5 ms·

Why isn't `-march=native -mtune=native` enabled by-default for every piece of software compiled unless explicitly specified otherwise?

loeg 8y ago
You'd have to ask the compiler communities that. As an observer, I notice that C compilers are extremely conservative about changing defaults in new versions. And I could imagine some benefits of that approach, and downsides. But I am not involved in e.g. gcc or clang development.
jsnell 8y ago
Compiling with -march=native makes for brittle binaries, since the binary might contain instructions that are not be available on slightly older CPUs or on virtual machines. So it's only suitable for software that's performance sensitive but won't be distributed to other machines (or at least won't be distributed outside of a strictly controlled environment). It wouldn't make for a good default.
dman 8y ago
When you are shipping binaries, you usually do not have control over the cpus your users are using. Setting march/mtune to native breaks if the developers machine is noticeably newer than the users machine. Also letting the build machine configuration decide which level of simd to target is too flakey, so most software specifies the target explicitly.
ndesaulniers 8y ago
Because your cpu has a base ISA, plus extensions. The compiler doesn't know if you're going to only run this binary on your machine, or distribute it to others that may share your base ISA but possibly not your extensions, so it takes the conservative approach and doesn't use them unless signaled to do so via those compiler flags. Also, I think -march implies -mtune.
- AndyKelley 8y ago
  In Zig the default target is native, which turns on all the applicable CPU features. If you want to target something other than the native machine you can use --target-os --target-arch parameters. I haven't exposed extra CPU options for cross compiling yet.
  - saagarjha 8y ago
    Do you consider the default architecture targetted by GCC (e.g. some old Intel) to be cross compiling? That is, can I make a binary that supports most x86 processors rather than just those with the particular extensions I support, with the current Zig compiler?
    - AndyKelley 8y ago
      Yes. You can pass the native OS and native arch as the cross compilation target, and produce a binary that runs on your machine and others that don't have the same extra CPU features.
  - loeg 8y ago
    That explains the difference in the SHA256 code case. Case closed ;-).
black-tea 8y ago
It is if you compile everything yourself on every system you use. Every Gentoo user does it (or at least, the equivalent of it using explicit flags in case they use distcc). But most people don't do that. They use software compiled by other people on very different machines.

loeg 8y ago

You'd have to ask the compiler communities that. As an observer, I notice that C compilers are extremely conservative about changing defaults in new versions. And I could imagine some benefits of that approach, and downsides. But I am not involved in e.g. gcc or clang development.

jsnell 8y ago

Compiling with -march=native makes for brittle binaries, since the binary might contain instructions that are not be available on slightly older CPUs or on virtual machines. So it's only suitable for software that's performance sensitive but won't be distributed to other machines (or at least won't be distributed outside of a strictly controlled environment). It wouldn't make for a good default.

dman 8y ago

When you are shipping binaries, you usually do not have control over the cpus your users are using. Setting march/mtune to native breaks if the developers machine is noticeably newer than the users machine. Also letting the build machine configuration decide which level of simd to target is too flakey, so most software specifies the target explicitly.

ndesaulniers 8y ago

Because your cpu has a base ISA, plus extensions. The compiler doesn't know if you're going to only run this binary on your machine, or distribute it to others that may share your base ISA but possibly not your extensions, so it takes the conservative approach and doesn't use them unless signaled to do so via those compiler flags. Also, I think -march implies -mtune.

AndyKelley 8y ago
In Zig the default target is native, which turns on all the applicable CPU features. If you want to target something other than the native machine you can use --target-os --target-arch parameters. I haven't exposed extra CPU options for cross compiling yet.
- saagarjha 8y ago
  Do you consider the default architecture targetted by GCC (e.g. some old Intel) to be cross compiling? That is, can I make a binary that supports most x86 processors rather than just those with the particular extensions I support, with the current Zig compiler?
  - AndyKelley 8y ago
    Yes. You can pass the native OS and native arch as the cross compilation target, and produce a binary that runs on your machine and others that don't have the same extra CPU features.
- loeg 8y ago
  That explains the difference in the SHA256 code case. Case closed ;-).

black-tea 8y ago

It is if you compile everything yourself on every system you use. Every Gentoo user does it (or at least, the equivalent of it using explicit flags in case they use distcc). But most people don't do that. They use software compiled by other people on very different machines.