Switching callstack for C++ functions

Question

Here's my previous question about switching C callstacks. However, C++ uses a different calling convention (thiscall) and may require some different asm code. Can someone explain the differences and point to or supply some code snippets that switch C++ callstacks (preferably in GCC inline asm)?

Thanks, James

Answer 1

The code given in the previous question should work fine.

The thiscall calling convention differs only in who is responsible for popping the arguments off the stack. Under the thiscall calling convention, the callee pops the arguments (and additionally, the this pointer is passed in ecx); under the C calling convention, the caller pops the arguments. This does not affect context switches.

However, if you're going to do context switches yourself, note that you need to save and restore the registers as well (probably on the stack) in addition to switching stacks.

Note, by the way, that C++ doesn't always use thiscall -- it's only used for methods with a fixed number of arguments (and apart from that, it's a Microsoftism... g++ doesn't use it).

Answer 2

Note the ABI for C++ is not explicitly defined.

The idea was that compiler manufactures are able to use the optimal calling convention for the situation and thus make C++ faster.

The down side of this is that each compiler has its own calling convention thus code from different compilers are not compatable (even code form different versions (or even different optimization flags) of the same compiler can be incompatable).