Stream: git-wasmtime

Topic: wasmtime / issue #11725 Missed optimization: `clz` and `ctz`

view this post on Zulip Wasmtime GitHub notifications bot (Sep 20 2025 at 00:27):

khagankhan opened issue #11725:

Missed optimization for leading (clz) and trailing (ctz) zeros.

Absolute minimum WAT file:

(module
  (func $main (export "main") (result i32)
    i32.const -1
    i32.clz
  )
)

Not optimized for a constant matching execution (0). Dumping CLIF from /tmp/.tmpZ8O1nN/wasm[0]--function[0]--main.clif:

=================== CLIF BEGIN ===================
;; Intermediate Representation of function <wasm[0]::function[0]::main>:
function u0:0(i64 vmctx, i64) -> i32 tail {
    gv0 = vmctx
    gv1 = load.i64 notrap aligned readonly gv0+8
    gv2 = load.i64 notrap aligned gv1+16
    stack_limit = gv2

                                block0(v0: i64, v1: i64):
@0025                               jump block1

                                block1:
@0022                               v3 = iconst.i32 -1
@0024                               v4 = clz v3  ; v3 = -1
@0025                               return v4
}

==================== CLIF END ====================
Aborted

P.S. I want to create issues and then create PRs, which, if deemed fit, may be added about missed optimizations. I would also suggest having a "missed-optimization" label like LLVM has, which can be helpful.

Last updated: Dec 06 2025 at 06:05 UTC