Increase the Rust ABI by-value arg/return size limit from 2*usize to 3*usize.

Author: eddyb

compiler/rustc_middle/src/ty/layout.rs

@@ -3214,18 +3214,36 @@ 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;
+                // Pass and return structures up to 3 pointers in size by value,
+                // similar to `ScalarPair`'s 2 values, but more widely applicable.
+                // LLVM will usually pass these in 3 registers, which is more efficient than by-ref.
+                let pointer_size = Pointer.size(self);
+                let max_by_val_size = pointer_size * 3;
                 let size = arg.layout.size;
 
                 if arg.layout.is_unsized() || size > max_by_val_size {
                     arg.make_indirect();
                 } else {
                     // We want to pass small aggregates as immediates, but using
                     // a LLVM aggregate type for this leads to bad optimizations,
-                    // so we pick an appropriately sized integer type instead.
-                    arg.cast_to(Reg { kind: RegKind::Integer, size });
+                    // so we pick an appropriately sized integer type instead,
+                    // for anything small enough.
+                    let cast_target = if size > pointer_size * 2 {
+                        // This will end up as either a mixed aggregate,
+                        // or `[usize; N]` if `size == pointer_size * N`.
+                        call::Uniform {
+                            unit: Reg { kind: RegKind::Integer, size: pointer_size },
+                            total: size,
+                        }
+                    } else {
+                        // FIXME(eddyb) what's the impact of this combining
+                        // several *unrelated* fields into one integer with a
+                        // non-physical size (e.g. `i96`)? this is likely
+                        // alleviated in common cases by `ScalarPair`, which is
+                        // handled more directly, and before "adjustments".
+                        Reg { kind: RegKind::Integer, size }.into()
+                    };
+                    arg.cast_to(cast_target);
                 }
             };
             fixup(&mut fn_abi.ret);

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

@@ -19,7 +19,7 @@ pub fn modify(s: S) -> S {
 
 #[repr(packed)]
 pub struct TooBig {
-    a: u64,
+    a: u128,
     b: u32,
     c: u32,
     d: u8,

src/test/codegen/array-equality.rs

@@ -25,11 +25,11 @@ pub fn array_eq_ref(a: &[u16; 6], b: &[u16; 6]) -> bool {
 
 // CHECK-LABEL: @array_eq_value_still_passed_by_pointer
 #[no_mangle]
-pub fn array_eq_value_still_passed_by_pointer(a: [u16; 9], b: [u16; 9]) -> bool {
+pub fn array_eq_value_still_passed_by_pointer(a: [u16; 13], b: [u16; 13]) -> bool {
     // CHECK-NEXT: start:
     // CHECK-NEXT: bitcast
     // CHECK-NEXT: bitcast
-    // CHECK-NEXT: %[[CMP:.+]] = tail call i32 @{{bcmp|memcmp}}(i8* {{.*}} dereferenceable(18) %{{.+}}, i8* {{.*}} dereferenceable(18) %{{.+}}, i64 18)
+    // CHECK-NEXT: %[[CMP:.+]] = tail call i32 @{{bcmp|memcmp}}(i8* {{.*}} dereferenceable(26) %{{.+}}, i8* {{.*}} dereferenceable(26) %{{.+}}, i64 26)
     // CHECK-NEXT: %[[EQ:.+]] = icmp eq i32 %[[CMP]], 0
     // CHECK-NEXT: ret i1 %[[EQ]]
     a == b

src/test/codegen/issue-15953.rs

@@ -1,29 +1,32 @@
 // Test that llvm generates `memcpy` for moving a value
 // inside a function and moving an argument.
 
+// NOTE(eddyb) this has to be large enough to never be passed in registers.
+type BigWithDrop = [String; 2];
+
 struct Foo {
-    x: Vec<i32>,
+    x: BigWithDrop,
 }
 
 #[inline(never)]
 #[no_mangle]
 // CHECK: memcpy
-fn interior(x: Vec<i32>) -> Vec<i32> {
+fn interior(x: BigWithDrop) -> BigWithDrop {
     let Foo { x } = Foo { x: x };
     x
 }
 
 #[inline(never)]
 #[no_mangle]
 // CHECK: memcpy
-fn exterior(x: Vec<i32>) -> Vec<i32> {
+fn exterior(x: BigWithDrop) -> BigWithDrop {
     x
 }
 
 fn main() {
-    let x = interior(Vec::new());
+    let x = interior(BigWithDrop::default());
     println!("{:?}", x);
 
-    let x = exterior(Vec::new());
+    let x = exterior(BigWithDrop::default());
     println!("{:?}", x);
 }