Bug 277703 – [WASM] Subsequent comparisons and conditional branches should be fused in BBQ

• Status: RESOLVED
• Resolution: FIXED
• Component: WebAssembly

[David Degazio]

Since WASM only has br_if and if - which branch on a boolean i32 value - as conditional branch primitives, it's common in WASM to write sequences like these:
  
    local.get 0
    i32.eqz
    br_if 0

    local.get 1
    i32.const 10
    i32.lt_s
    if

In BBQ, since we compile in a single pass, we currently handle these cases super naively, emitting each opcode independently. So for that first example with the eqz, we'll emit something like the following on ARM:

    ldur    w0, [fp, #-12]
    cmp     w0, #0
    cset    w0, eq
    mov     x8, x0
    cbz     w8, ...

This is pretty nasty - we're using cset to store the result of comparing our local against 0, only to use it in a cbz a few instructions later.

In a single-pass compiler our options for improving on this are a little bit limited. We could consider some kind of abstract representation of a condition, making partially-evaluated conditions a member of BBQJIT::Value. This has a lot of weird implications for the register allocator and I'm not sure it's super worth it. In the absence of that, we should at least combine obviously combinable instructions like these that are literally adjacent in the source. Due to WASM's stack-based nature, this is almost always how such comparisons are emitted, so even a simple approach like one bytecode of lookahead should capture many of the combinable cases.

[Radar WebKit Bug Importer]

<rdar://problem/133317430>

[David Degazio]

Pull request: https://github.com/WebKit/WebKit/pull/31802

[EWS]

Committed 282799@main (406449de0ae5): <https://commits.webkit.org/282799@main>

Reviewed commits have been landed. Closing PR #31802 and removing active labels.