Auto merge of #79547 - erikdesjardins:byval, r=nagisa

Pass arguments up to 2*usize by value

In https://github.com/rust-lang/rust/pull/77434#discussion_r498719533, `@eddyb` said:

> I wonder if it makes sense to limit this to returns [...]

Let's do a perf run and find out.

It seems the `extern "C"` ABI will pass arguments up to 2*usize in registers: https://godbolt.org/z/n8E6zc. (modified from https://github.com/rust-lang/rust/issues/26494#issuecomment-619506345)

r? `@nagisa`

compiler/rustc_middle/src/ty/layout.rs

@@ -2848,7 +2848,7 @@ where
             || abi == SpecAbi::RustIntrinsic
             || abi == SpecAbi::PlatformIntrinsic
         {
-            let fixup = |arg: &mut ArgAbi<'tcx, Ty<'tcx>>, is_ret: bool| {
+            let fixup = |arg: &mut ArgAbi<'tcx, Ty<'tcx>>| {
                 if arg.is_ignore() {
                     return;
                 }
@@ -2886,9 +2886,9 @@ where
                     _ => return,
                 }
 
-                // Return structures up to 2 pointers in size by value, matching `ScalarPair`. LLVM
-                // will usually return these in 2 registers, which is more efficient than by-ref.
-                let max_by_val_size = if is_ret { Pointer.size(cx) * 2 } else { Pointer.size(cx) };
+                // 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(cx) * 2;
                 let size = arg.layout.size;
 
                 if arg.layout.is_unsized() || size > max_by_val_size {
@@ -2900,9 +2900,9 @@ where
                     arg.cast_to(Reg { kind: RegKind::Integer, size });
                 }
             };
-            fixup(&mut self.ret, true);
+            fixup(&mut self.ret);
             for arg in &mut self.args {
-                fixup(arg, false);
+                fixup(arg);
             }
             return;
         }

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

@@ -1,4 +1,4 @@
-//! This test checks that types of up to 128 bits are returned by-value instead of via out-pointer.
+//! Check that types of up to 128 bits are passed and returned by-value instead of via pointer.
 
 // compile-flags: -C no-prepopulate-passes -O
 // only-x86_64
@@ -11,7 +11,7 @@ pub struct S {
     c: u32,
 }
 
-// CHECK: define i128 @modify(%S* noalias nocapture dereferenceable(16) %s)
+// CHECK: define i128 @modify(i128{{( %0)?}})
 #[no_mangle]
 pub fn modify(s: S) -> S {
     S { a: s.a + s.a, b: s.b + s.b, c: s.c + s.c }

src/test/codegen/union-abi.rs

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