sunnuntai 17. marraskuuta 2019

What is -pipe and should you use it?

Every now and then you see people using the -pipe compiler argument. This is particularly common on vintage handwritten makefiles. Meson even uses the argument by default, but what does it actually do? GCC manpages say the following:

-pipe
    Use pipes rather than temporary files for communication
    between the various stages of compilation.  This fails
    to work on some systems where the assembler is unable to
    read from a pipe; but the GNU assembler has no trouble.

So presumably this is a compile speed optimization. What sort of an improvement does it actually provide? I tested this by compiling LLVM on my desktop machine both with and without the -pipe command line argument. Without it I got the following time:

ninja  14770,75s user 799,50s system 575% cpu 45:04,98 total

Adding the argument produced the following timing:

ninja  14874,41s user 791,95s system 584% cpu 44:41,22 total

This is an improvement of less than one percent. Given that I was using my machine for other things at the same time, the difference is most likely statistically insignificant.

This argument may have been needed in the ye olden times of supporting tens of broken commercial unixes. Nowadays the only platform where this might make a difference is Windows, given that its file system is a lot slower than Linux's. But is its pipe implementation any faster? I don't know, and I'll let other people measure that.

The "hindsight is perfect" design lesson to be learned

Looking at this now, it is fairly easy to see that this command line option should not exist. Punting the responsibility of knowing whether files or pipes are faster (or even work) on any given platform to the user is poor usability. Most people don't know that and performance characteristics of operating systems change over time. Instead this should be handled inside the compiler with logic roughly like the following:

if(assembler_supports_pipes(...) &&
   pipes_are_faster_on_this_platform(...)) {
    communicate_with_pipes();
} else {
    communicate_with_files();
}

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