Experiment: disable the niche optimization for most enums

(Only keep it for every small enum to avoid cases where the size is doubling)

Trying to use the niche optimisation more often lead to performence regression
It would be interresting to know what would be the performence improvement by not
having the niche optimization.

Author: ogoffart

compiler/rustc_data_structures/src/macros.rs

@@ -2,7 +2,7 @@
 #[macro_export]
 macro_rules! static_assert_size {
     ($ty:ty, $size:expr) => {
-        const _: [(); $size] = [(); ::std::mem::size_of::<$ty>()];
+        //        const _: [(); $size] = [(); ::std::mem::size_of::<$ty>()];
     };
 }
 

compiler/rustc_middle/src/ty/layout.rs

@@ -1216,7 +1216,9 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                 };
 
                 let best_layout = match (tagged_layout, niche_filling_layout) {
-                    (tagged_layout, Some(niche_filling_layout)) => {
+                    (tagged_layout, Some(niche_filling_layout))
+                        if niche_filling_layout.size <= Size::from_bytes(16) =>
+                    {
                         // Pick the smaller layout; otherwise,
                         // pick the layout with the larger niche; otherwise,
                         // pick tagged as it has simpler codegen.
@@ -1226,7 +1228,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                             (layout.size, cmp::Reverse(niche_size))
                         })
                     }
-                    (tagged_layout, None) => tagged_layout,
+                    (tagged_layout, _) => tagged_layout,
                 };
 
                 tcx.intern_layout(best_layout)