Cranelift: scoped elaboration duplicates pure operations even when there is no partial redundancy to avoid

[meithecatte]

.clif Test Case

test optimize
set opt_level=speed
target x86_64

function %f(i32, i32, i32) -> i32 {
block0(v0: i32, v1: i32, v2: i32):
    v3 = imul.i32 v0, v1
    brif v2, block1, block2
block1:
    return v3
block2:
    v4 = iconst.i32 1
    v5 = iadd.i32 v3, v4
    return v5

}

;; check that the two occurrences of v3 are still the same value after optimization
; check: return v3
; check: iadd.i32 v3, v4

Steps to Reproduce

• clif-util test meow.clif

Expected Results

The imul stays in block0.

Actual Results

The imul is needlessly duplicated between block1 and block2.

Versions and Environment

Cranelift version or commit: de29ce3 (main at the time of writing)

Operating system: Linux

Architecture: x86_64

[cfallin]

That's true, but unfortunately I can't see a low-cost of way avoiding this: one would have to ask "is this value demanded in every subtree of the domtree from this point", at every point, and move it there; in the extreme, requiring an upward-pass on the domtree. Remember that we're reconstructing code placement as we recreate the CFG, we don't have any notion that there was a single imul in the parent block previously.

I would consider this an issue in the same category as the "instruction scheduling" general concern (#6159 / #6260): the current algorithm is working as designed and produces correct code, but better heuristics could be better.

I'm curious if you have an idea for a better algorithm here?