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c-variadic: use emit_ptr_va_arg for va_arg on sparc #160660

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Merged
rust-bors merged 1 commit into from
Sep 4, 2026
+33 −8
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c-variadic: use emit_ptr_va_arg for va_arg on sparc #160660

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24 changes: 23 additions & 1 deletion compiler/rustc_codegen_llvm/src/va_arg.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1233,6 +1233,28 @@ pub(super) fn emit_va_arg<'ll, 'tcx>(
// sparc64 is a big-endian target and stores variable arguments right-adjusted.
ForceRightAdjust::Yes,
),
Arch::Sparc => {
std::assert_matches!(stability, CVariadicStatus::Unstable { .. });

// f128 is passed indirectly.
let pass_mode = match layout.layout.backend_repr() {
BackendRepr::Scalar(scalar) => match scalar.primitive() {
Primitive::Float(Float::F128) => PassMode::Indirect,
_ => PassMode::Direct,
},
_ => PassMode::Direct,
};
Comment on lines +1239 to +1246

@folkertdev folkertdev Sep 3, 2026 •
edited by rustbot
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f128 does not yet implement VaArgSafe, so this is unreachable in practice, but:

View changes since the review


emit_ptr_va_arg(
bx,
addr,
target_ty,
pass_mode,
SlotSize::Bytes4,
AllowHigherAlign::No,
ForceRightAdjust::Yes,
)
}
Comment on lines +1236 to +1257

@folkertdev folkertdev Aug 6, 2026 •
edited by rustbot
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Sparc is still gated by the c_variadic_experimental_arch feature.

The arguments to emit_ptr_va_arg here are based on the LLVM implementation. Many targets have an implementation of va_arg in Clang, but for sparc it is really LLVM that expands va_arg.

https://github.com/llvm/llvm-project/blob/5194e33faad9d84fd8d21c0305621a17e0d8a49f/llvm/lib/Target/Sparc/SparcISelLowering.cpp#L2768-L2790

  • it just loads from a pointer regardless of the size of the argument, hence unconditional PassMode::Direct
  • The pointer is increased by just the VT size in bytes. Due to argument promotion, the smallest type that can actually be read using va_arg is 4 bytes, and no attempt is made to align to something higher than that. So SlotSize::Bytes4.
  • AllowHigherAlign::Yes would align e.g. an i128 to a 16-byte boundary. This function does not take the alignment into account at all. So, AllowHigherAlign::No
  • This setting is true on BE targets with a slot size of 8, where a 4-byte value could be either in the low or high bytes. Despite being a BE target, the setting is not relevant for sparc because all values that could be passed divide cleanly into 4-byte slots.

View changes since the review

Arch::Mips | Arch::Mips32r6 | Arch::Mips64 | Arch::Mips64r6 => emit_ptr_va_arg(
bx,
addr,
Expand All @@ -1256,7 +1278,7 @@ pub(super) fn emit_va_arg<'ll, 'tcx>(
Arch::Bpf => bug!("bpf does not support c-variadic functions"),
Arch::SpirV => bug!("spirv does not support c-variadic functions"),

Arch::Sparc | Arch::Avr | Arch::M68k | Arch::Msp430 => {
Arch::Avr | Arch::M68k | Arch::Msp430 => {
std::assert_matches!(stability, CVariadicStatus::Unstable { .. });

// Clang uses the LLVM implementation for these architectures.
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15 changes: 9 additions & 6 deletions tests/assembly-llvm/c-variadic/sparc.rs
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Expand Up @@ -90,14 +90,17 @@ unsafe extern "C" fn read_i64(ap: &mut VaList<'_>) -> i64 {
// CHECK-LABEL: read_i64
//
// SPARC: ld [%o0], %o1
// SPARC-NEXT: add %o1, 4, %o2
// SPARC-NEXT: add %o1, 8, %o2
// SPARC-NEXT: st %o2, [%o0]
// SPARC-NEXT: ld [%o1], %o2
// SPARC-NEXT: add %o1, 8, %o3
// SPARC-NEXT: st %o3, [%o0]
// SPARC-NEXT: ld [%o1+4], %o1
// SPARC-NEXT: ld [%o1+4], %o0
// SPARC-NEXT: add %sp, 96, %o2
// SPARC-NEXT: or %o2, 4, %o2
// SPARC-NEXT: st %o0, [%o2]
// SPARC-NEXT: ld [%o1], %o0
// SPARC-NEXT: st %o0, [%sp+96]
// SPARC-NEXT: ldd [%sp+96], %o0
// SPARC-NEXT: retl
// SPARC-NEXT: mov %o2, %o0
// SPARC-NEXT: add %sp, 104, %sp
//
// SPARC64: ldx [%o0], %o1
// SPARC64-NEXT: add %o1, 8, %o2
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2 changes: 1 addition & 1 deletion tests/run-make/c-link-to-rust-va-list-fn/checkrust.rs
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@@ -1,5 +1,5 @@
#![crate_type = "staticlib"]
#![feature(c_variadic_int128)]
#![feature(c_variadic_int128, c_variadic_experimental_arch)]

use core::ffi::{CStr, VaList, c_char, c_double, c_int, c_long, c_longlong};

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