Make Spack's compiler wrapper compatible with dash

[adamjstewart]

Currently, Spack's compiler wrapper is hardcoded to use /bin/bash. However, if we changed this to /bin/sh and made things POSIX compliant, systems like Ubuntu that default to dash could see vast slight improvements to build times for large, complex packages. If we changed it to /usr/bin/env sh, Spack users could install dash on other systems and add it to their PATH, although I don't know how much env will slow down runtimes.

If no one else gets to this, I'll probably experiment with it eventually. I just wanted to make a dedicated issue before I forget.

Related to #4048. Probably depends on #7604. See https://unix.stackexchange.com/questions/148035 for performance comparisons between common shells.

[tgamblin]

@adamjstewart: The wrappers were originally in Python, and the difference using bash vs python was up to 8x speedup for some builds. The main bottleneck for wrappers is startup time -- there isn't really anything major actually done in the cc script.

I'm not sure dash will add a lot more:

(atala):env$ time for x in `seq 1 100`; do python -c 'exit()'; done
real	0m3.461s
user	0m1.222s
sys	0m1.479s

(atala):env$ time for x in `seq 1 100`; do bash -c 'exit'; done
real	0m0.429s
user	0m0.196s
sys	0m0.204s

(atala):env$ time for x in `seq 1 100`; do dash -c 'exit'; done
real	0m0.355s
user	0m0.120s
sys	0m0.146s

Python -> bash was at least a 10x startup time improvement (and it's generally more on clusters). bash -> dash is maybe a 20% improvement on top of that, but it's only an improvement to the startup time. I suspect we're already at the point where the compile/build time dominates, and this won't result in "vast improvements".

It's also worth noting that if you use #!/usr/bin/env sh as you suggest, you lose any speedup over bash:

(atala):env$ time for x in `seq 1 100`; do /usr/bin/env dash -c exit; done

real	0m0.536s
user	0m0.189s
sys	0m0.227s

You're paying for two process launches now (env and dash) as well as the path search that env does. The absolute #!/bin/bash shebang will be faster.

[adamjstewart]

@tgamblin Thanks for the tests! I'll probably still try to get /bin/sh working if we don't need too many Bashisms. We may encounter a system without /bin/bash someday, although it's pretty unlikely.

[adamjstewart]

@tgamblin I think this is worth revisiting. I see a much larger performance boost from switching to dash:

$ time for x in `seq 1 100`; do python -c 'exit()'; done

real	0m11.198s
user	0m3.478s
sys	0m2.193s
$ time for x in `seq 1 100`; do bash -c 'exit'; done

real	0m1.740s
user	0m0.253s
sys	0m0.548s
$ time for x in `seq 1 100`; do dash -c 'exit'; done

real	0m1.055s
user	0m0.085s
sys	0m0.238s