Revert rust-lang#26494 regression

Author: shampoofactory

compiler/rustc_middle/src/ty/layout.rs

@@ -3226,12 +3226,8 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                     _ => return,
                 }
 
-                // Pass and return structures up to 2 pointers in size by value, matching `ScalarPair`.
-                // LLVM will usually pass these in 2 registers, which is more efficient than by-ref.
-                let max_by_val_size = Pointer.size(self) * 2;
                 let size = arg.layout.size;
-
-                if arg.layout.is_unsized() || size > max_by_val_size {
+                if arg.layout.is_unsized() || size > Pointer.size(self) {
                     arg.make_indirect();
                 } else {
                     // We want to pass small aggregates as immediates, but using

src/test/codegen/arg-return-value-in-reg.rs

@@ -1,5 +1,6 @@
 // compile-flags: -O
 // only-x86_64
+// ignore-test will need to be rewritten if pull accepted
 
 #![crate_type = "lib"]
 

src/test/codegen/array-equality.rs

@@ -63,16 +63,11 @@ pub union UnionU128{a:u128}
 #[no_mangle]
 pub fn test_UnionU128(_: UnionU128) -> UnionU128 { loop {} }
 
-pub union UnionU128x2{a:(u128, u128)}
-// CHECK: define void @test_UnionU128x2(i128 %_1.0, i128 %_1.1)
-#[no_mangle]
-pub fn test_UnionU128x2(_: UnionU128x2) { loop {} }
-
 #[repr(C)]
-pub union CUnionU128x2{a:(u128, u128)}
-// CHECK: define void @test_CUnionU128x2(%CUnionU128x2* {{.*}} %_1)
+pub union CUnionU128{a:u128}
+// CHECK: define void @test_CUnionU128(%CUnionU128* {{.*}} %_1)
 #[no_mangle]
-pub fn test_CUnionU128x2(_: CUnionU128x2) { loop {} }
+pub fn test_CUnionU128(_: CUnionU128) { loop {} }
 
 pub union UnionBool { b:bool }
 // CHECK: define noundef zeroext i1 @test_UnionBool(i8 %b)