Stream: cranelift

Topic: Defining a variable in one branch and not the other, and usi

osa1 (May 29 2020 at 08:01):

What happens in cranelift if I do something like

b0:
    ...
    brz b2
    fallthrough b1

b1:
    a = ...
    jmp b3

b2:
    ... does not define a ...
    jmp b3


b3:
    use a

Is this a bug? Or will a be used uninitialized when b0->b2->b3 path is
taken? Where do I look for the relevant code that this is handled?

osa1 (May 29 2020 at 08:38):

I just tried this, and it seems like cranelift initializes the variable with 0 so it's initialized in all paths:

function u0:0() -> i64 system_v {
block3:
    v5 = iconst.i64 0
    v4 -> v5
    v0 = iconst.i64 123
    v1 = iconst.i64 456
    br_icmp eq v0, v1, block0
    jump block1

block0:
    jump block2(v5)

block1:
    v2 = iconst.i64 999
    jump block2(v2)

block2(v3: i64):
    return v3
}

I'd still like to see where this happens. Any pointers?

bjorn3 (May 29 2020 at 09:03):

This is done by cranelift_frontend::ssa::emit_zero.

Last updated: Dec 23 2024 at 12:05 UTC