reduce/extend combo on aarch64 · cranelift

hiya. i have some code that is building a little ffi argument adapter. one thing it does is handle that i8/i16 values have to be represented as i32 by the caller, so i insert ireduce on arguments and uextend/sextend on return values. this doesn't seem to be producing the expected result on aarch64 though, for example passing 256 through the adapter reducing to i8 i would expect to get 0 out, but i get 256. I don't know the specifics of arm64 but it behaves kind of like if you chose the wrong register size for mov in x64. Unfortunately I don't have an aarch64 machine so I've just been trying to debug it through CI, so I'm hoping someone here might know where the problem is. Here's the code in question: <https://github.com/denoland/deno/blob/725744955123bf63f22090e4117eec8280fa0d4b/ext/ffi/turbocall.rs#L30-L180>

snek (Feb 14 2025 at 08:43):

actually i guess for this specific case, extend isn't even used.

function u0:0(i64, i32) -> i32 system_v {
    sig0 = (i8) -> i32 system_v

block0(v0: i64, v1: i32):
    v3 = ireduce.i8 v1
    v4 = iconst.i64 0x7fe3_dc66_b090
    v5 = call_indirect sig0, v4(v3)
    return v5
}

bjorn3 (Feb 14 2025 at 09:43):

Apple's arm64 calling convention requires zero/sign extending integer arguments smaller than 32bit. You can use ArgumentExtension::Uext/Sext in the AbiParam of the function signature for this.

bjorn3 (Feb 14 2025 at 09:44):

https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms#Pass-arguments-to-functions-correctly

The caller of a function is responsible for signing or zero-extending any argument with fewer than 32 bits. The standard ABI expects the callee to sign or zero-extend those arguments.

snek (Feb 14 2025 at 09:47):

ooo perfect, ty! is it safe to just set this extension on all the int types as informed by their signedness?

bjorn3 (Feb 14 2025 at 09:49):

Yes, though for 32bit integers it would add some unnecessary overhead.

snek (Feb 14 2025 at 09:49):

oh like it will still insert extension instructions?

bjorn3 (Feb 14 2025 at 09:52):

I suspect it does. I haven't checked though.

Last updated: Feb 27 2025 at 23:03 UTC

Stream: cranelift

Topic: reduce/extend combo on aarch64