Implement SystemV struct argument passing

cranelift/codegen/meta/src/isa/x86/encodings.rs

@@ -465,6 +465,7 @@ fn define_moves(e: &mut PerCpuModeEncodings, shared_defs: &SharedDefinitions, r:
     let sextend = shared.by_name("sextend");
     let set_pinned_reg = shared.by_name("set_pinned_reg");
     let uextend = shared.by_name("uextend");
+    let dummy_sarg__ = shared.by_name("dummy_sarg__");
 
     // Shorthands for recipes.
     let rec_copysp = r.template("copysp");
@@ -482,6 +483,7 @@ fn define_moves(e: &mut PerCpuModeEncodings, shared_defs: &SharedDefinitions, r:
     let rec_umr_reg_to_ssa = r.template("umr_reg_to_ssa");
     let rec_urm_noflags = r.template("urm_noflags");
     let rec_urm_noflags_abcd = r.template("urm_noflags_abcd");
+    let rec_dummy_sarg__ = r.recipe("dummy_sarg__");
 
     // The pinned reg is fixed to a certain value entirely user-controlled, so it generates nothing!
     e.enc64_rec(get_pinned_reg.bind(I64), rec_get_pinned_reg, 0);
@@ -747,6 +749,8 @@ fn define_moves(e: &mut PerCpuModeEncodings, shared_defs: &SharedDefinitions, r:
         copy_to_ssa.bind(F32),
         rec_furm_reg_to_ssa.opcodes(&MOVSS_LOAD),
     );
+
+    e.enc_32_64_rec(dummy_sarg__, rec_dummy_sarg__, 0);
 }
 
 #[inline(never)]

cranelift/codegen/meta/src/isa/x86/recipes.rs

@@ -1270,6 +1270,12 @@ pub(crate) fn define<'shared>(
         );
     }
 
+    recipes.add_recipe(
+        EncodingRecipeBuilder::new("dummy_sarg__", &formats.nullary, 0)
+            .operands_out(vec![Stack::new(gpr)])
+            .emit(""),
+    );
+
     // XX+rd id with Abs4 function relocation.
     recipes.add_template_recipe(
         EncodingRecipeBuilder::new("fnaddr4", &formats.func_addr, 4)