Stream: git-wasmtime

Topic: wasmtime / Issue #1023 Predicates on patterns

view this post on Zulip Wasmtime GitHub notifications bot (Feb 03 2021 at 19:55):

bjorn3 commented on Issue #1023:

The new backend framework doesn't use legalizations as much as the old backend.

view this post on Zulip Wasmtime GitHub notifications bot (Feb 03 2021 at 20:05):

cfallin closed Issue #1023:

Suppose we want to compare 8-bit ints on a 32-bit RISC:

widen32.legalize(
    a << icmp('ult', x, y),
    Rtl(
        wx << uextend.i32(x),
        wy << uextend.i32(y),
        a << icmp('ult', wx, wy),
    ))

We want to generalize this pattern, but this transformation is only valid for the unsigned or sign-neutral condition codes, so this is wrong:

widen32.legalize(
    a << icmp(cc, x, y),
    Rtl(
        wx << uextend.i32(x),
        wy << uextend.i32(y),
        a << icmp(cc, wx, wy),
    ))

We need a way of specifying a predicate on the immediate cc. Ideally, this mechanism should share representation with the instruction predicates already supported by instruction encodings.

(Also note that the first example doesn't work either—we can't even require a fixed immediate value in the input pattern.)

Last updated: Nov 22 2024 at 16:03 UTC