[Bug libgomp/81377] New: [nvptx] Add possibility for libgomp nvptx plugin to use ptxas

["vries at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81377

            Bug ID: 81377
           Summary: [nvptx] Add possibility for libgomp nvptx plugin to
                    use ptxas
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: libgomp
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
                CC: jakub at gcc dot gnu.org
  Target Milestone: ---

An openacc executable contains ptx code, and uses the libgomp plugin to:
- compile the code using the JIT compiler into a cubin module
- load the cubin module
- execute one or more kernels in the cubin module

[ Context: There is a number of patches pending (
https://gcc.gnu.org/ml/gcc-patches/2017-07/msg00167.html ) which add
environment variables to control this process:
- GOMP_NVPTX_PTXRW=w: Write the embedded ptx to files
- GOMP_NVPTX_PTXRW=r: Ignore the embedded ptx, and read from files instead 
- GOMP_NVPTX_JIT: Add opions to JIT command line
- GOMP_NVPTX_SAVE_TEMPS: save the generated cubin module to file
- GOMP_NVPTX_DISASM: disassemble the generated cubin module to stdout
]

Instead of using the JIT compiler, we could use ptxas instead. That way, we can
see if a JIT bug also reproduces with ptxas. Furthermore, it's easier to try
out multiple versions of ptxas, since we don't have to install another driver
version.

We could redefine GOMP_NVPTX_SAVE_TEMPS into GOMP_NVPTX_CUBINRW=w, then add
GOMP_NVPTX_CUBINRW=r and do:
- GOMP_NVPTX_PTRXRW=w
- call ptxas to produce cubin
- GOMP_NVPTX_CUBINRW=r

But it would be easier from user perspective to add handling of a variable
GOMP_NVPTX_PTXAS, f.i. set to "/usr/local/cuda-8.0/bin/ptxas --return-at-end
-O2".