D842 - A program that reads its own address is translated for it

Status: decided Date: 2026-10-14

s_getpc_b64 writes the address of the instruction after it into a scalar register pair. A module built from a program containing it is translated for the guest address the program was loaded at. The address is a translation input, UserData::program_address. With it, the instruction writes that address plus the next instruction's offset as constants. Without it, the instruction is refused by name.

The pipeline sets the address only for a program that contains s_getpc_b64, and salts the module's key with it. Such a program is a different module at each place it is loaded, and every other program stays one module wherever it is. A computed buffer access (D753) carries the address too, so a prefix that forms a base from the program counter runs the same per draw.

Why: ACO places a shader's constant data after its code and reaches it through s_getpc_b64 and a carry-chained add. PPSA21564's vertex shaders do this. The address is the guest's own fact about the program, as the address a draw's registers name it at, so it is read, not assumed.

Rejected:

• Folding the address into every module's key: identical programs at two places would be two modules for no reason, and the kept translations would stop matching across runs that load a program somewhere else.
• Answering the counter with zero, or with a fixed address: the constant data would be read from somewhere the guest never wrote.