Auto merge of #74611 - Mark-Simulacrum:revert-74069-bad-niche, r=eddyb

Revert "Compare tagged/niche-filling layout and pick the best one"

Reverts rust-lang/rust#74069. It caused a performance regression, see https://github.com/rust-lang/rust/pull/74069#issuecomment-662166827. perf: https://perf.rust-lang.org/compare.html?start=d3df8512d2c2afc6d2e7d8b5b951dd7f2ad77b02&end=cfade73820883adf654fe34fd6b0b03a99458a51

r? @eddyb

cc @nnethercote

1 files changed, 4 insertions, 22 deletions

diff --git a/src/librustc_middle/ty/layout.rs b/src/librustc_middle/ty/layout.rs
index dc775b15927..215f44819b5 100644
--- a/src/librustc_middle/ty/layout.rs
+++ b/src/librustc_middle/ty/layout.rs
@@ -876,8 +876,6 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                     .iter_enumerated()
                     .all(|(i, v)| v.discr == ty::VariantDiscr::Relative(i.as_u32()));
 
-                let mut niche_filling_layout = None;
-
                 // Niche-filling enum optimization.
                 if !def.repr.inhibit_enum_layout_opt() && no_explicit_discriminants {
                     let mut dataful_variant = None;
@@ -974,7 +972,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                             let largest_niche =
                                 Niche::from_scalar(dl, offset, niche_scalar.clone());
 
-                            niche_filling_layout = Some(Layout {
+                            return Ok(tcx.intern_layout(Layout {
                                 variants: Variants::Multiple {
                                     tag: niche_scalar,
                                     tag_encoding: TagEncoding::Niche {
@@ -993,7 +991,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                                 largest_niche,
                                 size,
                                 align,
-                            });
+                            }));
                         }
                     }
                 }
@@ -1216,7 +1214,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
 
                 let largest_niche = Niche::from_scalar(dl, Size::ZERO, tag.clone());
 
-                let tagged_layout = Layout {
+                tcx.intern_layout(Layout {
                     variants: Variants::Multiple {
                         tag,
                         tag_encoding: TagEncoding::Direct,
@@ -1231,23 +1229,7 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                     abi,
                     align,
                     size,
-                };
-
-                let best_layout = match (tagged_layout, niche_filling_layout) {
-                    (tagged_layout, Some(niche_filling_layout)) => {
-                        // Pick the smaller layout; otherwise,
-                        // pick the layout with the larger niche; otherwise,
-                        // pick tagged as it has simpler codegen.
-                        cmp::min_by_key(tagged_layout, niche_filling_layout, |layout| {
-                            let niche_size =
-                                layout.largest_niche.as_ref().map_or(0, |n| n.available(dl));
-                            (layout.size, cmp::Reverse(niche_size))
-                        })
-                    }
-                    (tagged_layout, None) => tagged_layout,
-                };
-
-                tcx.intern_layout(best_layout)
+                })
             }
 
             // Types with no meaningful known layout.