Auto merge of #85158 - JulianKnodt:array_const_val, r=cjgillot

Mir-Opt for copying enums with large discrepancies

I have been meaning to make this for quite a while, based off of this [hackmd](https://hackmd.io/`@ft4bxUsFT5CEUBmRKYHr7w/rJM8BBPzD).`

I'm not sure where to put this opt now that I've made it, so I'd appreciate suggestions on that!
It's also one long chain of statements, not sure if there's a more friendly format to make it.

r? `@tmiasko`
I would `r` oli but he's on leave so he suggested I `r` tmiasko or wesleywiser.

2 files changed, 301 insertions, 0 deletions

diff --git a/compiler/rustc_mir_transform/src/large_enums.rs b/compiler/rustc_mir_transform/src/large_enums.rs
new file mode 100644
index 00000000000..89f8de23583
--- /dev/null
+++ b/compiler/rustc_mir_transform/src/large_enums.rs
@@ -0,0 +1,298 @@
+use crate::rustc_middle::ty::util::IntTypeExt;
+use crate::MirPass;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_middle::mir::interpret::AllocId;
+use rustc_middle::mir::*;
+use rustc_middle::ty::{self, AdtDef, Const, ParamEnv, Ty, TyCtxt};
+use rustc_session::Session;
+use rustc_target::abi::{HasDataLayout, Size, TagEncoding, Variants};
+
+/// A pass that seeks to optimize unnecessary moves of large enum types, if there is a large
+/// enough discrepancy between them.
+///
+/// i.e. If there is are two variants:
+/// ```
+/// enum Example {
+///   Small,
+///   Large([u32; 1024]),
+/// }
+/// ```
+/// Instead of emitting moves of the large variant,
+/// Perform a memcpy instead.
+/// Based off of [this HackMD](https://hackmd.io/@ft4bxUsFT5CEUBmRKYHr7w/rJM8BBPzD).
+///
+/// In summary, what this does is at runtime determine which enum variant is active,
+/// and instead of copying all the bytes of the largest possible variant,
+/// copy only the bytes for the currently active variant.
+pub struct EnumSizeOpt {
+    pub(crate) discrepancy: u64,
+}
+
+impl<'tcx> MirPass<'tcx> for EnumSizeOpt {
+    fn is_enabled(&self, sess: &Session) -> bool {
+        sess.opts.unstable_opts.unsound_mir_opts || sess.mir_opt_level() >= 3
+    }
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        // NOTE: This pass may produce different MIR based on the alignment of the target
+        // platform, but it will still be valid.
+        self.optim(tcx, body);
+    }
+}
+
+impl EnumSizeOpt {
+    fn candidate<'tcx>(
+        &self,
+        tcx: TyCtxt<'tcx>,
+        param_env: ParamEnv<'tcx>,
+        ty: Ty<'tcx>,
+        alloc_cache: &mut FxHashMap<Ty<'tcx>, AllocId>,
+    ) -> Option<(AdtDef<'tcx>, usize, AllocId)> {
+        let adt_def = match ty.kind() {
+            ty::Adt(adt_def, _substs) if adt_def.is_enum() => adt_def,
+            _ => return None,
+        };
+        let layout = tcx.layout_of(param_env.and(ty)).ok()?;
+        let variants = match &layout.variants {
+            Variants::Single { .. } => return None,
+            Variants::Multiple { tag_encoding, .. }
+                if matches!(tag_encoding, TagEncoding::Niche { .. }) =>
+            {
+                return None;
+            }
+            Variants::Multiple { variants, .. } if variants.len() <= 1 => return None,
+            Variants::Multiple { variants, .. } => variants,
+        };
+        let min = variants.iter().map(|v| v.size).min().unwrap();
+        let max = variants.iter().map(|v| v.size).max().unwrap();
+        if max.bytes() - min.bytes() < self.discrepancy {
+            return None;
+        }
+
+        let num_discrs = adt_def.discriminants(tcx).count();
+        if variants.iter_enumerated().any(|(var_idx, _)| {
+            let discr_for_var = adt_def.discriminant_for_variant(tcx, var_idx).val;
+            (discr_for_var > usize::MAX as u128) || (discr_for_var as usize >= num_discrs)
+        }) {
+            return None;
+        }
+        if let Some(alloc_id) = alloc_cache.get(&ty) {
+            return Some((*adt_def, num_discrs, *alloc_id));
+        }
+
+        let data_layout = tcx.data_layout();
+        let ptr_sized_int = data_layout.ptr_sized_integer();
+        let target_bytes = ptr_sized_int.size().bytes() as usize;
+        let mut data = vec![0; target_bytes * num_discrs];
+        macro_rules! encode_store {
+            ($curr_idx: expr, $endian: expr, $bytes: expr) => {
+                let bytes = match $endian {
+                    rustc_target::abi::Endian::Little => $bytes.to_le_bytes(),
+                    rustc_target::abi::Endian::Big => $bytes.to_be_bytes(),
+                };
+                for (i, b) in bytes.into_iter().enumerate() {
+                    data[$curr_idx + i] = b;
+                }
+            };
+        }
+
+        for (var_idx, layout) in variants.iter_enumerated() {
