Stream: git-cranelift

Topic: cranelift / Issue #27 Predicates on patterns

view this post on Zulip GitHub (Feb 28 2020 at 23:19):

alexcrichton transferred Issue #27:

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