Auto merge of rust-lang#96862 - oli-obk:enum_cast_mir, r=RalfJung

Change enum->int casts to not go through MIR casts. follow-up to rust-lang#96814 this simplifies all backends and even gives LLVM more information about the return value of `Rvalue::Discriminant`, enabling optimizations in more cases.
Dylan-DPC · Jul 5, 2022 · 53792b9 · 53792b9
2 parents 4045ce6 + 82c73af
commit 53792b9
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Showing 21 changed files with 238 additions and 143 deletions.
diff --git a/compiler/rustc_codegen_cranelift/src/base.rs b/compiler/rustc_codegen_cranelift/src/base.rs
@@ -635,29 +635,6 @@ fn codegen_stmt<'tcx>(
                             let (ptr, _extra) = operand.load_scalar_pair(fx);
                             lval.write_cvalue(fx, CValue::by_val(ptr, dest_layout))
                         }
-                    } else if let ty::Adt(adt_def, _substs) = from_ty.kind() {
-                        // enum -> discriminant value
-                        assert!(adt_def.is_enum());
-                        match to_ty.kind() {
-                            ty::Uint(_) | ty::Int(_) => {}
-                            _ => unreachable!("cast adt {} -> {}", from_ty, to_ty),
-                        }
-                        let to_clif_ty = fx.clif_type(to_ty).unwrap();
-
-                        let discriminant = crate::discriminant::codegen_get_discriminant(
-                            fx,
-                            operand,
-                            fx.layout_of(operand.layout().ty.discriminant_ty(fx.tcx)),
-                        )
-                        .load_scalar(fx);
-
-                        let res = crate::cast::clif_intcast(
-                            fx,
-                            discriminant,
-                            to_clif_ty,
-                            to_ty.is_signed(),
-                        );
-                        lval.write_cvalue(fx, CValue::by_val(res, dest_layout));
                     } else {
                         let to_clif_ty = fx.clif_type(to_ty).unwrap();
                         let from = operand.load_scalar(fx);

diff --git a/compiler/rustc_codegen_llvm/src/builder.rs b/compiler/rustc_codegen_llvm/src/builder.rs
@@ -28,7 +28,7 @@ use std::ffi::CStr;
 use std::iter;
 use std::ops::Deref;
 use std::ptr;
-use tracing::debug;
+use tracing::{debug, instrument};
 
 // All Builders must have an llfn associated with them
 #[must_use]
@@ -464,15 +464,15 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         }
     }
 
+    #[instrument(level = "trace", skip(self))]
     fn load_operand(&mut self, place: PlaceRef<'tcx, &'ll Value>) -> OperandRef<'tcx, &'ll Value> {
-        debug!("PlaceRef::load: {:?}", place);
-
         assert_eq!(place.llextra.is_some(), place.layout.is_unsized());
 
         if place.layout.is_zst() {
             return OperandRef::new_zst(self, place.layout);
         }
 
+        #[instrument(level = "trace", skip(bx))]
         fn scalar_load_metadata<'a, 'll, 'tcx>(
             bx: &mut Builder<'a, 'll, 'tcx>,
             load: &'ll Value,

diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
@@ -1619,7 +1619,7 @@ extern "C" {
         B: &Builder<'a>,
         Val: &'a Value,
         DestTy: &'a Type,
-        IsSized: bool,
+        IsSigned: bool,
     ) -> &'a Value;
 
     // Comparisons

diff --git a/compiler/rustc_codegen_ssa/src/mir/place.rs b/compiler/rustc_codegen_ssa/src/mir/place.rs
@@ -204,6 +204,7 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
     }
 
     /// Obtain the actual discriminant of a value.
+    #[instrument(level = "trace", skip(bx))]
     pub fn codegen_get_discr<Bx: BuilderMethods<'a, 'tcx, Value = V>>(
         self,
         bx: &mut Bx,
@@ -420,12 +421,12 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
 }
 
 impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
+    #[instrument(level = "trace", skip(self, bx))]
     pub fn codegen_place(
         &mut self,
         bx: &mut Bx,
         place_ref: mir::PlaceRef<'tcx>,
     ) -> PlaceRef<'tcx, Bx::Value> {
-        debug!("codegen_place(place_ref={:?})", place_ref);
         let cx = self.cx;
         let tcx = self.cx.tcx();
 

diff --git a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
@@ -12,17 +12,15 @@ use rustc_middle::ty::cast::{CastTy, IntTy};
 use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf};
 use rustc_middle::ty::{self, adjustment::PointerCast, Instance, Ty, TyCtxt};
 use rustc_span::source_map::{Span, DUMMY_SP};
-use rustc_target::abi::{Abi, Int, Variants};
 
 impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
+    #[instrument(level = "trace", skip(self, bx))]
     pub fn codegen_rvalue(
         &mut self,
         mut bx: Bx,
         dest: PlaceRef<'tcx, Bx::Value>,
         rvalue: &mir::Rvalue<'tcx>,
     ) -> Bx {
-        debug!("codegen_rvalue(dest.llval={:?}, rvalue={:?})", dest.llval, rvalue);
-
         match *rvalue {
             mir::Rvalue::Use(ref operand) => {
                 let cg_operand = self.codegen_operand(&mut bx, operand);
@@ -285,74 +283,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             CastTy::from_ty(operand.layout.ty).expect("bad input type for cast");
                         let r_t_out = CastTy::from_ty(cast.ty).expect("bad output type for cast");
                         let ll_t_in = bx.cx().immediate_backend_type(operand.layout);
-                        match operand.layout.variants {
-                            Variants::Single { index } => {
-                                if let Some(discr) =
-                                    operand.layout.ty.discriminant_for_variant(bx.tcx(), index)
-                                {
-                                    let discr_layout = bx.cx().layout_of(discr.ty);
-                                    let discr_t = bx.cx().immediate_backend_type(discr_layout);
-                                    let discr_val = bx.cx().const_uint_big(discr_t, discr.val);
-                                    let discr_val =
-                                        bx.intcast(discr_val, ll_t_out, discr.ty.is_signed());
-
-                                    return (
-                                        bx,
-                                        OperandRef {
-                                            val: OperandValue::Immediate(discr_val),
-                                            layout: cast,
-                                        },
-                                    );
-                                }
-                            }
-                            Variants::Multiple { .. } => {}
-                        }
                         let llval = operand.immediate();
 
-                        let mut signed = false;
-                        if let Abi::Scalar(scalar) = operand.layout.abi {
-                            if let Int(_, s) = scalar.primitive() {
-                                // We use `i1` for bytes that are always `0` or `1`,
-                                // e.g., `#[repr(i8)] enum E { A, B }`, but we can't
-                                // let LLVM interpret the `i1` as signed, because
-                                // then `i1 1` (i.e., E::B) is effectively `i8 -1`.
-                                signed = !scalar.is_bool() && s;
-
-                                if !scalar.is_always_valid(bx.cx())
-                                    && scalar.valid_range(bx.cx()).end
-                                        >= scalar.valid_range(bx.cx()).start
-                                {
-                                    // We want `table[e as usize ± k]` to not
-                                    // have bound checks, and this is the most
-                                    // convenient place to put the `assume`s.
-                                    if scalar.valid_range(bx.cx()).start > 0 {
-                                        let enum_value_lower_bound = bx.cx().const_uint_big(
-                                            ll_t_in,
-                                            scalar.valid_range(bx.cx()).start,
-                                        );
-                                        let cmp_start = bx.icmp(
-                                            IntPredicate::IntUGE,
-                                            llval,
-                                            enum_value_lower_bound,
-                                        );
-                                        bx.assume(cmp_start);
-                                    }
-
-                                    let enum_value_upper_bound = bx
-                                        .cx()
-                                        .const_uint_big(ll_t_in, scalar.valid_range(bx.cx()).end);
-                                    let cmp_end = bx.icmp(
-                                        IntPredicate::IntULE,
-                                        llval,
-                                        enum_value_upper_bound,
-                                    );
-                                    bx.assume(cmp_end);
-                                }
-                            }
-                        }
-
                         let newval = match (r_t_in, r_t_out) {
-                            (CastTy::Int(_), CastTy::Int(_)) => bx.intcast(llval, ll_t_out, signed),
+                            (CastTy::Int(i), CastTy::Int(_)) => {
+                                bx.intcast(llval, ll_t_out, i.is_signed())
+                            }
                             (CastTy::Float, CastTy::Float) => {
                                 let srcsz = bx.cx().float_width(ll_t_in);
                                 let dstsz = bx.cx().float_width(ll_t_out);
@@ -364,8 +300,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                                     llval
                                 }
                             }
-                            (CastTy::Int(_), CastTy::Float) => {
-                                if signed {
+                            (CastTy::Int(i), CastTy::Float) => {
+                                if i.is_signed() {
                                     bx.sitofp(llval, ll_t_out)
                                 } else {
                                     bx.uitofp(llval, ll_t_out)
@@ -374,8 +310,9 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             (CastTy::Ptr(_) | CastTy::FnPtr, CastTy::Ptr(_)) => {
                                 bx.pointercast(llval, ll_t_out)
                             }
-                            (CastTy::Int(_), CastTy::Ptr(_)) => {
-                                let usize_llval = bx.intcast(llval, bx.cx().type_isize(), signed);
+                            (CastTy::Int(i), CastTy::Ptr(_)) => {
+                                let usize_llval =
+                                    bx.intcast(llval, bx.cx().type_isize(), i.is_signed());
                                 bx.inttoptr(usize_llval, ll_t_out)
                             }
                             (CastTy::Float, CastTy::Int(IntTy::I)) => {

