RFR: 8360389: Support printing from C2 compiled code

[Tobias Hartmann]

On Fri, 25 Jul 2025 09:39:00 GMT, Benoît Maillard <bmaillard at openjdk.org> wrote:

> This PR adds support for printf-style debugging from C2 compiled code. This is implemented as a runtime call to a C++ method that prints the values of the nodes passed as arguments. The runtime C++ method is implemented with the help of variadic templates, as it is expected to support various combinations of argument types.
> 
> ## Usage
> 
> Suppose we have this piece of Java code, that simply computes an arithmetic operation, and
> that we compile `square` with C2.
> 
> class Square {
>     static int square(int a) {
>         return a * a;
>     }
> 
>     public static void main(String[] args) {
>         square(9);
>     }
> }
> 
> 
> We would like to inspect the node that contains the value returned in this method.
> We can add a call to `Compile::make_debug_print` and pass a message, the IGVN instance, as well as the node(s) that we would like to inspect.
> 
> ```c++
> void Compile::return_values(JVMState* jvms) {
>   GraphKit kit(jvms);
>   Node* ret = new ReturnNode(TypeFunc::Parms,
>                              kit.control(),
>                              kit.i_o(),
>                              kit.reset_memory(),
>                              kit.frameptr(),
>                              kit.returnadr());
>   // Add zero or 1 return values
>   int ret_size = tf()->range()->cnt() - TypeFunc::Parms;
> 
>   
>   Node* return_value;
>   if (ret_size > 0) {
>     kit.inc_sp(-ret_size);  // pop the return value(s)
>     kit.sync_jvms();
>     return_value = kit.argument(0);
>     ret->add_req(return_value);
> 
>     // <-------------------- Simply insert this
>     C->make_debug_print_new<jint>("return:", initial_gvn(), return_value);
>   }
>   
>   // bind it to root
>   root()->add_req(ret);
>   record_for_igvn(ret);
>   initial_gvn()->transform(ret);
> }
> 
> 
> We can then call run our code with `-XX:CompileCommand="compileonly,Square::square`
> and we get the following output:
> 
> 
> return:
> int 81
> 
> 
> This case is of course trivial, but more useful examples are shown later.
> 
> ## Implementation
> ## Implementation
> 
> The debugging capability is implemented as a runtime call to a C++ printing method. For this, `Compile::make_debug_print` inserts a `CallLeafNode` into the graph and rewires control flow as needed.
> 
> The actual printing is handled by the `SharedRuntime::debug_print` method, written with a variadic template to support various argument type combinations. A pointer to this runtime method is obtained at compile time and is passed to the `CallLeafNode` constructor.
> 
> The first argument to the runtime printing method is the printing message, a static ...

Great work Benoît! This will be very useful for debugging.

With the current changes, we only support adding printing during parsing, i.e. when `GraphKit` is available. I think it would be great if we could add printing after parsing, for example only once a specific IGVN or loop optimization happened. Do you think that's feasible?

src/hotspot/share/opto/graphKit.hpp line 784:

> 782:     address call_addr = CAST_FROM_FN_PTR(address, SharedRuntime::debug_print<TT...>);
> 783: 
> 784:     Node* str_node = new ConPNode(TypeRawPtr::make(((address) str)));

Don't you need a `transform` here?

src/hotspot/share/opto/runtime.cpp line 1787:

> 1785:   switch (parm->bottom_type()->basic_type()) {
> 1786:     case T_BOOLEAN:
> 1787:       fields[(*argp)++] = TypeInt::BOOL;

I think you can use `Type::get_const_basic_type(parm->bottom_type()->basic_type())` here and special case `T_DOUBLE` / `T_LONG`.

src/hotspot/share/opto/runtime.cpp line 1813:

> 1811:       break;
> 1812:     case T_OBJECT:
> 1813:       fields[(*argp)++] = TypePtr::NOTNULL;

I think passing null should be okay as well, right? I would use `TypeInstPtr::BOTTOM` here, which will be taking care of by `Type::get_const_basic_type`.

-------------

PR Review: https://git.openjdk.org/jdk/pull/26475#pullrequestreview-3060479577
PR Review Comment: https://git.openjdk.org/jdk/pull/26475#discussion_r2234764495
PR Review Comment: https://git.openjdk.org/jdk/pull/26475#discussion_r2234762106
PR Review Comment: https://git.openjdk.org/jdk/pull/26475#discussion_r2234763252