Binary size could be smaller

[smklein]

Rust Version

rustc 1.31.0-nightly (e7f5d4805 2018-10-18)

Affected Version of clap

2.32.0

Bug or Feature Request Summary

I compared the binary size of a "hello world" application, with and without clap, and came up with the following data:

Using a Cargo.toml:

  [profile.release]                                                                                                                                                                                                
  opt-level = 3                                                                                                                                                                                                    
  lto = true                                                                                                                                                                                                       
  panic ='abort'      

And a main.rs (of either this, or a version which only calls println!("Hello world"):

#![feature(alloc_system)]
extern crate alloc_system;
extern crate clap;                                                                                                                                                                                                                   
use clap::{App, Arg};

fn main() {
    let matches = App::new("Hello")                                                                                                                                                                              
                    .arg(Arg::with_name("foo")                                                                                                                                                                   
                         .short("f")                                                                                                                                                                             
                         .takes_value(true))                                                                                                                                                                     
                    .get_matches();                                                                                                                                                                              
    let foo = matches.value_of("foo").unwrap();                                                                                                                                                  
    println!("Hello, world: {}", foo);                                                                                                                                                                           }

I ran the following:

$ cargo rustc --release -- -C link-args=-static && strip target/release/hello

The "Non-clap" version of hello world resulted in the following output from bloaty:

     VM SIZE                      FILE SIZE
 --------------                --------------
  78.3%   283Ki .text            283Ki  78.3%
   8.2%  29.8Ki .rodata         29.8Ki   8.2%
   6.0%  21.7Ki .eh_frame       21.7Ki   6.0%
   3.7%  13.6Ki .data.rel.ro    13.7Ki   3.8%
   1.1%  4.16Ki .bss                 0   0.0%
   0.2%     568 [ELF Headers]   2.37Ki   0.7%
   0.6%  2.16Ki .dynsym         2.16Ki   0.6%
   0.0%       0 [Unmapped]      1.91Ki   0.5%
   0.4%  1.36Ki .dynstr         1.36Ki   0.4%
   0.3%  1.17Ki .rela.dyn       1.17Ki   0.3%
   0.2%     712 .got               712   0.2%
   0.2%     672 .rela.plt          672   0.2%
   0.2%     576 .dynamic           576   0.2%
   0.1%     534 [12 Others]        525   0.1%
   0.1%     464 .plt               464   0.1%
   0.1%     416 .gnu.hash          416   0.1%
   0.1%     320 .gnu.version_r     320   0.1%
   0.1%     272 .data              272   0.1%
   0.0%       0 .shstrtab          250   0.1%
   0.0%     184 .gnu.version       184   0.0%
   0.0%     152 .plt.got           152   0.0%
 100.0%   362Ki TOTAL            362Ki 100.0%

Where the clap version resulted in the following:

     VM SIZE                      FILE SIZE
 --------------                --------------
  82.6%   620Ki .text            620Ki  82.7%
   7.0%  52.6Ki .rodata         52.6Ki   7.0%
   5.4%  40.3Ki .eh_frame       40.3Ki   5.4%
   3.2%  23.8Ki .data.rel.ro    23.9Ki   3.2%
   0.6%  4.29Ki .bss                 0   0.0%
   0.1%     568 [ELF Headers]   2.37Ki   0.3%
   0.3%  2.27Ki .dynsym         2.27Ki   0.3%
   0.2%  1.41Ki .rela.dyn       1.41Ki   0.2%
   0.2%  1.38Ki .dynstr         1.38Ki   0.2%
   0.0%       0 [Unmapped]      1.12Ki   0.1%
   0.1%     760 .got               760   0.1%
   0.1%     576 .dynamic           576   0.1%
   0.1%     576 .rela.plt          576   0.1%
   0.1%     542 [12 Others]        501   0.1%
   0.1%     476 .gnu.hash          476   0.1%
   0.1%     400 .plt               400   0.1%
   0.0%     320 .gnu.version_r     320   0.0%
   0.0%     272 .data              272   0.0%
   0.0%       0 .shstrtab          250   0.0%
   0.0%     194 .gnu.version       194   0.0%
   0.0%     184 .plt.got           184   0.0%
 100.0%   751Ki TOTAL            750Ki 100.0%

(Diff for visibility)

