Update compiler/rustc_codegen_llvm/src/va_arg.rs

Co-authored-by: Jubilee <workingjubilee@gmail.com>

Author: folkertdev

compiler/rustc_codegen_llvm/src/va_arg.rs

@@ -810,6 +810,9 @@ fn x86_64_sysv64_va_arg_from_memory<'ll, 'tcx>(
     // byte boundary if alignment needed by type exceeds 8 byte boundary.
     // It isn't stated explicitly in the standard, but in practice we use
     // alignment greater than 16 where necessary.
+    // The AMD64 psABI leaves unspecified what to do for alignments above 16, but
+    // this behavior for 32+ alignment matches clang.
+    // It currently (2026 July) can only occur for 16-byte-aligned types.
     let overflow_arg_area_v = if layout.layout.align.bytes() > 8 {
         round_pointer_up_to_alignment(bx, overflow_arg_area_v, layout.layout.align.abi)
     } else {