llvm-project

00653431 - [NVPTX] Improve device function byval parameter lowering (#129188)

[NVPTX] Improve device function byval parameter lowering (#129188) PTX supports 2 methods of accessing device function parameters: - "simple" case: If a parameters is only loaded, and all loads can address the parameter via a constant offset, then the parameter may be loaded via the ".param" address space. This case is not possible if the parameters is stored to or has it's address taken. This method is preferable when possible. - "move param" case: For more complex cases the address of the param may be placed in a register via a "mov" instruction. This mov also implicitly moves the param to the ".local" address space and allows for it to be written to. This essentially defers the responsibilty of the byval copy to the PTX calling convention. The handling of these cases in the NVPTX backend for byval pointers has some major issues. We currently attempt to determine if a copy is necessary in NVPTXLowerArgs and either explicitly make an additional copy in the IR, or insert "addrspacecast" to move the param to the param address space. Unfortunately the criteria for determining which case is possible are not correct, leading to miscompilations (https://godbolt.org/z/Gq1fP7a3G). Further, the criteria for the "simple" case aren't enforceable in LLVM IR across other transformations and instruction selection, making deciding between the 2 cases in NVPTXLowerArgs brittle and buggy. This patch aims to fix these issues and improve address space related optimization. In NVPTXLowerArgs, we conservatively assume that all parameters will use the "move param" case and the local address space. Responsibility for switching to the "simple" case is given to a new MachineIR pass, NVPTXForwardParams, which runs once it has become clear whether or not this is possible. This ensures that the correct address space is known for the "move param" case allowing for optimization, while still using the "simple" case where ever possible.