diff --git a/compiler/rustc_codegen_ssa/src/mir/statement.rs b/compiler/rustc_codegen_ssa/src/mir/statement.rs
@@ -6,9 +6,8 @@ use crate::traits::BuilderMethods;
 use crate::traits::*;
 
 impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
+    #[instrument(level = "debug", skip(self, bx))]
     pub fn codegen_statement(&mut self, mut bx: Bx, statement: &mir::Statement<'tcx>) -> Bx {
-        debug!("codegen_statement(statement={:?})", statement);
-
         self.set_debug_loc(&mut bx, statement.source_info);
         match statement.kind {
             mir::StatementKind::Assign(box (ref place, ref rvalue)) => {

diff --git a/compiler/rustc_const_eval/src/interpret/cast.rs b/compiler/rustc_const_eval/src/interpret/cast.rs
@@ -8,7 +8,7 @@ use rustc_middle::mir::CastKind;
 use rustc_middle::ty::adjustment::PointerCast;
 use rustc_middle::ty::layout::{IntegerExt, LayoutOf, TyAndLayout};
 use rustc_middle::ty::{self, FloatTy, Ty, TypeAndMut};
-use rustc_target::abi::{Integer, Variants};
+use rustc_target::abi::Integer;
 use rustc_type_ir::sty::TyKind::*;
 
 use super::{
@@ -128,12 +128,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             Float(FloatTy::F64) => {
                 return Ok(self.cast_from_float(src.to_scalar()?.to_f64()?, cast_ty).into());
             }
-            // The rest is integer/pointer-"like", including fn ptr casts and casts from enums that
-            // are represented as integers.
+            // The rest is integer/pointer-"like", including fn ptr casts
             _ => assert!(
                 src.layout.ty.is_bool()
                     || src.layout.ty.is_char()
-                    || src.layout.ty.is_enum()
                     || src.layout.ty.is_integral()
                     || src.layout.ty.is_any_ptr(),
                 "Unexpected cast from type {:?}",
@@ -143,25 +141,6 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
         // # First handle non-scalar source values.
 
-        // Handle cast from a ZST enum (0 or 1 variants).
-        match src.layout.variants {
-            Variants::Single { index } => {
-                if src.layout.abi.is_uninhabited() {
-                    // This is dead code, because an uninhabited enum is UB to
-                    // instantiate.
-                    throw_ub!(Unreachable);
-                }
-                if let Some(discr) = src.layout.ty.discriminant_for_variant(*self.tcx, index) {
-                    assert!(src.layout.is_zst());
-                    let discr_layout = self.layout_of(discr.ty)?;
-
-                    let scalar = Scalar::from_uint(discr.val, discr_layout.layout.size());
-                    return Ok(self.cast_from_int_like(scalar, discr_layout, cast_ty)?.into());
-                }
-            }
-            Variants::Multiple { .. } => {}
-        }
-
         // Handle casting any ptr to raw ptr (might be a fat ptr).
         if src.layout.ty.is_any_ptr() && cast_ty.is_unsafe_ptr() {
             let dest_layout = self.layout_of(cast_ty)?;

