ptxas is a proprietary compiler from Nvidia that can compile PTX to
machine code (SASS). It has a lot of diagnostics to catch errors
in PTX, which can be used to verify PTX output from llc.
Set -DPXTAS_EXECUTABLE=/path/to/ptxas CMake option to enable it.
If this option is not set, then ptxas is substituted to true which
effectively disables all ptxas RUN lines.
LLVM_PTXAS_EXECUTABLE environment variable takes precedence over
the CMake option, and allows to override ptxas executable that is used for LIT
without complete re-configuration.
Here and everywhere -- I'd run conditional ptxas testing as the last step.
We should make sure that the actual tests have succeeded, first. No point running ptxas on the output that we already know is wrong.