Auto merge of #138582 - scottmcm:option-ssa-2, r=<try>

Don't require `alloca`s for consuming simple enums

Well, 4 months later I'm finally back to this.

For example, if you pass an `Option<u32>` to a function, don't immediately write it to an `alloca` then read it again.

Author: bors

compiler/rustc_abi/src/lib.rs

@@ -1821,7 +1821,7 @@ pub enum Variants<FieldIdx: Idx, VariantIdx: Idx> {
 
     /// Single enum variants, structs/tuples, unions, and all non-ADTs.
     Single {
-        /// Always `0` for types that cannot have multiple variants.
+        /// Always [`FIRST_VARIANT`] for types that cannot have multiple variants.
         index: VariantIdx,
     },
 

compiler/rustc_codegen_ssa/src/mir/analyze.rs

@@ -1,12 +1,13 @@
 //! An analysis to determine which locals require allocas and
 //! which do not.
 
+use rustc_abi as abi;
 use rustc_data_structures::graph::dominators::Dominators;
 use rustc_index::bit_set::DenseBitSet;
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::{self, DefLocation, Location, TerminatorKind, traversal};
-use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf};
+use rustc_middle::ty::layout::{LayoutOf, TyAndLayout};
 use rustc_middle::{bug, span_bug};
 use tracing::debug;
 
@@ -99,63 +100,117 @@ impl<'a, 'b, 'tcx, Bx: BuilderMethods<'b, 'tcx>> LocalAnalyzer<'a, 'b, 'tcx, Bx>
         context: PlaceContext,
         location: Location,
     ) {
-        let cx = self.fx.cx;
+        if !place_ref.projection.is_empty() {
+            const COPY_CONTEXT: PlaceContext =
+                PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy);
+
+            // `PlaceElem::Index` is the only variant that can mention other `Local`s,
+            // so check for those up-front before any potential short-circuits.
+            for elem in place_ref.projection {
+                if let mir::PlaceElem::Index(index_local) = *elem {
+                    self.visit_local(index_local, COPY_CONTEXT, location);
+                }
+            }
 
-        if let Some((place_base, elem)) = place_ref.last_projection() {
-            let mut base_context = if context.is_mutating_use() {
-                PlaceContext::MutatingUse(MutatingUseContext::Projection)
-            } else {
-                PlaceContext::NonMutatingUse(NonMutatingUseContext::Projection)
-            };
+            // If our local is already memory, nothing can make it *more* memory
+            // so we don't need to bother checking the projections further.
+            if self.locals[place_ref.local] == LocalKind::Memory {
+                return;
+            }
 
-            // Allow uses of projections that are ZSTs or from scalar fields.
-            let is_consume = matches!(
-                context,
-                PlaceContext::NonMutatingUse(
-                    NonMutatingUseContext::Copy | NonMutatingUseContext::Move,
-                )
-            );
-            if is_consume {
-                let base_ty = place_base.ty(self.fx.mir, cx.tcx());
-                let base_ty = self.fx.monomorphize(base_ty);
-
-                // ZSTs don't require any actual memory access.
-                let elem_ty = base_ty.projection_ty(cx.tcx(), self.fx.monomorphize(elem)).ty;
-                let span = self.fx.mir.local_decls[place_ref.local].source_info.span;
-                if cx.spanned_layout_of(elem_ty, span).is_zst() {
-                    return;
+            if place_ref.is_indirect_first_projection() {
+                // If this starts with a `Deref`, we only need to record a read of the
+                // pointer being dereferenced, as all the subsequent projections are
+                // working on a place which is always supported. (And because we're
+                // looking at codegen MIR, it can only happen as the first projection.)
+                self.visit_local(place_ref.local, COPY_CONTEXT, location);
+                return;
+            }
+
+            if context.is_mutating_use() {
+                // If it's a mutating use it doesn't matter what the projections are,
+                // if there are *any* then we need a place to write. (For example,
+                // `_1 = Foo()` works in SSA but `_2.0 = Foo()` does not.)
+                let mut_projection = PlaceContext::MutatingUse(MutatingUseContext::Projection);
+                self.visit_local(place_ref.local, mut_projection, location);
+                return;
+            }
+
+            // Scan through to ensure the only projections are those which
+            // `FunctionCx::maybe_codegen_consume_direct` can handle.
+            let base_ty = self.fx.monomorphized_place_ty(mir::PlaceRef::from(place_ref.local));
+            let mut layout = self.fx.cx.layout_of(base_ty);
+            for elem in place_ref.projection {
+                if layout.is_zst() {
+                    // Any further projections must still be ZSTs, so we're good.
+                    break;
                 }
 