diff --git a/compiler/rustc_const_eval/src/transform/validate.rs b/compiler/rustc_const_eval/src/transform/validate.rs
@@ -7,9 +7,9 @@ use rustc_middle::mir::interpret::Scalar;
 use rustc_middle::mir::visit::NonUseContext::VarDebugInfo;
 use rustc_middle::mir::visit::{PlaceContext, Visitor};
 use rustc_middle::mir::{
-    traversal, AggregateKind, BasicBlock, BinOp, Body, BorrowKind, Local, Location, MirPass,
-    MirPhase, Operand, Place, PlaceElem, PlaceRef, ProjectionElem, Rvalue, SourceScope, Statement,
-    StatementKind, Terminator, TerminatorKind, UnOp, START_BLOCK,
+    traversal, AggregateKind, BasicBlock, BinOp, Body, BorrowKind, CastKind, Local, Location,
+    MirPass, MirPhase, Operand, Place, PlaceElem, PlaceRef, ProjectionElem, Rvalue, SourceScope,
+    Statement, StatementKind, Terminator, TerminatorKind, UnOp, START_BLOCK,
 };
 use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::{self, InstanceDef, ParamEnv, Ty, TyCtxt, TypeFoldable};
@@ -361,6 +361,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                     );
                 }
             }
+            Rvalue::Ref(..) => {}
             Rvalue::Len(p) => {
                 let pty = p.ty(&self.body.local_decls, self.tcx).ty;
                 check_kinds!(
@@ -503,7 +504,30 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                 let a = operand.ty(&self.body.local_decls, self.tcx);
                 check_kinds!(a, "Cannot shallow init type {:?}", ty::RawPtr(..));
             }
-            _ => {}
+            Rvalue::Cast(kind, operand, target_type) => {
+                match kind {
+                    CastKind::Misc => {
+                        let op_ty = operand.ty(self.body, self.tcx);
+                        if op_ty.is_enum() {
+                            self.fail(
+                                location,
+                                format!(
+                                    "enum -> int casts should go through `Rvalue::Discriminant`: {operand:?}:{op_ty} as {target_type}",
+                                ),
+                            );
+                        }
+                    }
+                    // Nothing to check here
+                    CastKind::PointerFromExposedAddress
+                    | CastKind::PointerExposeAddress
+                    | CastKind::Pointer(_) => {}
+                }
+            }
+            Rvalue::Repeat(_, _)
+            | Rvalue::ThreadLocalRef(_)
+            | Rvalue::AddressOf(_, _)
+            | Rvalue::NullaryOp(_, _)
+            | Rvalue::Discriminant(_) => {}
         }
         self.super_rvalue(rvalue, location);
     }

diff --git a/compiler/rustc_middle/src/ty/cast.rs b/compiler/rustc_middle/src/ty/cast.rs
@@ -15,6 +15,12 @@ pub enum IntTy {
     Char,
 }
 
+impl IntTy {
+    pub fn is_signed(self) -> bool {
+        matches!(self, Self::I)
+    }
+}
+
 // Valid types for the result of a non-coercion cast
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum CastTy<'tcx> {

diff --git a/compiler/rustc_mir_build/src/build/expr/as_rvalue.rs b/compiler/rustc_mir_build/src/build/expr/as_rvalue.rs
@@ -1,6 +1,7 @@
 //! See docs in `build/expr/mod.rs`.
 
 use rustc_index::vec::Idx;
+use rustc_middle::ty::util::IntTypeExt;
 
 use crate::build::expr::as_place::PlaceBase;
 use crate::build::expr::category::{Category, RvalueFunc};
@@ -190,7 +191,30 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
             ExprKind::Cast { source } => {
                 let source = &this.thir[source];
-                let from_ty = CastTy::from_ty(source.ty);
+
+                // Casting an enum to an integer is equivalent to computing the discriminant and casting the
+                // discriminant. Previously every backend had to repeat the logic for this operation. Now we
+                // create all the steps directly in MIR with operations all backends need to support anyway.
+                let (source, ty) = if let ty::Adt(adt_def, ..) = source.ty.kind() && adt_def.is_enum() {
+                    let discr_ty = adt_def.repr().discr_type().to_ty(this.tcx);
+                    let place = unpack!(block = this.as_place(block, source));
+                    let discr = this.temp(discr_ty, source.span);
+                    this.cfg.push_assign(
+                        block,
+                        source_info,
+                        discr,
+                        Rvalue::Discriminant(place),
+                    );
+
+                    (Operand::Move(discr), discr_ty)
+                } else {
+                    let ty = source.ty;
+                    let source = unpack!(
+                        block = this.as_operand(block, scope, source, None, NeedsTemporary::No)
+                    );
+                    (source, ty)
+                };
+                let from_ty = CastTy::from_ty(ty);
                 let cast_ty = CastTy::from_ty(expr.ty);
                 let cast_kind = match (from_ty, cast_ty) {
                     (Some(CastTy::Ptr(_) | CastTy::FnPtr), Some(CastTy::Int(_))) => {
@@ -201,9 +225,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     }
                     (_, _) => CastKind::Misc,
                 };
-                let source = unpack!(
-                    block = this.as_operand(block, scope, source, None, NeedsTemporary::No)
-                );
                 block.and(Rvalue::Cast(cast_kind, source, expr.ty))
             }
             ExprKind::Pointer { cast, source } => {