Lunderberg opened a new pull request, #17103:
URL: https://github.com/apache/tvm/pull/17103
Prior to this PR, a TIR PrimFunc could return an `int64`, `float64`, or
void. Returning any other type, even if supported by TVM's FFI, would raise an
exception during `MakePackedAPI`. With this PR, string literals can be
returned from TIR PrimFuncs.
Enabling this TIR functionality requires a change to the TVM FFI.
Previously, a `TVMRetValue` holding a string would allocate a
`std::string*` and store it to `value_.v_handle`. This return value depends
on the C++ runtime, and cannot be constructed on all backends that only require
the C API. With this commit, this is instead represented as a `const char*` in
`value_.v_str`, matching the representation in `TVMArgValue`. Unlike
`TVMArgValue`, since p`TVMRetValue` may own its return value, two new member
variables `void (*f_deleter_)(void*)` and `void* f_deleter_arg_`. These are
designed to be easy to call from a C API, and are set to a deleter function
that will free the allocation (if required), and an argument to be passed into
the deleter function.
With this new representation, the return value for different use cases would
be set as follows:
* Return string from C++ PackedFunc. The backing allocation is made using
`new std::string(std::move(value))`. This heap allocation is stored in
`f_deleter_arg_`, so that `f_deleter_` can then call `delete
static_cast<std::string*>(arg)`. Finally, `new_str->data()` is stored as
`value.v_str`.
* Return pointer to static string from C. A pointer to the static string is
stored in `v_str`. Both `f_deleter_` and `f_deleter_arg_`are left with their
default value of `nullptr`, indicating that the `TVMRetValue` does not need to
free any memory on destruction.
This is the return value used by `T.ret(T.StringImm("my_string"))` in a
TIR PrimFunc.
* Return pointer to heap allocation made in C. Assuming the allocation is
made using `malloc()`, a pointer to the heap allocation is stored in both
`value_.v_str` and `f_deleter_arg_`. The `f_deleter_` is set to `free`,
indicating that `free(f_deleter_arg_)` should be called when the `TVMRetValue`
is destructed.
This functionality is possibly within the updated `TVMRetValue` API, but
is not currently used.
This commit adds unit tests for the return of strings from compiled TIR
PrimFuncs, when either running the function locally, or over a RPC connection.
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