-                if let mir::ProjectionElem::Field(..) = elem {
-                    let layout = cx.spanned_layout_of(base_ty.ty, span);
-                    if cx.is_backend_immediate(layout) || cx.is_backend_scalar_pair(layout) {
-                        // Recurse with the same context, instead of `Projection`,
-                        // potentially stopping at non-operand projections,
-                        // which would trigger `not_ssa` on locals.
-                        base_context = context;
+                #[track_caller]
+                fn compatible_projection(src: TyAndLayout<'_>, tgt: TyAndLayout<'_>) -> bool {
+                    fn compatible_initness(a: abi::Scalar, b: abi::Scalar) -> bool {
+                        !a.is_uninit_valid() || b.is_uninit_valid()
+                    }
+
+                    use abi::BackendRepr::*;
+                    match (src.backend_repr, tgt.backend_repr) {
+                        _ if tgt.is_zst() => true,
+                        (Scalar(a), Scalar(b))
+                        | (SimdVector { element: a, .. }, SimdVector { element: b, .. }) => {
+                            compatible_initness(a, b)
+                        }
+                        (ScalarPair(a0, a1), Scalar(b)) => {
+                            compatible_initness(a0, b) && compatible_initness(a1, b)
+                        }
+                        (ScalarPair(a0, a1), ScalarPair(b0, b1)) => {
+                            compatible_initness(a0, b0) && compatible_initness(a1, b1)
+                        }
+                        // This arm is a hack; remove it as part of
+                        // <https://github.com/rust-lang/compiler-team/issues/838>
+                        (SimdVector { .. }, Memory { .. }) => true,
+                        _ => bug!("Mismatched layouts in analysis\nsrc: {src:?}\ntgt: {tgt:?}"),
                     }
                 }
-            }
 
-            if let mir::ProjectionElem::Deref = elem {
-                // Deref projections typically only read the pointer.
-                base_context = PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy);
-            }
+                match *elem {
+                    mir::PlaceElem::Field(fidx, ..) => {
+                        let field_layout = layout.field(self.fx.cx, fidx.as_usize());
+                        if compatible_projection(layout, field_layout) {
+                            layout = field_layout;
+                            continue;
+                        }
+                    }
+                    mir::PlaceElem::Downcast(_, vidx) => {
+                        let variant_layout = layout.for_variant(self.fx.cx, vidx);
+                        if compatible_projection(layout, variant_layout) {
+                            layout = variant_layout;
+                            continue;
+                        }
+                    }
 
-            self.process_place(&place_base, base_context, location);
-            // HACK(eddyb) this emulates the old `visit_projection_elem`, this
-            // entire `visit_place`-like `process_place` method should be rewritten,
-            // now that we have moved to the "slice of projections" representation.
-            if let mir::ProjectionElem::Index(local) = elem {
-                self.visit_local(
-                    local,
-                    PlaceContext::NonMutatingUse(NonMutatingUseContext::Copy),
-                    location,
-                );
+                    mir::PlaceElem::Index(..)
+                    | mir::PlaceElem::ConstantIndex { .. }
+                    | mir::PlaceElem::Subslice { .. }
+                    | mir::PlaceElem::OpaqueCast(..)
+                    | mir::PlaceElem::UnwrapUnsafeBinder(..)
+                    | mir::PlaceElem::Subtype(..) => {}
+
+                    mir::PlaceElem::Deref => bug!("Deref after first position"),
+                }
+
+                // If the above didn't `continue`, we can't handle the projection.
+                self.locals[place_ref.local] = LocalKind::Memory;
+                return;
             }
-        } else {
-            self.visit_local(place_ref.local, context, location);
+            debug_assert!(
+                !self.fx.cx.is_backend_ref(layout),
+                "Post-projection {place_ref:?} layout should be non-Ref, but it's {layout:?}",
+            );
         }
+
+        // Even with supported projections, we still need to have `visit_local`
+        // check for things that can't be done in SSA (like `SharedBorrow`).
+        self.visit_local(place_ref.local, context, location);
     }
 }
 

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -1,6 +1,6 @@
 use std::cmp;
 
-use rustc_abi::{Align, BackendRepr, ExternAbi, HasDataLayout, Reg, Size, WrappingRange};
+use rustc_abi::{Align, BackendRepr, ExternAbi, FieldIdx, HasDataLayout, Reg, Size, WrappingRange};
 use rustc_ast as ast;
 use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
 use rustc_data_structures::packed::Pu128;
@@ -1038,7 +1038,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             let (idx, _) = op.layout.non_1zst_field(bx).expect(
                                 "not exactly one non-1-ZST field in a `DispatchFromDyn` type",
                             );
-                            op = op.extract_field(self, bx, idx.as_usize());
+                            op = op.extract_field(self, bx, None, idx);
                         }
 
                         // Now that we have `*dyn Trait` or `&dyn Trait`, split it up into its
@@ -1598,7 +1598,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         } else {
             // If the tuple is immediate, the elements are as well.
             for i in 0..tuple.layout.fields.count() {
-                let op = tuple.extract_field(self, bx, i);
+                let op = tuple.extract_field(self, bx, None, FieldIdx::from_usize(i));
                 self.codegen_argument(bx, op, llargs, &args[i], lifetime_ends_after_call);
             }
         }

compiler/rustc_codegen_ssa/src/mir/locals.rs

@@ -12,6 +12,7 @@ use tracing::{debug, warn};
 use crate::mir::{FunctionCx, LocalRef};
 use crate::traits::BuilderMethods;
 
+#[derive(Debug)]
 pub(super) struct Locals<'tcx, V> {
     values: IndexVec<mir::Local, LocalRef<'tcx, V>>,
 }

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -136,6 +136,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
     }
 }
 
+#[derive(Debug)]
 enum LocalRef<'tcx, V> {
     Place(PlaceRef<'tcx, V>),
     /// `UnsizedPlace(p)`: `p` itself is a thin pointer (indirect place).