`br_if` vs `if` performance · general

I've been looking through some WASM code generated by compiling a Rust program using wasm32-wasip1 target and I've noticed that it contains a lot of blocks that are used as ifs. As far as I understand instead of an if like:

(if (i32.eqz (local.get $foo))
  (then
    (call $bar)))

(block $b1
  ;; if $foo is *not* equal zero, we skip to the end of the block,
  ;; otherwise we handle the if case
  (br_if $b1 (i32.ne (i32.const 0) (local.get $foo)))
  (call $bar)
)

    (if (i32.eqz (i32.const 0)) (then
    (if (i32.eqz (i32.const 0)) (then
      ;; 248 more ifs here
    ))))
 ```

and the following benchmark for blocks with `if_br`:

```wat
(block $foo1
(block $foo2
;; 248 more blocks
(br_if $foo2 (i32.ne (i32.const 1) (i32.const 0))))
(br_if $foo1 (i32.ne (i32.const 1) (i32.const 0))))

Then I looped both pieces of code and I measured the runtime. When executing the files in Wasmtime I consistently see the block version being faster then the if version. full runtime is ~300ms for blocks vs ~540ms for ifs. I've also checked on node.js and it's ~1s vs ~1.5s, so it seems like it's not only a detail of Wasmtime implementation.

Is it something that's expected? Anyone more familiar with the VM implementation knows why it would be the case?

Chris Fallin (Nov 18 2024 at 19:19):

Definitely not expected. One thing I notice is that the conditions aren't the same -- though both should be constant-folded away. Could you dump the CLIF and/or machine code from this? (For the latter, wasmtime compile then objdump)

Piotr Sarnacki (Nov 18 2024 at 21:08):

Yeah, in order to go through all of the br_if statements the statement has to be reversed.

Chris Fallin (Nov 18 2024 at 21:30):

Piotr Sarnacki (Nov 18 2024 at 21:54):

How do I do that? I tried wasm-objdump, but it works only with .wasm files. Wasmtime doesn't seem to have objdump as a subcommand and when I try wasm-tools objdump it says:

error: failed to parse `blocks.cwasm`: input bytes aren't valid utf-8

Joel Dice (Nov 18 2024 at 21:56):

I think he means the objdump from e.g. GNU Binutils or similar, i.e. one meant for native binaries, not Wasm.

Piotr Sarnacki (Nov 18 2024 at 22:11):

Oh, ok, I thought it needs to be a WASM specific tool :sweat_smile: I updated the gist

Chris Fallin (Nov 18 2024 at 22:22):

woah, fascinating -- in the first (blocks) case all the branches are elided, while in the second case they're still there. We don't do any branch-folding in the mid-end, so probably something is different in the Wasm translation itself

Chris Fallin (Nov 18 2024 at 22:23):

I don't have a ton of time to dig into this right now but if you could file an issue so we can track this, that'd be very useful!

Stream: general

Topic: `br_if` vs `if` performance

Piotr Sarnacki (Nov 18 2024 at 12:57):