const C &) noexcept' elidable
>                 |   `-MaterializeTemporaryExpr 0x7fc1e20021a0 <col:23, 
> col:26> 'const C' lvalue
>                 |     `-ImplicitCastExpr 0x7fc1e2002188 <col:23, 
> col:26> 'const C' <NoOp>
>                 |       `-CXXFunctionalCastExpr 0x7fc1e2002078 
> <col:23, col:26> 'C' functional cast to class C <ConstructorConversion>
>                 |         `-CXXBindTemporaryExpr 0x7fc1e2002058 
> <col:23, col:26> 'C' (CXXTemporary 0x7fc1e2002050)
>                 |           `-CXXConstructExpr 0x7fc1e2002018 <col:23, 
> col:26> 'C' 'void (int)'
>                 |             `-IntegerLiteral 0x7fc1e2001e60 <col:25> 
> 'int' 1
>                 `-CXXBindTemporaryExpr 0x7fc1e20022d8 <col:30, col:33> 
> 'C' (CXXTemporary 0x7fc1e20022d0)
>                   `-CXXConstructExpr 0x7fc1e2002298 <col:30, col:33> 
> 'C' 'void (const C &) noexcept' elidable
>                     `-MaterializeTemporaryExpr 0x7fc1e2002280 <col:30, 
> col:33> 'const C' lvalue
>                       `-ImplicitCastExpr 0x7fc1e2002268 <col:30, 
> col:33> 'const C' <NoOp>
>                         `-CXXFunctionalCastExpr 0x7fc1e2002130 
> <col:30, col:33> 'C' functional cast to class C <ConstructorConversion>
>                           `-CXXBindTemporaryExpr 0x7fc1e2002110 
> <col:30, col:33> 'C' (CXXTemporary 0x7fc1e2002108)
>                             `-CXXConstructExpr 0x7fc1e20020d0 <col:30, 
> col:33> 'C' 'void (int)'
>                               `-IntegerLiteral 0x7fc1e20020b0 <col:32> 
> 'int' 2
>
> Each branch contains two constructors: the temporary and the elidable 
> copy. The temporaries are surrounded with their respective BTEs and 
> copy-elision-kind MTEs, which indicates that they need to be either 
> destroyed as temporaries, or, if copy elision is chosen, have their 
> lifetime decided upon by the surrounding AST. The elidable copy 
> constructors also, being temporaries, have their respective BTEs. 
> Note, however, that there is only one MTE for both BTEs for the 
> elidable constructors.
>
> So after the conditional operator is resolved (which is the first 
> thing we need to do, according to the CFG), we'd go ahead and perform 
> the constructors, and their trigger would be their respective BTE in 
> the non-elide case, and the single top-level MTE in the elide case. In 
> the non-elide case, copy constructors would be triggered by the 
> top-level MTE.
>
> It means that, once again, copy elision would prevent us from handling 
> both the BTE and the copy-elision-kind MTE in the single construction, 
> allowing the "predictable target region" trick to work: when we need 
> the temporary destructor, we construct directly into 
> CXXTempObjectRegion of the BTE and it gets automatically picked up 
> during destruction, and when we need the automatic destructor, we 
> construct directly into CXXTempObjectRegion of the MTE and we can 
> easily compute the value of the MTE. But when we don't do copy 
> elision, we'd have to keep at least one of those in the program state 
> map.
