NVIDIA’s Experience with Open64 Mike Murphy NVIDIA

© NVIDIA Corporation 2008 Outline Why Open64 How we use Open64 What we did to Open64 Future work in Open64

© NVIDIA Corporation 2008 Compiling CUDA for GPUs NVCC C/C++ CUDA Application GPU CodeCPU CodeGPU Code executable

© NVIDIA Corporation 2008 Why Open64 We had a low-level code generator for graphics codes, but for CUDA needed high-level optimization for C/C++ codes. owngccopen64

© NVIDIA Corporation 2008 Why Open64 We had a low-level code generator for graphics codes, but for CUDA needed high-level optimization for C/C++ codes. owngccopen64 take too long

© NVIDIA Corporation 2008 Why Open64 We had a low-level code generator for graphics codes, but for CUDA needed high-level optimization for C/C++ codes. owngccopen64 take too long good long- term support

© NVIDIA Corporation 2008 Why Open64 We had a low-level code generator for graphics codes, but for CUDA needed high-level optimization for C/C++ codes. owngccopen64 take too long good long- term support best performance (kudos to PathScale)

© NVIDIA Corporation 2008 NVCC processing of GPU code cudafe C code for GPU nvopencc (Open64) ptx OCG object code

© NVIDIA Corporation 2008 Changes: Rehosting Open64 Our compiler has to run on 32 & 64bit Linux, 32 & 64bit Windows, and Mac OS. Main Open64 source tree is only for Linux. This is an area where sharing our changes can help grow the user base by making it easier to port Open64. For Windows we build using Cygwin’s MINGW

© NVIDIA Corporation 2008 Changes: Memory and registers We don’t have a stack or fast memory Therefore want to keep data in registers Inline everything and optimize as much as possible Try to keep small structs in registers by expanding struct copies into field copies (versus taking address and generating loop to do byte copy)

© NVIDIA Corporation 2008 Changes: Vector loads and stores Coalesce adjacent loads and stores for performance Do this in CG: Iterate through ops, trying to add to vectors Check for intervening kills Change alignment and use dummy regs for padding if helps to create wider vector (e.g. may use 4-word vector for 3-word struct).

© NVIDIA Corporation 2008 Changes: 16bit optimization Cheaper to use 16bit registers and operations But C converts shorts to int. So add pass in CG that converts back to 16bit: Mark 16bit loads, stores, and converts Propagate 16bit-ness forwards and backwards Unmark 16bit-ness if cannot be 16bit Change remaining registers and instructions to be 16bit.

© NVIDIA Corporation 2008 Future work 1 person -> 4 people working with Open64 New application TBA Merging changes into trunk Thanks to Sun Chan and Shin! Investigating register pressure in WOPT Want better control of register pressure during optimization Investigating using other features (LNO, IPA, etc)

© NVIDIA Corporation 2008 Questions?