{"id":729,"date":"2018-07-15T18:25:34","date_gmt":"2018-07-15T18:25:34","guid":{"rendered":"http:\/\/www.vidarholen.net\/contents\/blog\/?p=729"},"modified":"2018-07-17T17:34:03","modified_gmt":"2018-07-17T17:34:03","slug":"shellcheck-on-armv6hf-crossless-compilers-and-the-value-of-split-sections","status":"publish","type":"post","link":"https:\/\/www.vidarholen.net\/contents\/blog\/?p=729","title":{"rendered":"So what exactly is -ffunction-sections and how does it reduce binary size?"},"content":{"rendered":"

If you’d like a more up-to-date version of ShellCheck<\/a> than what Raspbian provides, you can build your own on a Raspberry Pi Zero in a little over 21 hours.<\/p>\n

Alternatively, as of last week, you can also download RPi compatible, statically linked armv6hf binaries<\/a> of every new commit and stable release.<\/p>\n

It’s statically linked — i.e. the executable has all its library dependencies built in — so you can expect it to be pretty big. However, I didn’t expect it to be 67MB:<\/p>\n

build@d1044ff3bf67:\/mnt\/shellcheck# ls -l shellcheck\r\n-rwxr-xr-x 1 build build 66658032 Jul 14 16:04 shellcheck<\/pre>\n
This is for a tool intended to run on devices with 512MiB RAM. strip<\/code> helps shed a lot of that weight, and the post-stripped number is the one we’ll use from now on, but 36MB is still more than I expected, especially given that the x86_64 build is 23MB.<\/p>\n
build@d1044ff3bf67:\/mnt\/shellcheck# strip --strip-all shellcheck\r\nbuild@d1044ff3bf67:\/mnt\/shellcheck# ls -l shellcheck\r\n-rwxr-xr-x 1 build build 35951068 Jul 14 16:22 shellcheck<\/pre>\n
So now what? Optimize for size? Here’s ghc -optlo-Os<\/code> to enable LLVM opt<\/code> size optimizations, including a complete three hour Qemu emulated rebuild of all dependencies:<\/p>\n
build@31ef6588fdf1:\/mnt\/shellcheck# ls -l shellcheck\r\n-rwxr-xr-x 1 build build 32051676 Jul 14 22:38 shellcheck<\/pre>\n
Welp, that’s not nearly enough.<\/p>\n
The real problem is that we’re linking in both C and Haskell dependencies, from the JSON formatters and Regex libraries to bignum implemenations and the Haskell runtime itself. These have tons of functionality that ShellCheck doesn’t use, but which is still included as part of the package.<\/p>\n
Fortunately, GCC and GHC allow eliminating this kind of dead code through function sections<\/em>. Let’s look at how that works, and why dead code can’t just be eliminated as a matter of course:<\/p>\n
An ELF binary contains a lot of different things, each stored in a section<\/em>. It can have any number of these sections, each of which has a pile of attributes including a name:<\/p>\n