+            let curr_idx =
+                target_bytes * adt_def.discriminant_for_variant(tcx, var_idx).val as usize;
+            let sz = layout.size;
+            match ptr_sized_int {
+                rustc_target::abi::Integer::I32 => {
+                    encode_store!(curr_idx, data_layout.endian, sz.bytes() as u32);
+                }
+                rustc_target::abi::Integer::I64 => {
+                    encode_store!(curr_idx, data_layout.endian, sz.bytes());
+                }
+                _ => unreachable!(),
+            };
+        }
+        let alloc = interpret::Allocation::from_bytes(
+            data,
+            tcx.data_layout.ptr_sized_integer().align(&tcx.data_layout).abi,
+            Mutability::Not,
+        );
+        let alloc = tcx.create_memory_alloc(tcx.intern_const_alloc(alloc));
+        Some((*adt_def, num_discrs, *alloc_cache.entry(ty).or_insert(alloc)))
+    }
+    fn optim<'tcx>(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        let mut alloc_cache = FxHashMap::default();
+        let body_did = body.source.def_id();
+        let param_env = tcx.param_env(body_did);
+
+        let blocks = body.basic_blocks.as_mut();
+        let local_decls = &mut body.local_decls;
+
+        for bb in blocks {
+            bb.expand_statements(|st| {
+                if let StatementKind::Assign(box (
+                    lhs,
+                    Rvalue::Use(Operand::Copy(rhs) | Operand::Move(rhs)),
+                )) = &st.kind
+                {
+                    let ty = lhs.ty(local_decls, tcx).ty;
+
+                    let source_info = st.source_info;
+                    let span = source_info.span;
+
+                    let (adt_def, num_variants, alloc_id) =
+                        self.candidate(tcx, param_env, ty, &mut alloc_cache)?;
+                    let alloc = tcx.global_alloc(alloc_id).unwrap_memory();
+
+                    let tmp_ty = tcx.mk_ty(ty::Array(
+                        tcx.types.usize,
+                        Const::from_usize(tcx, num_variants as u64),
+                    ));
+
+                    let size_array_local = local_decls.push(LocalDecl::new(tmp_ty, span));
+                    let store_live = Statement {
+                        source_info,
+                        kind: StatementKind::StorageLive(size_array_local),
+                    };
+
+                    let place = Place::from(size_array_local);
+                    let constant_vals = Constant {
+                        span,
+                        user_ty: None,
+                        literal: ConstantKind::Val(
+                            interpret::ConstValue::ByRef { alloc, offset: Size::ZERO },
+                            tmp_ty,
+                        ),
+                    };
+                    let rval = Rvalue::Use(Operand::Constant(box (constant_vals)));
+
+                    let const_assign =
+                        Statement { source_info, kind: StatementKind::Assign(box (place, rval)) };
+
+                    let discr_place = Place::from(
+                        local_decls
+                            .push(LocalDecl::new(adt_def.repr().discr_type().to_ty(tcx), span)),
+                    );
+
+                    let store_discr = Statement {
+                        source_info,
+                        kind: StatementKind::Assign(box (discr_place, Rvalue::Discriminant(*rhs))),
+                    };
+
+                    let discr_cast_place =
+                        Place::from(local_decls.push(LocalDecl::new(tcx.types.usize, span)));
+
+                    let cast_discr = Statement {
+                        source_info,
+                        kind: StatementKind::Assign(box (
+                            discr_cast_place,
+                            Rvalue::Cast(
+                                CastKind::IntToInt,
+                                Operand::Copy(discr_place),
+                                tcx.types.usize,
+                            ),
+                        )),
+                    };
+
+                    let size_place =
+                        Place::from(local_decls.push(LocalDecl::new(tcx.types.usize, span)));
+
+                    let store_size = Statement {
+                        source_info,
+                        kind: StatementKind::Assign(box (
+                            size_place,
+                            Rvalue::Use(Operand::Copy(Place {
+                                local: size_array_local,
+                                projection: tcx.intern_place_elems(&[PlaceElem::Index(
+                                    discr_cast_place.local,
+                                )]),