     VM SIZE                    FILE SIZE
 --------------              --------------
  +119%  +336Ki .text         +336Ki  +119%
   +77% +22.8Ki .rodata      +22.8Ki   +77%
   +85% +18.6Ki .eh_frame    +18.6Ki   +85%
   +75% +10.2Ki .data.rel.ro +10.2Ki   +75%
   +20%    +240 .rela.dyn       +240   +20%
  +3.0%    +128 .bss               0  [ = ]
  +5.4%    +120 .dynsym         +120  +5.4%
   +14%     +60 .gnu.hash        +60   +14%
  +6.7%     +48 .got             +48  +6.7%
   +21%     +32 .plt.got         +32   +21%
   +24%     +32 .tbss              0  [ = ]
  +1.9%     +26 .dynstr          +26  +1.9%
  +5.4%     +10 .gnu.version     +10  +5.4%
 -28.9%     -24 [LOAD [RX]]      -24 -28.9%
 -13.8%     -64 .plt             -64 -13.8%
 -14.3%     -96 .rela.plt        -96 -14.3%
  [ = ]       0 [Unmapped]      -808 -41.2%
  +107%  +388Ki TOTAL         +388Ki  +107%

The amount of .text being generated here doubles the size of the binary.

Clap is really useful, and I'd love if I could use it on more constrained environments, where binary size might be expensive.

Chatting with @kbknapp , it seems that there are some changes to clap-rs underway to allow more flexible usage of the crate, potentially disabling some features at compile-time to shrink the binary, but I figured I'd file this bug to track.

Are there currently other ways of toggling clap features, to compare binary sizes for this evaluation?

[kbknapp]

Re-energizing this topic. Not necessarily for action prior to 3.0 release, but something to keep in our mind and begin to tackle as available.

My comment from #1891

Along those same lines but also a separate issue entirely, at some point I would like to have the discussion about aggressive cargo feature use. Ideally, all non-essential sections of clap could be disabled (but enabled by default) to allow small binary sizes when desired. I've had quite a few people / companies approach me about clap's binary size being their only issue. In fact, I believe that is the number one factor that led Google to work on argh, was binary size is their top concern (for Fuschia) and don't need a lot of the "extra" features that clap provides.

@CreepySkeleton's comment from that same issue:

On the other hand, people from this anti-dependency camp likely see clap as bloat-full and having too much allegedly unneeded deps irrespective of term_size. Clap already pulls 7 crates in --no-default-features mode, 22 in default-features mode, and 27 in all-features mode; one extra crate - very tiny and very useful btw - won't change their opinion. It's kind of all-or-nothing situation, 8 deps (including transitive) kind of count as "as small as possible". I really doubt any of them would become our user if we feature-gate one or two tiny crates.

That's not to mention that clap itself takes as much time to build as (almost) all of the deps combined, and the compilation time is a much more common complaint among users (in my view). Along with code bloat, which would only decrease without this feature.

Regarding modularization of clap.

I've been thinking about it too, for some time, and I've come to conclusion that, while it's doable - to some extent, it would require full-scale refactoring. And by full-scale I mean "rewrite the parser and the validator entirely" which is quite an endeavor.

Finally, @Dylan-DPC's comment from that issue:

Given how popular clap is, the number of dependencies is definitely a huge concern. I've been planing of tackling a lighter clap in some form or the other post the 3.0 release. That discussion is beyond the scope of this issue.

[kbknapp]

Just so my thoughts are known, I don't believe we need to remove all dependencies and features for the smallest possible size. What I do think is that we should know which dependencies have a low "weight to functionality" ratio.

"Weight" can assessed in two manners, code size added to clap, and compile time.

"Functionality" should be self evident.

I am definitely not in the camp of "make sure there are as few deps as possible." However, I am in the camp of I don't want to take on a dep that doesn't provide intrinsic value, or is a maintenance burden.

The harder crates to evaluate will be ones that don't add a lot to clap functionally, but at the same time don't add much if any compile time or code size either. I would propose for those style crates we look at either pulling the code internal, or dropping the crate.

I'm most concerned with the --no-default-features case, that case should be as small as possible, with as few high ratio deps as possible. I'm not concerned with the default features, or even --all-features cases, because at that point the consumer as chosen to go a heavier weight route, while there are lighter weight options.

This would be a great first issue, as it's not a lot of code changing, and doesn't require too much in depth knowledge of clap. At first, it will primarily be measuring the default crates, or seeing how widespread their use inside clap is.