>
> == Return by value ==
>
> Returning C++ objects by value is actually very similar to 
> constructing it. Consider:
>
>      1    class C {
>      2    public:
>      3      C() {}
>      4      ~C() {}
>      5    };
>      6
>      7    C bar() {
>      8      C c;
>      9      return c;
>     10    }
>     11
>     12    void foo() {
>     13      const C &c = bar();
>     14    }
>
> With the respective AST for DeclStmt in foo():
>
>       DeclStmt 0x7fe62c84f8e8 <line:13:3, col:21>
>       `-VarDecl 0x7fe62c84f6b8 <col:3, col:20> col:12 c 'const C &' cinit
>         `-ExprWithCleanups 0x7fe62c84f878 <col:16, col:20> 'const C' 
> lvalue
>           `-MaterializeTemporaryExpr 0x7fe62c84f848 <col:16, col:20> 
> 'const C' lvalue extended by Var 0x7fe62c84f6b8 'c' 'const C &'
>             `-ImplicitCastExpr 0x7fe62c84f830 <col:16, col:20> 'const 
> C' <NoOp>
>               `-CXXBindTemporaryExpr 0x7fe62c84f810 <col:16, col:20> 
> 'C' (CXXTemporary 0x7fe62c84f808)
>                 `-CallExpr 0x7fe62c84f7e0 <col:16, col:20> 'C'
>                   `-ImplicitCastExpr 0x7fe62c84f7c8 <col:16> 'C (*)()' 
> <FunctionToPointerDecay>
>                     `-DeclRefExpr 0x7fe62c84f770 <col:16> 'C ()' 
> lvalue Function 0x7fe62c84f190 'bar' 'C ()'
>
> And for the ReturnStmt in bar():
>
>       ReturnStmt 0x7fe62c84f5b0 <line:9:3, col:10>
>       `-CXXConstructExpr 0x7fe62c84f578 <col:10> 'C' 'void (const C &) 
> noexcept' elidable
>         `-ImplicitCastExpr 0x7fe62c84f518 <col:10> 'const C' lvalue 
> <NoOp>
>           `-DeclRefExpr 0x7fe62c84f4f0 <col:10> 'C' lvalue Var 
> 0x7fe62c84f280 'c' 'C'
>
> Since https://reviews.llvm.org/D42875 we can already realize that the 
> constructor in bar(), assuming that we're inlining bar() during 
> analysis, would be constructed into something that is a return value 
> of bar(). This allows us, by looking that the StackFrameContext's call 
> site, to figure out that it is being constructed into the CallExpr in 
> foo(). Now if only we knew that that the call site is a 
> lifetime-extended temporary, i.e. if only we had a pointer to the 
> foo()'s MTE at the CallExpr's CFG element, we'd be able to find the 
> correct target region for construction: the CXXTempObjectRegion for 
> the MTE in the StackFrameContext of foo(). So i'm proposing to add 
> some sort of construction context to not only constructors, but also 
> to functions that return objects, and then during construction perform 
> the lookup in three easy steps:
>
>   1. in the callee's CFG from constructor to return statement,
>   2. through the location from the return statement to the call site,
>   3. then through the caller's CFG from the call site to the MTE.
>
> If the function is not inlined, we can still make use of the 
> construction context to represent the return value as a 
> LazyCompoundValue of the MTE's temporary. It would eliminate the need 
> to replace the return value with another value while evaluating the 
> MTE, and of course the need to re-bind the object to a different 
> this-region.
>
> So i believe that this is a good way to eliminate the need for the 
> "createTemporaryRegionIfNeeded" thing in the function calls as well.
>
>
> On 06/02/2018 1:41 PM, Artem Dergachev wrote:
>> A bit of an update.
>>
>> == Temporary destructors ==
>>
>> Adding some initial support for temporary destructors seems pretty 
>> easy and straightforward at this point, given the awesome work done 
>> by Manuel Klimek on our CFG a few years ago.
>>
>> 1. We already have the awesome CFGTemporaryDtor elements, which have 
>> the backreference to the CXXBindTemporaryExpr for their temporaries.
>>
>> 2. In simple cases CXXBindTemporaryExprs have an immediate 
>> constructor within them, and therefore we can provide the 
>> CXXBindTemporaryExprs as the construction context's trigger 
>> statements, and therefore have a reliable CXXTempObjectRegion for 
>> constructors.
>>
>> 3. Then we already track the CXXBindTemporaryExprs for the active 
>> temporaries in the program state. We can attach any constructor 
>> information to them, such as the target region, if we need to 
>> (probably we can reconstruct it by looking at the expression and the 
>> location context, not sure if we want to).
