Implement SystemV struct argument passing

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -2152,6 +2152,8 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             panic!("x86-specific opcode in supposedly arch-neutral IR!");
         }
 
+        Opcode::DummySarg => unreachable!(),
+
         Opcode::AvgRound => unimplemented!(),
         Opcode::Iabs => unimplemented!(),
         Opcode::Snarrow

cranelift/codegen/src/isa/x86/abi.rs

@@ -108,6 +108,19 @@ impl Args {
 
 impl ArgAssigner for Args {
     fn assign(&mut self, arg: &AbiParam) -> ArgAction {
+        if let ArgumentPurpose::StructArgument(size) = arg.purpose {
+            if self.call_conv != CallConv::SystemV {
+                panic!(
+                    "The sarg argument purpose is not yet implemented for non-systemv call conv {:?}",
+                    self.call_conv,
+                );
+            }
+            let loc = ArgumentLoc::Stack(self.offset as i32);
+            self.offset += size;
+            debug_assert!(self.offset <= i32::MAX as u32);
+            return ArgAction::AssignAndChangeType(loc, types::SARG__);
+        }
+
         let ty = arg.value_type;
 
         if ty.bits() > u16::from(self.pointer_bits) {