[cfe-dev] Determining Static vs Dynamic Dispatch

[Gregory Malecha via cfe-dev]

Thanks, David, that's very helpful! I haven't been digging into the
code-generation portion of the code.

On Thu, May 28, 2020 at 8:02 PM David Blaikie <dblaikie at gmail.com> wrote:

> I found this by setting a breakpoint on "llvm::CallInst::CallInst" and
> looking around at the two call instructions created in your example - it
> looks like Clang's IR generation tests whether the call is qualified to
> decide whether it can call virtually or not:
>
>
> https://github.com/llvm-mirror/clang/blob/master/lib/CodeGen/CGExprCXX.cpp#L197
>
> On Thu, May 28, 2020 at 4:35 PM Gregory Malecha via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> Hello --
>>
>> I'm working on a tool that gives a formal semantics to the clang C++
>> abstract syntax tree (the project is available here:
>> github.com/bedrocksystems/cpp2v). I'm working on adding support for
>> virtual functions and I'm trying to understand whether a call in the AST is
>> a static call or a dynamic call and I'm having difficulty figuring out how
>> to do it reliably. Here's a simple program:
>>
>> struct A {
>>   virtual int foo() { return 100; }
>> };
>> struct B : public A {
>>   virtual int foo() { return 10; }
>> };
>> int test(B* b) {
>>   return b->foo() + b->B::foo();
>> }
>>
>> From what I understand (and what the compiler does) the left call
>> `b->foo()` uses dynamic dispatch to resolve `foo` in the most derived class
>> of the argument B. But the call on the right always calls B::foo (returning
>> 10) regardless of the most derived class of B. However, I don't understand
>> what function I can call on the CXXMemberCallExpr node that
>> distinguishes these two cases. getMethodDecl, getRecordDecl, and
>> getImplicitObjectArgument all seem to return the exact same information.
>>
>> How does clang differentiate between these two calls in order to get the
>> correct behavior?
>>
>> Thanks in advance for any pointers.
>>
>> --
>> gregory malecha
>> gmalecha.github.io
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
>

-- 
gregory malecha
gmalecha.github.io
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