Replace serde json wasm

[webmaster128]

Closes #110

At webmaster128/serde_json_wasm#1 there is a diff showing what needs to be done to remote float instructions from serde_json. I think this does the job.

• Patch serde_json and integrate
• Publish fork as replace git reference with crates.io package

[ethanfrey]

Wow. I need to look at this.

Can you put this repo under confio or cosmwasm? Let me know if you need permissions.

Two main questions:

• it seems you recompiled the contract and it worked with check_wasm. Awesome!
• gas was not affected significantly:+1:
• this increased contract size by 70kb. I wonder if you can do some size profiling. I used twilly on the output before wasm-opt. Eg just compile with the rustflags argument and see what is taking up the space... maybe there is something easy to trim out

[webmaster128]

The current state is that cargo wasm works perfectly but cargo +nightly wasm still has disallowed instructions in it. I think it is related to how the different compilers handle the many #[inline] statements from the 'serde-rs/json'. I'm in the process of kicking out much more code. Let's see

[webmaster128]

No matter what I do, starting from Rust 1.43 a seemingly unused function is created

(func $_ZN4core3fmt5float52_$LT$impl$u20$core..fmt..Display$u20$for$u20$f64$GT$3fmt17h0f5e2db0ac6eccddE (type 1) (param i32 i32) (result i32)

which then calls

  (func $_ZN4core3fmt5float29float_to_decimal_common_exact17hd8511abc64c7c0afE (type 14) (param i32 f64 i32 i32) (result i32)
    (local i32 i64 i32 i64 i32 i64 i64 i64 i32 i32 i32)
    global.get 0
    i32.const 1136
    i32.sub
    local.tee 4
    global.set 0
    block  ;; label = @1
      block  ;; label = @2
        local.get 1
        i64.reinterpret_f64
        local.tee 5
        i64.const 9223372036854775807
        i64.and
        i64.eqz
        i32.eqz
        br_if 0 (;@2;)
        i32.const 4
        local.set 6
        br 1 (;@1;)
      end
      local.get 5
      i64.const 4503599627370495

where we get the float instructions from. My old Rust stable did not have this issue.

[webmaster128]

Turns out that an implementation of float_to_decimal_common_exact was included in both Rust versions, but between 1.42.0 and 1.43.0 the Wasm output changed. In the libcore source diff I don't see a significant difference. The changed Wasm instructions look like an optimization, where the reference to the number was a pointer before and is the actual value now:

Look at the second parameter as well as the code around local.get 1.

---

That explains the difference between the old stable builds and the nightly builds. No idea if we can trace down where impl Display for f32 { and impl Display for f64 { are used.

[webmaster128]

@ethanfrey this is more or less what I do to test the resulting instructions from a local version of serde_json:

# - Checkout serde_json_wasm next to cosmwasm
# - Delete local replace-serde-json-wasm if it exists and is outdated. Simon does happy rabasing on this branch.
$ git fetch --all && git checkout replace-serde-json-wasm
$ cd contracts/hackatom
$ echo "" >> Cargo.toml
$ echo "[patch.crates-io]" >> Cargo.toml
$ echo "serde_json = { path = '../../../serde_json_wasm' }" >> Cargo.toml
$ echo "" >> Cargo.toml
$ echo "[patch.'https://github.com/webmaster128/serde_json_wasm']" >> Cargo.toml
$ echo "serde_json = { path = '../../../serde_json_wasm' }" >> Cargo.toml
$ cargo wasm
$ wasm2wat target/wasm32-unknown-unknown/release/hackatom.wasm | grep 'float_to_decimal_common_exact' -C 40
$ wasm2wat target/wasm32-unknown-unknown/release/hackatom.wasm | grep 'f64' -C 20