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Do not mark BPF extern C returns as zeroext #158030

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Do not mark BPF extern C returns as zeroext #158030

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3 changes: 1 addition & 2 deletions compiler/rustc_target/src/callconv/bpf.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,8 +6,6 @@ use crate::callconv::{ArgAbi, FnAbi};
fn classify_ret<Ty>(ret: &mut ArgAbi<'_, Ty>) {
if ret.layout.is_aggregate() || ret.layout.size.bits() > 64 {
ret.make_indirect();
} else {
ret.extend_integer_width_to(32);
}
}

Expand Down Expand Up @@ -45,5 +43,6 @@ where
pub(crate) fn compute_rust_abi_info<Ty>(fn_abi: &mut FnAbi<'_, Ty>) {
if !fn_abi.ret.is_ignore() {
classify_ret(&mut fn_abi.ret);
fn_abi.ret.extend_integer_width_to(32);

@bjorn3 bjorn3 Jun 17, 2026 •
edited by rustbot
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I don't think this is necessary. We can do whatever we want for the rust ABI and it probably doesn't matter for performance if the C ABI doesn't do it either.

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}
}
50 changes: 50 additions & 0 deletions tests/codegen-llvm/bpf-abi-extern-narrow-ret.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,50 @@
//@ add-minicore
//@ needs-llvm-components: bpf
//@ compile-flags: --target bpfel-unknown-none

#![feature(no_core)]
#![crate_type = "lib"]
#![no_core]

extern crate minicore;
use minicore::*;

unsafe extern "C" {
fn c_ret_u8() -> u8;
fn c_ret_u16() -> u16;
fn c_ret_i8() -> i8;
fn c_ret_i16() -> i16;
}

// CHECK-LABEL: define {{.*}} @observe_u8(
// CHECK: call noundef i8 @c_ret_u8()
#[unsafe(no_mangle)]
pub unsafe extern "C" fn observe_u8() -> u64 {
c_ret_u8() as u64
}

// CHECK-LABEL: define {{.*}} @observe_u16(
// CHECK: call noundef i16 @c_ret_u16()
#[unsafe(no_mangle)]
pub unsafe extern "C" fn observe_u16() -> u64 {
c_ret_u16() as u64
}

// CHECK-LABEL: define {{.*}} @observe_i8(
// CHECK: call noundef i8 @c_ret_i8()
#[unsafe(no_mangle)]
pub unsafe extern "C" fn observe_i8() -> i64 {
c_ret_i8() as i64
}

// CHECK-LABEL: define {{.*}} @observe_i16(
// CHECK: call noundef i16 @c_ret_i16()
#[unsafe(no_mangle)]
pub unsafe extern "C" fn observe_i16() -> i64 {
c_ret_i16() as i64
}

// CHECK: declare noundef i8 @c_ret_u8()
// CHECK: declare noundef i16 @c_ret_u16()
// CHECK: declare noundef i8 @c_ret_i8()
// CHECK: declare noundef i16 @c_ret_i16()
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