Tweak the threshold for chunked swapping

Thanks to 98892 for the tests I brought in here, as it demonstrated that 3×usize is currently suboptimal.

Author: AngelicosPhosphoros

library/core/src/mem/mod.rs

@@ -736,7 +736,7 @@ pub const fn swap<T>(x: &mut T, y: &mut T) {
         // tends to copy the whole thing to stack rather than doing it one part
         // at a time, so instead treat them as one-element slices and piggy-back
         // the slice optimizations that will split up the swaps.
-        if size_of::<T>() / align_of::<T>() > 4 {
+        if const { size_of::<T>() / align_of::<T>() > 2 } {
             // SAFETY: exclusive references always point to one non-overlapping
             // element and are non-null and properly aligned.
             return unsafe { ptr::swap_nonoverlapping(x, y, 1) };

tests/codegen/swap-small-types.rs

@@ -26,12 +26,15 @@ pub fn swap_rgb48_manually(x: &mut RGB48, y: &mut RGB48) {
 // CHECK-LABEL: @swap_rgb48
 #[no_mangle]
 pub fn swap_rgb48(x: &mut RGB48, y: &mut RGB48) {
-    // FIXME: See #115212 for why this has an alloca again
+    // CHECK-NOT: alloca
 
-    // CHECK: alloca [3 x i16], align 2
-    // CHECK: call void @llvm.memcpy.p0.p0.i64({{.+}}, i64 6, i1 false)
-    // CHECK: call void @llvm.memcpy.p0.p0.i64({{.+}}, i64 6, i1 false)
-    // CHECK: call void @llvm.memcpy.p0.p0.i64({{.+}}, i64 6, i1 false)
+    // Whether `i8` is the best for this is unclear, but
+    // might as well record what's actually happening right now.
+
+    // CHECK: load i8
+    // CHECK: load i8
+    // CHECK: store i8
+    // CHECK: store i8
     swap(x, y)
 }
 
@@ -41,10 +44,39 @@ type RGBA64 = [u16; 4];
 #[no_mangle]
 pub fn swap_rgba64(x: &mut RGBA64, y: &mut RGBA64) {
     // CHECK-NOT: alloca
-    // CHECK-DAG: %[[XVAL:.+]] = load <4 x i16>, ptr %x, align 2
-    // CHECK-DAG: %[[YVAL:.+]] = load <4 x i16>, ptr %y, align 2
-    // CHECK-DAG: store <4 x i16> %[[YVAL]], ptr %x, align 2
-    // CHECK-DAG: store <4 x i16> %[[XVAL]], ptr %y, align 2
+    // CHECK-DAG: %[[XVAL:.+]] = load i64, ptr %x, align 2
+    // CHECK-DAG: %[[YVAL:.+]] = load i64, ptr %y, align 2
+    // CHECK-DAG: store i64 %[[YVAL]], ptr %x, align 2
+    // CHECK-DAG: store i64 %[[XVAL]], ptr %y, align 2
+    swap(x, y)
+}
+
+// CHECK-LABEL: @swap_vecs
+#[no_mangle]
+pub fn swap_vecs(x: &mut Vec<u32>, y: &mut Vec<u32>) {
+    // CHECK-NOT: alloca
+    // There are plenty more loads and stores than just these,
+    // but at least one sure better be 64-bit (for size or capacity).
+    // CHECK: load i64
+    // CHECK: load i64
+    // CHECK: store i64
+    // CHECK: store i64
+    // CHECK: ret void
+    swap(x, y)
+}
+
+// CHECK-LABEL: @swap_slices
+#[no_mangle]
+pub fn swap_slices<'a>(x: &mut &'a [u32], y: &mut &'a [u32]) {
+    // CHECK-NOT: alloca
+    // CHECK: load ptr
+    // CHECK: load i64
+    // CHECK: load ptr
+    // CHECK: load i64
+    // CHECK: store ptr
+    // CHECK: store i64
+    // CHECK: store ptr
+    // CHECK: store i64
     swap(x, y)
 }
 
@@ -55,9 +87,9 @@ type RGB24 = [u8; 3];
 // CHECK-LABEL: @swap_rgb24_slices
 #[no_mangle]
 pub fn swap_rgb24_slices(x: &mut [RGB24], y: &mut [RGB24]) {
-// CHECK-NOT: alloca
-// CHECK: load <{{[0-9]+}} x i8>
-// CHECK: store <{{[0-9]+}} x i8>
+    // CHECK-NOT: alloca
+    // CHECK: load <{{[0-9]+}} x i8>
+    // CHECK: store <{{[0-9]+}} x i8>
     if x.len() == y.len() {
         x.swap_with_slice(y);
     }
@@ -69,9 +101,9 @@ type RGBA32 = [u8; 4];
 // CHECK-LABEL: @swap_rgba32_slices
 #[no_mangle]
 pub fn swap_rgba32_slices(x: &mut [RGBA32], y: &mut [RGBA32]) {
-// CHECK-NOT: alloca
-// CHECK: load <{{[0-9]+}} x i32>
-// CHECK: store <{{[0-9]+}} x i32>
+    // CHECK-NOT: alloca
+    // CHECK: load <{{[0-9]+}} x i32>
+    // CHECK: store <{{[0-9]+}} x i32>
     if x.len() == y.len() {
         x.swap_with_slice(y);
     }
@@ -84,10 +116,24 @@ const _: () = assert!(!std::mem::size_of::<String>().is_power_of_two());
 // CHECK-LABEL: @swap_string_slices
 #[no_mangle]
 pub fn swap_string_slices(x: &mut [String], y: &mut [String]) {
-// CHECK-NOT: alloca
-// CHECK: load <{{[0-9]+}} x i64>
-// CHECK: store <{{[0-9]+}} x i64>
+    // CHECK-NOT: alloca
+    // CHECK: load <{{[0-9]+}} x i64>
+    // CHECK: store <{{[0-9]+}} x i64>
     if x.len() == y.len() {
         x.swap_with_slice(y);
     }
 }
+
+#[repr(C, packed)]
+pub struct Packed {
+    pub first: bool,
+    pub second: usize,
+}
+
+// CHECK-LABEL: @swap_packed_structs
+#[no_mangle]
+pub fn swap_packed_structs(x: &mut Packed, y: &mut Packed) {
+    // CHECK-NOT: alloca
+    // CHECK: ret void
+    swap(x, y)
+}