+                            })),
+                        )),
+                    };
+
+                    let dst =
+                        Place::from(local_decls.push(LocalDecl::new(tcx.mk_mut_ptr(ty), span)));
+
+                    let dst_ptr = Statement {
+                        source_info,
+                        kind: StatementKind::Assign(box (
+                            dst,
+                            Rvalue::AddressOf(Mutability::Mut, *lhs),
+                        )),
+                    };
+
+                    let dst_cast_ty = tcx.mk_mut_ptr(tcx.types.u8);
+                    let dst_cast_place =
+                        Place::from(local_decls.push(LocalDecl::new(dst_cast_ty, span)));
+
+                    let dst_cast = Statement {
+                        source_info,
+                        kind: StatementKind::Assign(box (
+                            dst_cast_place,
+                            Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(dst), dst_cast_ty),
+                        )),
+                    };
+
+                    let src =
+                        Place::from(local_decls.push(LocalDecl::new(tcx.mk_imm_ptr(ty), span)));
+
+                    let src_ptr = Statement {
+                        source_info,
+                        kind: StatementKind::Assign(box (
+                            src,
+                            Rvalue::AddressOf(Mutability::Not, *rhs),
+                        )),
+                    };
+
+                    let src_cast_ty = tcx.mk_imm_ptr(tcx.types.u8);
+                    let src_cast_place =
+                        Place::from(local_decls.push(LocalDecl::new(src_cast_ty, span)));
+
+                    let src_cast = Statement {
+                        source_info,
+                        kind: StatementKind::Assign(box (
+                            src_cast_place,
+                            Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(src), src_cast_ty),
+                        )),
+                    };
+
+                    let deinit_old =
+                        Statement { source_info, kind: StatementKind::Deinit(box dst) };
+
+                    let copy_bytes = Statement {
+                        source_info,
+                        kind: StatementKind::Intrinsic(
+                            box NonDivergingIntrinsic::CopyNonOverlapping(CopyNonOverlapping {
+                                src: Operand::Copy(src_cast_place),
+                                dst: Operand::Copy(dst_cast_place),
+                                count: Operand::Copy(size_place),
+                            }),
+                        ),
+                    };
+
+                    let store_dead = Statement {
+                        source_info,
+                        kind: StatementKind::StorageDead(size_array_local),
+                    };
+                    let iter = [
+                        store_live,
+                        const_assign,
+                        store_discr,
+                        cast_discr,
+                        store_size,
+                        dst_ptr,
+                        dst_cast,
+                        src_ptr,
+                        src_cast,
+                        deinit_old,
+                        copy_bytes,
+                        store_dead,
+                    ]
+                    .into_iter();
+
+                    st.make_nop();
+                    Some(iter)
+                } else {
+                    None
+                }
+            });
+        }
+    }
+}
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index 9070a7368b1..45cd4024c9f 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/compiler/rustc_mir_transform/src/lib.rs
@@ -1,6 +1,7 @@
 #![allow(rustc::potential_query_instability)]
 #![feature(box_patterns)]
 #![feature(drain_filter)]
+#![feature(box_syntax)]
 #![feature(let_chains)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]
@@ -73,6 +74,7 @@ mod function_item_references;
 mod generator;
 mod inline;
 mod instcombine;
+mod large_enums;
 mod lower_intrinsics;
 mod lower_slice_len;
 mod match_branches;
@@ -583,6 +585,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &simplify::SimplifyLocals::new("final"),
             &multiple_return_terminators::MultipleReturnTerminators,
             &deduplicate_blocks::DeduplicateBlocks,
+            &large_enums::EnumSizeOpt { discrepancy: 128 },
             // Some cleanup necessary at least for LLVM and potentially other codegen backends.
             &add_call_guards::CriticalCallEdges,
             // Dump the end result for testing and debugging purposes.