(implicit) argument display problem about uftrace

[namhyung]

In Itanium C++ ABI (which is the basis of the linux C++ ABI):

3.1.4 Return Values

In general, C++ return values are handled just like C return values. This includes class type results returned in registers. However, if the return value type has a non-trivial copy constructor or destructor, the caller allocates space for a temporary, and passes a pointer to the temporary as an implicit first parameter preceding both the this parameter and user parameters. The callee constructs the return value into this temporary.

https://refspecs.linuxfoundation.org/cxxabi-1.86.html#calls

[honggyukim]

Hi @namhyung , thanks for the specific answer. But my concern is that if it's possible to know whether I have to give -A bar@arg1 or -A bar@arg2 for such function to print the first argument.

[namhyung]

I think it should be handled in uftrace if it knows about the debug info. But I don't know how much info would be provided from the debuginfo.

[honggyukim]

So it seems that it's not possible to handle this case as of now. Debuginfo support may be required to fix this in the future.

[namhyung]

With gcc 8.1 and -O2 option, it can find the correct argument of bar from the debug info. (foo was optimized out). But it still gets an incorrect value when -O0.

$ g++ -O2 -g -pg test.cc

$ uftrace -A bar -R bar a.out
# DURATION    TID     FUNCTION
            [  3345] | main() {
  15.967 us [  3345] |   bar(10) = 0x7ffd64b2f410;
   1.777 us [  3345] |   operator delete();
  23.460 us [  3345] | } /* main */