---

Taking a quick look at our default crates I see:

Crate (+ transitive deps)	Functionality	Notes
bitflags	Underpin of AppSettings
unicode-width	Heavily used in Help Messages	If we made auto generated help optional, this could become an optional feature
textwrap	Used for Help Messages	same as unicode-width
indexmap (autocfg)	used in ArgMatcher/ArgMatches	Perhaps we could use vec_map instead
os_str_bytes	used in parsing	clap fundamentally messes with OsStrs or bytes heavily...so this is probably well worth the dep
vec_map	Used throughout Arg and usage parser	Used to be an optional feature. Has perf gains over BTreeMap...maybe those gains need to be evaluated again to see if they're worth it

[kbknapp]

Doing a --no-default-feautres --features std build on a simple:

App::new("foo").get_matches();

Gives us this output via cargo bloat in release mode:

cargo bloat --crates --release
[..]
    Analyzing target/release/fake

 File  .text     Size Crate
 9.3%  56.5% 285.5KiB clap
 4.8%  29.3% 148.1KiB std
 1.9%  11.7%  59.2KiB [Unknown]
 0.1%   0.5%   2.6KiB indexmap
 0.1%   0.3%   1.6KiB textwrap
 0.0%   0.1%     307B fake
 0.0%   0.0%      41B os_str_bytes
16.5% 100.0% 505.5KiB .text section size, the file size is 3.0MiB

Note: numbers above are a result of guesswork. They are not 100% correct and never will be.

Discounting clap and std, it looks like textwrap and indexmap need a close eye. If either has better alternatives, or is simple enough to not use/pull internal I'd say those may be on the chopping block.

Also, if we did something drastic like make auto help messages optional, that alone would cut a huge portion of the code along with size. Additionally and oddly enough, that may the easiest thing to modularize first.

"But wait!" I can hear you say. "Auto help generation is like THE thing clap is used for, right?" Sure. It's one of the things, but I'd argue not the most important at all. In many of the contexts where code size matters the most, help generation is one the things they don't care about because it's super easy to just write a help string manually for a small CLI.

[CreepySkeleton]

Before we more any further, would you please test it in release mode 😄? Nobody cares about debug binaries, release the ones of concern here.

[kbknapp]

Literally just edited the comment with that info 😆

[pksunkara]

Did you enable lto? I think we should assume with that.

[CreepySkeleton]

main.rs

use clap::*;

fn main() {
    App::new("app").get_matches();
}

With lto enabled (release build, no default features)

CreepySkeleton@builder:~/workspace/probe$ cargo bloat --crates --release
    [...]

 File  .text     Size Crate
19.4%  60.3% 277.7KiB clap
 9.0%  27.9% 128.5KiB std
 3.0%   9.2%  42.3KiB [Unknown]
 0.3%   1.0%   4.7KiB probe
 0.2%   0.6%   2.5KiB indexmap
 0.0%   0.1%     394B textwrap
32.2% 100.0% 460.3KiB .text section size, the file size is 1.4MiB

What interesting here is that the size of the main.rs crate increased from 307B to 4.7KiB. Basically, some code from std has been inlined and this is the biggest difference.

The overall difference is less than 10Kb (clap and its deps only) and it will thin as more features are actually used. All the further runs will be performed without LTO.

See also https://stackoverflow.com/a/52297790. TL;DR: LTO is not about reducing code size. z opt level is about it.

Another interesting read: https://internals.rust-lang.org/t/rust-staticlibs-and-optimizing-for-size/5746. No TL;DR here, go read, it's worthy.

---

Kevin, just to make sure we're on the same page: I wasn't counting you as a no-deps guy, I was just thinking aloud "how would we sell clap to those guys" and I'm pretty sure the answer is - we wouldn't.

If their wariness comes from concern that the deps affect binary size, we can righteously tell them that all the deps put together (in --no-default-features mode) take less than 5KiB, the next rustc release will likely change much more of that. The same can be said about compilation time: ~2sec difference, essentially a rounding error.

At the very least, if 5KiB do mater to you, you're working on embedded, and clap will likely never suit you. Take a look at getopts/argh or write it yourself.

I agree regarding help messages. It also drags error messages along.

What not clear to me is your logic about "either pulling the code internal, or dropping the crate". It sounds like "Let's copy-paste code from third party crates (we can't just drop them) in order to decrease our maintenance burden". But from that point on, we will be the ones who maintain this copy-pasted code. A contradiction. Could you please elaborate on that?