>>
>> 4. So when we reach the CFGTemporaryDtor element, we can just lookup 
>> all the info we need, and perform the destruction properly.
>>
>> 5. There's a bit of a liveness problem, because it seems that our 
>> liveness analysis tends to throw away the temporary too early. I can 
>> easily hack this problem away by marking all regions that correspond 
>> to active temporaries as live. I'll see if there's a better solution.
>>
>> == CXXDefaultArgExpr problems ==
>>
>> There's a known problem with those. Consider:
>>
>>   void foo(const C &c = C()) {
>>   }
>>
>>   void bar() {
>>     foo();
>>     foo();
>>   }
>>
>> Each call of foo() contains a CXXDefaultArgExpr for c. The default 
>> argument value C() is constructed before we enter foo() and destroyed 
>> after we leave foo(). However, c's initializer, "= C()", is *not part 
>> of the AST of bar()*. In particular, when foo() is called twice, the 
>> initializer for the two calls is the same, only CXXDefaultArgExprs 
>> are different. This screws a lot of invariants in the analyzer: 
>> program points start coinciding (causing the analysis to loop and 
>> cache out), Environment doesn't expect the same expression in the 
>> same location context have two different values (suppose calls are 
>> nested into each other), analysis taking wrong branches, and so on.
>>
>> Luckily, default-arg expressions that aren't zero integers or null 
>> pointers are pretty rare. Still, we'd need to eventually think how to 
>> avoid any really bad practical problems with them.
>>
>> On 25/01/2018 9:08 AM, Artem Dergachev wrote:
>>> Handling C++ temporary object construction and destruction seems to 
>>> be the biggest cause of false positives on C++ code at the moment. 
>>> I'd be looking into this, even though for now i don't see the whole 
>>> scale of problem.
>>>
>>> == CFG, destructors, and ProgramPoints ==
>>>
>>> We should probably enable `-analyzer-config 
>>> cfg-temporary-dtors=true` by default soon. It is a fairly low-impact 
>>> change because it only alters the CFG but the analyzer rarely 
>>> actually uses the new nodes. Destructors for the most part are still 
>>> evaluated conservatively, with improper object regions. So it causes 
>>> almost no changes in the analyzer's positives for now, but it 
>>> definitely opens up room for further improvements.
>>>
>>> I'm observing a couple of problems with this mode at the moment, 
>>> namely the rich CFG destructor element hierarchy is not currently 
>>> complemented by an equally rich ProgramPoint hierarchy. This causes 
>>> the analysis to merge nodes which are not equivalent, for example 
>>> two implicit destructors of the same type (with the same function 
>>> declaration) may sometimes cause the ExplodedGraph to coil up and 
>>> stop analysis (lost coverage) because of having the same program 
>>> state at the erroneously-same program point. Because situations when 
>>> we can guarantee a change in the program state are pretty rare, we'd 
>>> have to produce more program point kinds to handle this correctly.
>>>
>>> CallEvent hierarchy is also very much affected in a similar manner - 
>>> because apparently we're constructing program points by looking at 
>>> CallEvents, so they'd need to carry all the information that's 
>>> needed to construct the pre-call/post-call program point.
>>>
>>> == Construction contexts ==
>>>
>>> We are correctly modeling "this" object region during 
>>> construction/destruction of variables with automatic storage 
>>> duration, fields and base classes, and on operator new() since 
>>> recently, as long as these aren't arrays of objects. It was not yet 
>>> implemented for other cases such as temporaries, initializer lists, 
>>> fields or C++17 base classes within aggregates, and pass-by-value 
>>> from/to functions (the latter being a slightly different problem 
>>> than temporaries).
>>>
>>> First of all, by "not yet implemented" i mean that instead of 
>>> constructing into (destroying) the correct object (in the correct 
>>> memory region), we come up with a "temporary region", which looks 
>>> exactly like a region of a valid C++ temporary but is only used for 
>>> communicating that it is not the right region. Then we look at the 
>>> region, see that it is a temporary, and avoid inlining constructors, 
>>> because it would make little sense when the region is not correct. 
