diff --git a/compiler/rustc_codegen_llvm/src/va_arg.rs b/compiler/rustc_codegen_llvm/src/va_arg.rs
index 3cd4e490fd5b..cfdd47b3a8c8 100644
--- a/compiler/rustc_codegen_llvm/src/va_arg.rs
+++ b/compiler/rustc_codegen_llvm/src/va_arg.rs
@@ -88,11 +88,30 @@ enum SlotSize {
     Bytes1 = 1,
 }
 
+/// Whether to respect a value alignment that is higher than the slot alignment.
+///
+/// When `No` the argument is in the next slot, when `Yes` there will be empty slots
+/// until a slot's starting address has the required alignment.
 enum AllowHigherAlign {
     No,
     Yes,
 }
 
+/// Determines where in the slot the value is located. Only takes effect on big-endian targets.
+///
+/// with 8-byte slots, a 32-bit integer is either stored right-adjusted:
+///
+/// ```text
+/// [0x0, 0x0, 0x0, 0x0, 0xaa, 0xaa, 0xaa, 0xaa]
+/// ```
+///
+/// or left-adjusted:
+///
+/// ```text
+/// [0xaa, 0xaa, 0xaa, 0xaa, 0x0, 0x0, 0x0, 0x0]
+/// ```
+///
+/// Most big-endian targets store values as right-adjusted.
 enum ForceRightAdjust {
     No,
     Yes,
@@ -1169,7 +1188,8 @@ pub(super) fn emit_va_arg<'ll, 'tcx>(
             if target_ty_size > 2 * 8 { PassMode::Indirect } else { PassMode::Direct },
             SlotSize::Bytes8,
             AllowHigherAlign::Yes,
-            ForceRightAdjust::No,
+            // sparc64 is a big-endian target and stores variable arguments right-adjusted.
+            ForceRightAdjust::Yes,
         ),
         Arch::Mips | Arch::Mips32r6 | Arch::Mips64 | Arch::Mips64r6 => emit_ptr_va_arg(
             bx,
diff --git a/tests/assembly-llvm/c-variadic-arm.rs b/tests/assembly-llvm/c-variadic/arm.rs
similarity index 73%
rename from tests/assembly-llvm/c-variadic-arm.rs
rename to tests/assembly-llvm/c-variadic/arm.rs
index 1cfc115badee..682e9736958b 100644
--- a/tests/assembly-llvm/c-variadic-arm.rs
+++ b/tests/assembly-llvm/c-variadic/arm.rs
@@ -7,7 +7,9 @@
 #![crate_type = "lib"]
 #![feature(c_variadic)]
 
-// Check that the assembly that rustc generates matches what clang emits.
+// Check that the assembly that rustc generates matches what clang emits. This example in particular
+// is related to https://github.com/rust-lang/rust/pull/144549 and shows the effect of us correctly
+// emitting annotations that start and end the lifetime of the va_list.
 
 #[unsafe(no_mangle)]
 unsafe extern "C" fn variadic(a: f64, mut args: ...) -> f64 {
diff --git a/tests/assembly-llvm/c-variadic-mips.rs b/tests/assembly-llvm/c-variadic/mips.rs
similarity index 100%
rename from tests/assembly-llvm/c-variadic-mips.rs
rename to tests/assembly-llvm/c-variadic/mips.rs
diff --git a/tests/assembly-llvm/c-variadic/sparc.rs b/tests/assembly-llvm/c-variadic/sparc.rs
new file mode 100644
index 000000000000..59f039e7df28
--- /dev/null
+++ b/tests/assembly-llvm/c-variadic/sparc.rs
@@ -0,0 +1,112 @@
+//@ add-minicore
+//@ assembly-output: emit-asm
+//
+//@ revisions: SPARC SPARC64
+//@ [SPARC] compile-flags: -Copt-level=3 --target sparc-unknown-linux-gnu
+//@ [SPARC] needs-llvm-components: sparc
+//@ [SPARC64] compile-flags: -Copt-level=3 --target sparc64-unknown-linux-gnu
+//@ [SPARC64] needs-llvm-components: sparc
+#![feature(c_variadic, no_core, lang_items, intrinsics, rustc_attrs, asm_experimental_arch)]
+#![no_core]
+#![crate_type = "lib"]
+
+// Check that the assembly that rustc generates matches what clang emits.
+
+extern crate minicore;
+use minicore::*;
+
+#[lang = "va_arg_safe"]
+pub unsafe trait VaArgSafe {}
+
+unsafe impl VaArgSafe for i32 {}
+unsafe impl VaArgSafe for i64 {}
+unsafe impl VaArgSafe for f64 {}
+unsafe impl<T> VaArgSafe for *const T {}
+
+#[repr(transparent)]
+struct VaListInner {
+    ptr: *const c_void,
+}
+
+#[repr(transparent)]
+#[lang = "va_list"]
+pub struct VaList<'a> {
+    inner: VaListInner,
+    _marker: PhantomData<&'a mut ()>,
+}
+
+#[rustc_intrinsic]
+#[rustc_nounwind]
+pub const unsafe fn va_arg<T: VaArgSafe>(ap: &mut VaList<'_>) -> T;
+
+#[unsafe(no_mangle)]
+unsafe extern "C" fn read_f64(ap: &mut VaList<'_>) -> f64 {
+    // CHECK-LABEL: read_f64
+    //
+    // SPARC: ld [%o0], %o1
+    // SPARC-NEXT: add %o1, 8, %o2
+    // SPARC-NEXT: st %o2, [%o0]
+    // 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], %f0
+    // SPARC-NEXT: retl
+    // SPARC-NEXT: add %sp, 104, %sp
+    //
+    // SPARC64: ldx [%o0], %o1
+    // SPARC64-NEXT: add %o1, 8, %o2
+    // SPARC64-NEXT: stx %o2, [%o0]
+    // SPARC64-NEXT: retl
+    // SPARC64-NEXT: ldd [%o1], %f0
+    va_arg(ap)
+}
+
+#[unsafe(no_mangle)]
+unsafe extern "C" fn read_i32(ap: &mut VaList<'_>) -> i32 {
+    // CHECK-LABEL: read_i32
+    //
+    // SPARC: ld [%o0], %o1
+    // SPARC-NEXT: add %o1, 4, %o2
+    // SPARC-NEXT: st %o2, [%o0]
+    // SPARC-NEXT: retl
+    // SPARC-NEXT: ld [%o1], %o0
+    //
+    // SPARC64: ldx [%o0], %o1
+    // SPARC64-NEXT: add %o1, 8, %o2
+    // SPARC64-NEXT: stx %o2, [%o0]
+    // SPARC64-NEXT: retl
+    // SPARC64-NEXT: ldsw [%o1+4], %o0
+    va_arg(ap)
+}
+
+#[unsafe(no_mangle)]
+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: 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: retl
+    // SPARC-NEXT: mov %o2, %o0
+    //
+    // SPARC64: ldx [%o0], %o1
+    // SPARC64-NEXT: add %o1, 8, %o2
+    // SPARC64-NEXT: stx %o2, [%o0]
+    // SPARC64-NEXT: retl
+    // SPARC64-NEXT: ldx [%o1], %o0
+    va_arg(ap)
+}
+
+#[unsafe(no_mangle)]
+unsafe extern "C" fn read_ptr(ap: &mut VaList<'_>) -> *const u8 {
+    // SPARC: read_ptr = read_i32
+    // SPARC64: read_ptr = read_i64
+    va_arg(ap)
+}