[cfe-dev] Assignment expression in C++ returns its right operand (A possible bug)

[Xu Zhongxing]

clang -cc1 -ast-dump
would show there is an lvalue-to-rvalue cast for 'x=s;'

The relevant code is in ScalarExprEmitter::VisitBinAssign(const 
BinaryOperator *E).

于 2012/2/17 6:37, Xiaolong Tang 写道:
> Hi everyone,
>
> I found a surprise in the llvm code generated by clang. (I am using
> clang version 3.0. And the revision is git-svn-id:
> https://llvm.org/svn/llvm-project/cfe/trunk@137823). Though my
> original example is the swap function for STL string, I created one
> simple example as below to show the problem I found.
>
> class A {
> public:
>    int *foo(int *s) {
>      return x = s;
>    }
> private:
>    int *x;
> };
>
> int main() {
>    A a;
>    int x;
>    int *px =&x;
>    a.foo(px);
>    return 0;
> }
>
> In the above example, foo supposes to return x. However, its
> corresponding llvm bitcode (by clang) seems to return s. The llvm
> bitcode is as below:
>
> define linkonce_odr i32* @_ZN1A3fooEPi(%class.A* %this, i32* %s) nounwind uwtable ssp align 2 {
> entry:
>    %this.addr = alloca %class.A*, align 8, !tbaa !10, !clang.vardecl.type !10
>    %s.addr = alloca i32*, align 8, !tbaa !5, !clang.vardecl.type !5
>    store %class.A* %this, %class.A** %this.addr, align 8, !tbaa !10
>    store i32* %s, i32** %s.addr, align 8, !tbaa !5
>    %this1 = load %class.A** %this.addr, !tbaa !10
>    %tmp = load i32** %s.addr, align 8, !tbaa !5
>    %x = getelementptr inbounds %class.A* %this1, i32 0, i32 0
>    store i32* %tmp, i32** %x, align 8, !tbaa !5
>    ret i32* %tmp
> }
>
> Note that the register value denoted by %tmp is returned, but %tmp is
> the value of the second parameter. (This does not agree with the c++
> semantics.)
>
> Again I am using clang version 3.0. And the revision is git-svn-id:
> https://llvm.org/svn/llvm-project/cfe/trunk@137823
>
> If I am correct and the above problem has been fixed, can someone
> point to me the commit where the problem is fixed? If I am somehow
> wrong, can someone give me any explanation?
>
> Thanks!
>
> Xiaolong
>
>
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