>>> However, if we are to model temporaries, we need another way of 
>>> communicating our failure to find the correct region, which is being 
>>> addressed by https://reviews.llvm.org/D42457
>>>
>>> Also in the cases when the correct region is used, it is being 
>>> computed in a funky way: in order to figure out where do we 
>>> construct the object, we walk forward in the CFG (or from child to 
>>> parent in the AST) to find the next/parent statement that would 
>>> accomodate the newly constructed object. This means that the CFG, 
>>> while perfectly telling us what to do in what order (which we, 
>>> unlike CodeGen, cannot easily take from AST because we cannot afford 
>>> recursive evaluation of statements, mainly because of inlining), 
>>> discards too much context to be helpful in understanding how to do it.
>>>
>>> I tried to add the information about such "trigger statements" for 
>>> constructors into CFG and it is extremely helpful and easy to both 
>>> use and extend. This assumes adding a new sub-class of CFGElement 
>>> for constructors, which maintain a "construction context" - for now 
>>> it's just a trigger statement. For instance, in
>>>
>>>   class C { ... };
>>>   void foo() {
>>>     C c;
>>>   }
>>>
>>> ...the trigger for constructor C() would be DeclStmt `C c`, and once 
>>> we know this we can immediately figure out that the construction 
>>> target is the region of variable `c`. Construction trigger is not 
>>> necessarily a statement - it may be a CXXCtorInitializer, which is 
>>> an AST node kind of its own. Additionally, when constructing 
>>> aggregates via initializer lists, we may have the same statement 
>>> trigger multiple constructors, eg. in
>>>
>>>   class C { public: C(int); ~C(); };
>>>   struct S { C c1, c2, c3; };
>>>   void foo() {
>>>     S s { 1, 2, 3 };
>>>   }
>>>
>>> ... we have three different constructors (actually six different 
>>> constructors if we include elidable copy constructors) for c1, c2, 
>>> c3 (and lack of constructor for `s` because of the aggregate thing). 
>>> It would be more natural to declare that the specific field or index 
>>> would be a part of the CFG's construction context, as well as the 
>>> intermediate InitListExpr, so even in these simple cases the 
>>> construction context may get quite bulky. And this example is quite 
>>> popular on actual code - probably the second worst cause of false 
>>> positives after temporaries.
>>>
>>> For now i have no specific plan on what would construction context 
>>> for temporaries contain in the general case. I might not be able to 
>>> get the data structures right from the start. In any case, it might 
>>> be necessary to perform additional memory allocations for these CFG 
>>> elements (for analyzer's CFG only, so it wouldn't affect compilation 
>>> time or warnings).
>>>
>>> I believe that getting the correct target region in as many cases as 
>>> possible would be the main part of my work for the nearest future. 
>>> And i want to move most this work to CFG, while letting the analyzer 
>>> pick up the trigger statement from it and trust it as blindly as 
>>> possible.
>>>
>>> == Workflow ==
>>>
>>> When working on operator new, i tried hard to maintain a reasonable 
>>> history, making around 15 local rebases. It was not awesome because 
>>> it was hard for the reviewers to understand the context of the new 
>>> changes, and changes could have easily kicked in during rebases. A 
>>> few lessons learned here would be to commit more aggressively, i.e. 
>>> avoiding stockpiling a large history of patches (essentially a large 
>>> branch), which in turn would be possible through trying harder to 
>>> avoid unrelated hard-to-test core changes (as long as it doesn't 
>>> require weird workarounds) that aren't covered by a flag (such as 
>>> those evalCast fixes), in order to make sure reviewing take less 
>>> responsibility. It's fine if some parts would later be fixed 
>>> (assuming they would indeed be fixed), if it means making the 
>>> turnaround faster and the tail of patches shorter - that's also the 
>>> attitude i'd try to maintain when reviewing stuff myself.
>>
>



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