Big wasm files · general · Zulip Chat Archive

Why is the .wasm files generated by wasmtime for just a simple print Hello World of about 2 MB

Akshat Agarwal (Mar 15 2020 at 02:16):

I guess it's not "generated" by wasmtime (sorry about that). But even then. I am asking here because I don't know where else to ask this question.

Chris Fallin (Mar 15 2020 at 04:43):

Are you looking at binaries built with Rust? I noticed this too, at first; it seems by default the .wasm files include debug info, and wasm-strip(part of the WABT, WebAssembly Binary Toolkit) can remove it. I see (using a hello-world built with rustc --target=wasm32-wasi -O ...):

$ wc -c *.wasm
1925510 helloworld-rs.wasm
  71138 helloworld-rs-stripped.wasm

I used Bloaty McBloatface (https://github.com/google/bloaty/), which supports WebAssembly, to look at where the space is spent in the original (non-stripped) file:

$ bloaty helloworld-rs.wasm
    FILE SIZE        VM SIZE
 --------------  --------------
  32.0%   602Ki   NAN%       0    .debug_str
  25.6%   481Ki   NAN%       0    .debug_info
  15.8%   296Ki   NAN%       0    .debug_line
  10.0%   188Ki   NAN%       0    .debug_ranges
   8.0%   149Ki   NAN%       0    .debug_pubnames
   3.0%  57.1Ki   NAN%       0    Code
   2.8%  53.6Ki   NAN%       0    .debug_pubtypes
   1.0%  17.9Ki   NAN%       0    .debug_aranges
   0.8%  16.0Ki   NAN%       0    name
   0.6%  11.6Ki   NAN%       0    Data
   0.2%  4.39Ki   NAN%       0    .debug_abbrev
   0.0%     251   NAN%       0    Function
   0.0%     238   NAN%       0    Import
   0.0%     131   NAN%       0    Element
   0.0%     116   NAN%       0    producers
   0.0%     108   NAN%       0    Type
   0.0%      73   NAN%       0    Export
   0.0%      27   NAN%       0    Global
   0.0%      25   NAN%       0    .debug_macinfo
   0.0%      12   NAN%       0    [2 Others]
   0.0%       8   NAN%       0    [WASM Header]
 100.0%  1.84Mi 100.0%       0    TOTAL

FWIW, I also built a hello-world in C with the WASI SDK (https://github.com/WebAssembly/wasi-sdk/), and the resulting file is 16306 bytes, so it does seem the bits of the Rust stdlib pulled in by println! are a bit larger than the libc slice for printf...

Till Schneidereit (Mar 15 2020 at 10:11):

Yeah, it's almost certainly debug info. It'd be nice if we could move that to outside the Wasm file, but I don't think that works yet.

Dan Gohman (Mar 15 2020 at 16:16):

Two additional things I can add here: One is that we use static linking right now, so executables include the pieces of libc they need, rather than dynamic linking them.

Dan Gohman (Mar 15 2020 at 16:18):

Another is that we've made some major improvements in the tools recently which aren't yet in a release. With a trunk build of wasi-sdk and friends, a C hello world with debug info is 29K.

Dan Gohman (Mar 15 2020 at 16:21):

Ah, and one other detail: C++ hello worlds using <iostream> are significantly bigger than C hello worlds using printf. This is because <iostream> ends up causing us to link in all kinds of locale support (that hello world itself doesn't need).

fitzgen (he/him) (Mar 16 2020 at 16:29):

There is also twiggy, which I wrote to better understand why some function was called and why the linker didn't gc it from the final binary: https://github.com/rustwasm/twiggy

Stream: general

Topic: Big wasm files